The Human Brain During the Early First Trimester (Atlas of Human Central Nervous System Development)

The HUMAN BRAIN during the EARLY FIRST TRIMESTER © 2008 by Taylor & Francis Group, LLC 1424_FM.indd 1 6/5/07 3:19:3...

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The

HUMAN BRAIN during the

EARLY FIRST TRIMESTER

© 2008 by Taylor & Francis Group, LLC 1424_FM.indd 1

6/5/07 3:19:35 PM

ATLAS OF HUMAN CENTRAL NERVOUS SYSTEM DEVELOPMENT SERIES Shirley A. Bayer and Joseph Altman

VOLUME 1 The Spinal Cord from Gestational Week 4 to the 4th Postnatal Month

VOLUME 2 The Human Brain during the Third Trimester

VOLUME 3 The Human Brain during the Second Trimester

VOLUME 4 The Human Brain during the Late First Trimester

VOLUME 5 The Human Brain during the Early First Trimester


6/5/07 3:19:35 PM

The

HUMAN BRAIN during the

EARLY FIRST TRIMESTER

Shirley A. Bayer and Joseph Altman

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business


6/5/07 3:19:35 PM

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2008 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-0-8493-1424-7 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

© 2008 by Taylor & Francis Group, LLC T&F_LOC_C_Master.indd 1424_FM.indd 4 1

6/4/07 3:19:36 6/5/07 2:20:46 PM

DEDICATION We dedicate this volume to the new generation of neuroscientists: those who use powerful molecular techniques to study the mechanisms of central nervous system (CNS) development, and those who use advanced scanning techniques to monitor the development of the CNS under normal and abnormal conditions.

ACKNOWLEDGMENTS We thank Dr. William DeMyer, pediatric neurologist at Indiana University Medical Center, for access to his personal library on human CNS development. We also thank the staff of the National Museum of Health and Medicine at the Armed Forces Institute of Pathology, Walter Reed Hospital, Washington, D.C.: Dr. Adrianne Noe, Director; Archibald J. Fobbs, Curator of the Yakovlev Collection; Elizabeth C. Lockett; and William Discher. We are most grateful to Dr. James M. Petras at the Walter Reed Institute of Research who made his darkroom facilities available so that we could develop all the photomicrographs on location rather than in our laboratory in Indiana. Finally, we thank Barbara Norwitz, Kari Budyk, and Suzanne Lassandro at CRC Press/Taylor & Francis for their personal attention to us and for expert help during production of the manuscript.

CONTENTS PART I. INTRODUCTION -------------------------------------------------------------------------------------------1 A. Organization of the Atlas ------------------------------------------------------------------------------1 B. Specimens -------------------------------------------------------------------------------------------------2 C. Photography and Computer Processing ------------------------------------------------------------2 D. Identification of Transient and Immature Brain Regions ---------------------------------------3 E. Major Developmental Features of the First Trimester Brain ----------------------------------3 PART II. GW7 CORONAL ---------------------------------------------------------------------------------------------4 Plates 1A, 1B (Level 1: Section 50) ----------------------------------------------------------------------6, 7 Plates 2A, 2B (Level 2: Section 116) --------------------------------------------------------------------8, 9 Plates 3A, 3B (Level 3: Section 164) -----------------------------------------------------------------10, 11 Plates 4A, 4B (Level 4: Section 201) -----------------------------------------------------------------12, 13 Plates 5A, 5B (Level 5: Section 242) -----------------------------------------------------------------14, 15 Plates 6A, 6B (Level 6: Section 283) -----------------------------------------------------------------16, 17 Plates 7A, 7B (Level 7: Section 325) -----------------------------------------------------------------18, 19 Plates 8A, 8B (Level 8: Section 375) -----------------------------------------------------------------20, 21 Plates 9A, 9B (Level 9: Section 410) -----------------------------------------------------------------22, 23 Plates 10A, 10B (Level 10: Section 424) -------------------------------------------------------------24, 25 Plates 11A, 11B (Level 11: Section 444) -------------------------------------------------------------26, 27 Plates 12A, 12B (Level 12: Section 500) -------------------------------------------------------------28, 29 Plates 13A, 13B (Level 13: Section 533) -------------------------------------------------------------30, 31 Plates 14A, 14B (Level 14: Section 572) -------------------------------------------------------------32, 33 Plates 15A, 15B (Level 15: Section 588) -------------------------------------------------------------34, 35 Plates 16A, 16B (Level 16: Section 628) -------------------------------------------------------------36, 37 Plates 17A, 17B (Level 17: Section 677) -------------------------------------------------------------38, 39 Plates 18A, 18B (Cerebral Cortex and Thalamus: Section 203) ----------------------------------40, 41 Plates 19A, 19B (Cerebral Cortex and Thalamus: Section 236) ----------------------------------42, 43 Plates 20A, 20B (Diencephalon and Mesencephalon: Section 390) ------------------------------44, 45 PART III. GW7 SAGITTAL ------------------------------------------------------------------------------------------- 46 Plates 21A, 21B (Level 1: Slide 13, Section 5) ------------------------------------------------------48, 49 Plates 22A, 22B (Level 2: Slide 12, Section 5) ------------------------------------------------------50, 51 Plates 23A, 23B (Level 3: Slide 11, Section 5) ------------------------------------------------------52, 53 Plates 24A, 24B (Level 4: Slide 9, Section 5) -------------------------------------------------------54, 55 Plates 25A, 25B (Level 5: Slide 8, Section 8) -------------------------------------------------------56, 57 Plates 26A, 26B (Level 6: Slide 8, Section 2) -------------------------------------------------------58, 59 Plates 27A, 27B (Level 7: Slide 6, Section 11) ------------------------------------------------------60, 61 Plates 28A, 28B (Pons/Medulla: Slides 9, 9, 8, Sections 5, 2, 8, respectively) -----------------62, 63 PART IV. GW7 HORIZONTAL ------------------------------------------------------------------------------------- 64 Plates 29A, 29B (Level 1: Section 9) ------------------------------------------------------------------66, 67 Plates 30A, 30B (Level 2: Section 29) ----------------------------------------------------------------68, 69 Plates 31A, 31B (Level 3: Section 36) ----------------------------------------------------------------70, 71 Plates 32A, 32B (Level 4: Section 39) ----------------------------------------------------------------72, 73 Plates 33A, 33B (Level 5: Section 45) ----------------------------------------------------------------74, 75 Plates 34A, 34B (Level 6: Section 50) ----------------------------------------------------------------76, 77 Plates 35A, 35B (Level 7: Section 55) ----------------------------------------------------------------78, 79 Plates 36A, 36B (Level 8: Section 65) ----------------------------------------------------------------80, 81 Plates 37A, 37B (Level 9: Section 71) ----------------------------------------------------------------82, 83 Plates 38A, 38B (Level 10: Section 87) --------------------------------------------------------------84, 85 Plates 39A, 39B (Level 11: Section 94) --------------------------------------------------------------86, 87 Plates 40A, 40B (Level 12: Section 112) -------------------------------------------------------------88, 89 Plates 41A, 41B (Level 13: Section 118) -------------------------------------------------------------90, 91 Plates 42A, 42B (Level 14: Section 141) -------------------------------------------------------------92, 93

CONTENTS Plates 43A, 43B (Level 15: Section 152) -------------------------------------------------------------94, 95 Plates 44A, 44B (Level 16: Section 169) -------------------------------------------------------------96, 97 Plates 45A, 45B (Level 17: Section 205) -------------------------------------------------------------98, 99 PART V. GW6.5 CORONAL ---------------------------------------------------------------------------------------100 Plates 46A, 46B (Level 1: Sections 66) ------------------------------------------------------------102, 103 Plates 47A, 47B (Level 2: Sections 107) ----------------------------------------------------------104, 105 Plates 48A, 48B (Level 3: Section 130) -----------------------------------------------------------106, 107 Plates 49A, 49B (Level 4: Section 159) -----------------------------------------------------------108, 109 Plates 50A, 50B (Level 5: Section 190) -----------------------------------------------------------110, 111 Plates 51A, 51B (Level 6: Section 241) -----------------------------------------------------------112, 113 Plates 52A, 52B (Level 7: Section 258) -----------------------------------------------------------114, 115 Plates 53A, 53B (Level 8: Section 285) -----------------------------------------------------------116, 117 Plates 54A, 54B (Level 9: Section 330) -----------------------------------------------------------118, 119 Plates 55A, 55B (Level 10: Section 357) ----------------------------------------------------------120, 121 Plates 56A-56B (Level 11: Section 384) ----------------------------------------------------------122, 123 Plates 57A, 57B (Level 12: Section 420) ----------------------------------------------------------124, 125 Plates 58A, 58B (Level 13: Section 438) ----------------------------------------------------------126, 127 Plates 59A, 59B (Level 14: Section 488) ----------------------------------------------------------128, 129 Plates 60A, 60B (Level 15: Section 553) ----------------------------------------------------------130, 131 Plates 61A, 61B (Level 16: Section 583) ----------------------------------------------------------132, 133 Plates 63A, 63B (Cerebral Cortex, Future Paracentral Lobule: Section 643) ----------------136, 137 PART VI. GW6.5 SAGITTAL ---------------------------------------------------------------------------------------138 Plates 64A, 64B (Level 1: Slide 27, Section 14) --------------------------------------------------140, 141 Plates 65A, 65B (Level 2: Slide 25, Section 9) ---------------------------------------------------142, 143 Plates 66A, 66B (Level 3: Slide 23, Section 8) ---------------------------------------------------144, 145 Plates 67A, 67B (Level 4: Slide 21, Section 2) ---------------------------------------------------146, 147 Plates 68A, 68B (Level 5: Slide 19, Section 8) ---------------------------------------------------148, 149 Plates 69A, 69B (Level 6: Slide 18, Section 8) ---------------------------------------------------150, 151 Plates 70A, 70B (Level 7: Slide 16, Section 8) ---------------------------------------------------152, 153 Plates 71A, 71B (Dorsal Neocortex: Slide 23, Section 8) ---------------------------------------154, 155 Plates 72A, 72B (Hippocampus and Thalamus: Slide 26, Section 9) -------------------------156, 157 Plates 73A, 73B (Hypothalamus: Slide 27, Section 14) -----------------------------------------158, 159 Plates 74A, 74B (Mesencephalic Tegmentum: Slides 27, 21, Sections 7. 8. respectively) --------------------------------------------------------- 160-161 Plates 75A, 75B (Mesencephalic Tectum, Isthmus, and Cerebellum: Slide 24, Section 8) --------------------------------------------------------------162, 163 Plates 76A, 76B (Cerebellum: Slides 27, 18, Sections 14, 5, respectively) ------------------164, 165 Plates 77A, 77B (Trigeminal Nerve Entry Zone: Slide 19, Section 3) ------------------------166, 167 Plates 78A, 78B (Lateral Cerebellum, Pons, and Medulla: Slide 16, Section 3) -------------168, 169 Plates 79A, 79B (Trigeminal and Vestibulo-Cochlear Nerve Entry Zones: Slide 16, Section 13) -----------------------------------------------------------------170, 171 Plates 80A, 80B (Entry Zones of Nerves IX and X: Slide 18, Section 13) -------------------172, 173 Plates 81A, 81B (Medial Pons and Medulla: Slide 21, Section 8) -----------------------------174, 175 Plates 82A, 82B (Neuroepithelium and Midline Raphe Glial Structure [Isthmus and Upper Pons]: Slide 27, Section 14) ------------------------------------------------176, 177 Plates 83A, 83B (Neuroepithelium and Midline Raphe Glial Structure [Near Pontine Flexure]: Slide 27, Section 14) ---------------------------------------------------178, 179 Plates 84A, 84B (Neuroepithelium and Midline Raphe Glial Structure [Near Medullary Flexure]: Slide 27, Section 14) ------------------------------------------------180, 181 PART VII. GW5.5 CORONAL ---------------------------------------------------------------------------------------182 Plates 85A, 85B (Level 1: Section 29) -------------------------------------------------------------184, 185

CONTENTS Plates 86A, 86B (Level 2: Section 42) -------------------------------------------------------------186, 187 Plates 87A, 87B (Level 3: Section 100) -----------------------------------------------------------188, 189 Plates 88A, 88B (Level 4: Section 128) -----------------------------------------------------------190, 191 Plates 89A, 89B (Level 5: Section 169) -----------------------------------------------------------192, 193 Plates 90A, 90B (Level 6: Section 192) -----------------------------------------------------------194, 195 Plates 91A, 91B (Level 7: Section 215) -----------------------------------------------------------196, 197 Plates 92A, 92B (Level 8: Section 237) -----------------------------------------------------------198, 199 Plates 93A, 93B (Level 9: Section 255) -----------------------------------------------------------200, 201 Plates 94A, 94B (Level 10: Section 269) ----------------------------------------------------------202, 203 Plates 95A, 95B (Level 11: Section 285) ----------------------------------------------------------204, 205 Plates 96A, 96B (Level 12: Section 308) ----------------------------------------------------------206, 207 Plates 97A, 97B (Level 13: Section 334) ----------------------------------------------------------208, 209 Plates 98A, 98B (Level 14: Section 376) ----------------------------------------------------------210, 211 Plates 99A, 99B (Telencephalon and Diencephalon: Sections 85, 83, 123) ------------------212, 213 PART VIII. GW5.5 SAGITTAL ---------------------------------------------------------------------------------------214 Plates 100A, 100B (Level 1: Slide 11, Section 6) -------------------------------------------------216, 217 Plates 101A, 101B (Level 2: Slide 9, Section 14) ------------------------------------------------218, 219 Plates 102A, 102B (Level 3: Slide 8, Section 14) ------------------------------------------------220, 221 Plates 103A, 103B (Level 4: Slide 7, Section 10) ------------------------------------------------222, 223 Plates 104A, 104B (Level 5: Slide 6, Section 15) ------------------------------------------------224, 225 Plates 105A, 105B (Dorsal Cerebral Cortex: Slide 18, Section 8) -----------------------------226, 227 Plates 106A, 106B (Basal Telencephalon: Slide 11, Section 6) --------------------------------228, 229 Plates 107A, 107B (Septum and Diencephalon: Slide 11, Section 6) -------------------------230, 231 Plates 108A, 109B (Midbrain Tegmentum: Slide 11, Section 6) -------------------------------232, 233 Plates 109A, 109B (Isthmus and Cerebellum: Slide 11, Section 6) ---------------------------234, 235 Plates 110A, 111A, 110B. 111B (Pons and Medulla: Slide 8, Section 10) ------------------- 236-239 Plates 112A, 112B (Rhombencephalon: Slide 7, Section 6) ------------------------------------240, 241 Plates 113A, 114A, 113B, 114B (Rhombencephalon: Slide 6, Section 11) ------------------ 242-245 PART IX. GW5 CORONAL ------------------------------------------------------------------------------------------246 Plates 115A, 115B (Levels 1-2: Sections 12, 42) ------------------------------------------------248, 249 Plates 116A, 116B (Level 3: Section 82) ----------------------------------------------------------250, 251 Plates 117A, 117B (Level 4: Section 97) ----------------------------------------------------------252, 253 Plates 118A, 118B (Level 5: Section 117) --------------------------------------------------------254, 255 Plates 119A, 119B (Level 6: Section 127) --------------------------------------------------------256, 257 Plates 120A, 120B (Level 7: Section 162) --------------------------------------------------------258, 259 Plates 121A, 121B (Level 8: Section 172) --------------------------------------------------------260, 261 Plates 122A, 122B (Level 9: Section 182) --------------------------------------------------------262, 263 Plates 123A, 123B (Level 10: Section 192) -------------------------------------------------------264, 265 Plates 124A, 124B (Level 11: Section 222) -------------------------------------------------------266, 267 PART X. GW5 SAGITTAL -------------------------------------------------------------------------------------------268 Plates 125A, 125B (Level 1: Slide 6, Section 2) --------------------------------------------------270, 271 Plates 126A, 126B (Level 2: Slide 5, Section 2) -------------------------------------------------272, 273 Plates 127A, 127B (Level 3: Slide 4, Section 7) -------------------------------------------------274, 275 Plates 128A, 128B (Level 4: Slide 3, Section 24) ------------------------------------------------276, 277 Plates 129A, 129B (Level 5: Slide 3, Section 12) ------------------------------------------------278, 279 Plates 130A, 130B (Level 6: Slide 3, Section 5) -------------------------------------------------280, 281 Plates 131A, 131B (Level 7: Slide 2, Section 22) ------------------------------------------------282, 283 Plates 132A, 132B (Hypothalamus, Mesencephalon, and Rhombencephalon: Slide 5, Section 22) -----------------------------------------------------------284, 285 Plates 133A, 133B (Cerebellum and Pons: Slide 3, Section 24) -------------------------------286, 287

CONTENTS PART XI. GW4.5 CORONAL ---------------------------------------------------------------------------------------288 Plates 134A, 134B (Levels 1-2: Sections 5, 35) --------------------------------------------------290, 291 Plates 135A, 135B (Level 3: Section 65) ----------------------------------------------------------292, 293 Plates 136A, 136B (Level 4: Section 75) ----------------------------------------------------------294, 295 Plates 137A, 137B (Level 5: Section 85) ----------------------------------------------------------296, 297 Plates 138A, 138B (Level 6: Section 95) ----------------------------------------------------------298, 299 Plates 139A, 139B (Level 7: Section 115) --------------------------------------------------------300, 301 Plates 140A, 140B (Level 8: Section 135) --------------------------------------------------------302, 303 Plates 141A, 141B (Level 9: Section 145) --------------------------------------------------------304, 305 Plates 142A, 142B (Level 10: Section 155) -------------------------------------------------------306, 307 Plates 143A, 143B (Level 11: Section 165) -------------------------------------------------------308, 309 Plates 144A, 144B (Level 12: Section 185) -------------------------------------------------------310, 311 Plates 145A, 145B (Level 13: Section 200) -------------------------------------------------------312, 313 Plates 146A, 146B (Level 12: Section 210) -------------------------------------------------------314, 315 PART XII. GW4 SAGITTAL ------------------------------------------------------------------------------------------316 Plates 147A, 147B (Level 1: Slide 4, Section 32) -------------------------------------------------318, 319 Plates 148A, 148B (Level 2: Slide 4, Section 24) ------------------------------------------------320, 321 Plates 149A, 149B (Level 3: Slide 4, Section 16) ------------------------------------------------322, 323 Plates 150A, 150B (Level 4: Slide 4, Section 8) -------------------------------------------------324, 325 Plates 151A, 151B (Level 5: Slide 3, Section 40) ------------------------------------------------326, 327 Plates 152A, 152B (Level 6: Slide 3, Section 32) ------------------------------------------------328, 329 Plates 153A, 153B (Level 7: Slide 3, Section 24) ------------------------------------------------330, 331 Plates 154A, 154B (Level 8: Slide 3, Section 16) ------------------------------------------------332, 333 Plates 155A, 155B (Level 9: Slide 3, Section 8) -------------------------------------------------334, 335 Plates 156A, 156B (Subdivisions of the Prosencephalic Neuroepithelium: Slide 4, Section 24) -------------------------------------------------------------336, 337 Plates 157A, 157B (Isthmus, Cerebellum, and Pons: Slide 4, Section 24) -------------------338, 339 Plates 158A, 158B (Rhombomeres in Pons and Medulla: Slide 3, Section 40) --------------340, 341 Plates 159A, 159B (Rhombencephalon and Sensory Cranial Nerve Entry Zones: Slide 3, Section 24) ----------------------------------------------------------342, 343 Plates 160A, 160B (Midline Raphe Glial Structure: Slide 4, Section 24) --------------------344, 345 PART XIII. GW4 CORONAL ------------------------------------------------------------------------------------------346 Plates 161A, 161B (Levels 1-2: Sections 9, 27) --------------------------------------------------348, 349 Plates 162A, 162B (Level 3: Section 36) ----------------------------------------------------------350, 351 Plates 163A, 163B (Level 4: Section 48) ----------------------------------------------------------352, 353 Plates 164A, 164B (Level 5: Section 63) ----------------------------------------------------------354, 355 Plates 165A, 165B (Level 6: Section 72) ----------------------------------------------------------356, 357 Plates 166A, 166B (Level 7: Section 75) ----------------------------------------------------------358, 359 Plates 167A, 167B (Level 8: Section 81) ----------------------------------------------------------360, 361 Plates 168A, 168B (Level 9: Section 84) ----------------------------------------------------------362, 363 Plates 169A, 169B (Level 10: Section 90) --------------------------------------------------------364, 365 Plates 170A, 170B (Level 11: Section 93) --------------------------------------------------------366, 367 Plates 171A, 171B (Level 12: Section 99) --------------------------------------------------------368, 369 PART XIV. GW3.8 SAGITTAL ---------------------------------------------------------------------------------------370 Plates 172A, 172B (Level 1: Slide 2, Section 30) -------------------------------------------------372, 373 Plates 173A, 173B (Level 2: Slide 2, Section 24) ------------------------------------------------374, 375 Plates 174A, 174B (Level 3: Slide 2, Section 20) ------------------------------------------------376, 377 Plates 175A, 175B (Level 4: Slide 2, Section 16) ------------------------------------------------378, 379 Plates 176A, 176B (Level 5: Slide 2, Section 12) ------------------------------------------------380, 381 Plates 177A, 177B (Level 6: Slide 2, Section 8) -------------------------------------------------382, 383 Plates 178A, 178B (Level 7: Slide 1, Section 38) ------------------------------------------------384, 385

CONTENTS Plates 179A, 179B (Level 8: Slide 1, Section 30) ------------------------------------------------386, 387 Plates 180A-182A, 180B-182B (Prosencephalon, Mesencephalon, and Anterior Rhombencephalon: Slide 2, Sections 33, 24, and 20 respectively) ---------------------------- 388-393 Plates 183A, 183B (Lateral Mesencephalon and Rhombencephalon: Slide 2, Section 12) 394, 395 Plates 184A, 184B (Lateral Prosencephalon, Mesencephalon, and Rhombencephalon: Slide 2, Section 42) -----------------------------------------------------------------------------------396, 397 PART XV. GW3.2 CORONAL ---------------------------------------------------------------------------------------398 Plates 185A, 185B (Levels 1-2: Sections 3, 13) --------------------------------------------------400, 401 Plates 186A, 186B (Level 3: Section 18) ----------------------------------------------------------402, 403 Plates 187A, 187B (Level 4: Section 28) ----------------------------------------------------------404, 405 Plates 188A, 188B (Level 5: Section 33) ----------------------------------------------------------406, 407 Plates 189A, 189B (Level 6: Section 38) ----------------------------------------------------------408, 409 Plates 190A, 190B (Level 7: Section 43) ----------------------------------------------------------410, 411 Plates 191A, 191B (Level 8: Section 58) ----------------------------------------------------------412, 413 Plates 192A, 192B (Level 9: Section 63) ----------------------------------------------------------414, 415 Plates 193A, 193B (Level 10: Section 68) --------------------------------------------------------416, 417 Plates 194A, 194B (Level 11: Section 73) --------------------------------------------------------418, 419 Plates 195A, 195B (Level 12: Section 78) --------------------------------------------------------420, 421 Plates 196A, 196B (Level 13: Section 83) --------------------------------------------------------422, 423 Plates 197A, 197B (Levels 14-15: Sections 88, 93) ---------------------------------------------424, 425 PART XVI. CONCLUDING ESSAY ---------------------------------------------------------------------------------426 A. Overview -----------------------------------------------------------------------------------------------426 B. The NEP Matrix: Stockbuilding NEP Cells and Differentiating NEP Cells --------------428 C. The Superventricles and the Superarachnoid Reticulum ------------------------------------430 D. Metamerism or Mosaicism as Principles of CNS Development -----------------------------441 E. Exogenous and Endogenous Mechanisms of NEP Cell Diversification --------------------447 F. Timespans of Neurogenesis -------------------------------------------------------------------------465 G. Cell Migration, Sojourn Zones, Secondary Germinal Matrices, and Fate-Restricted Glioepithelia ------------------------------------------------------------------466 H. Centro-Central Signaling and the Morphogenetic Maturation of the CNS----------------482 I. Summary: The Epochs, Phases, and Mechanisms of CNS Development -------------------484 J. A Note on the Functional Maturation of the Human CNS ------------------------------------485 APPENDIX Timespans of Neurogenesis ------------------------------------------------------------------------------490 REFERENCES ------------------------------------------------------------------------------------------------------------------498 GLOSSARY

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1

PART PARTII

INTRODUCTION INTRODUCTION A. Organization of the Atlas This is the last volume in the Atlas of Human Central Nervous System Development series. It deals with human brain development during the early first trimester from the 3rd through the 7th gestational weeks (GW3-GW7). Volume 1 (Bayer and Altman, 2002) records the development of the spinal cord from GW4 to the 4th postnatal month. Volumes 2 through 5 deal with prenatal brain development. The analysis procedes in reverse (older-toyounger) order: from more recognizable brain structures in the third trimester to progressively less familiar structures in the second trimester to often uncertain or hypothetical structures in the first trimester. Volume 2 (Bayer and Altman, 2004a) records brain development during the third trimester, with specimens ranging in age from GW37 to GW26; its major theme is the maturation of the brain’s settled and enduring neuron populations. Volume 3 (Bayer and Altman, 2005) deals with brain development during the second trimester, with specimens ranging in age from GW24 to GW13.5; its major theme is the migration, sojourning, and settling of the brain’s diverse neuron populations. Volume 4 (Bayer and Altman, 2006) presents brain development during the late first trimester, with specimens ranging in age from GW11 to GW7.5; its major theme is the neuroepithelial mosaics that generate different populations of neurons and glia. This volume presents brain development during the early first trimester, with specimens ranging in age from GW7.0 to GW3.2, and has four major themes: (1) growth of the stockbuilding neuroepithelium along the expanding shorelines of the brain’s superventricles, (2) early neurogenesis, (3) the onset of brain parenchymal development related to the expansion and decline of the superarachnoid reticulum, and (4) the inductive and signaling interactions between the brain and peripheral structures in the skull. The present volume features 14 normal specimens. Approximately two specimens near the same age were selected for analysis, one cut in the transverse (mainly coronal) plane, the other cut in the sagittal plane. For the oldest age group (GW7), there is a third specimen sectioned mainly in the horizontal plane. (Younger horizontally sectioned specimens are not in any of the collections we examined.) Each specimen is presented as a series of grayscale photographs of its Nissl-stained brain sections

including the surrounding skull (Parts II through XV). The photographs are shown from anterior to posterior (coronal specimens), medial to lateral (sagittal specimens), and dorsal to ventral (horizontal GW7 specimen). Portrait orientation is used for the coronal specimens; the dorsal part of each section is toward the top of the page, the ventral part at the bottom, and the midline is in the vertical center of each section. Landscape orientation is used for the horizontal specimen; the anterior part of each section is facing to the left (bottom of page), posterior to the right (top of page), and the midline is in the horizontal center of each section. All coronal and horizontal specimes have computer-aided 3-dimensional reconstructions of their brains showing each section’s location. That reconstruction clears up the ambiguity about the exact plane of sectioning through each brain; a problem we addressed in Volume 4. Portrait orientation is used for all sagittal specimens; the anterior part of each section is facing left, posterior right, dorsal top, and ventral bottom. Parts II through XV contain companion plates, designated as A and B on facing pages. Part A on the left page shows the full contrast photograph with labels of the skull and peripheral neural structures; part B on the right page shows low contrast copies of the same photograph with superimposed outlines of the labeled brain parts. The low magnification plates show entire sections to identify the large structures and subdivisions of the brain. The high magnification plates feature enlarged views of the brain core to identify smaller structures. For ease of interpretation in all plates, the ventricles are labeled in capitals, the neuroepithelium and other germinal zones in Helvetica bold, transient structures in Times bold italic, and permanent structures in Times Roman or Times bold. Fixation artifacts are usually outlined with dashed lines in part B of each plate, but few specimens in this volume have artifacts. Since this is the final volume in the series, a Concluding Essay (Part XVI) links major themes of brain development at the cell and tissue level (described in the Atlas Series) with current neuro-developmental studies on gene expression and other molecular markers. Figure 15 to Figure 43 in the essay bring together photographs of individual brain structures at various ages so that the sequence of development is immediately apparent. An Appendix contains tables listing the estimated timespans of neurogenesis for the major populations in the human central ner-

2 vous system (CNS) based on experimentally determined data in rats. References follow the appendix, and the Atlas concludes with an alphabetized Glossary that defines the developmental structures labeled in the plates.

B. Specimens All specimens are from the collections of human embryos and fetal brains currently kept at the National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington, D.C. Nine specimens are from the Carnegie Collection and are designated by their respective numbers with the prefix C. The Carnegie Collection was started by Franklin P. Mall (1862-1917) and expanded at the Carnegie Institution of Washington under the direction of George L. Streeter (1873-1948) and George W. Corner (1889-1981). Five specimens are from the Minot Collection and are designated by their respective numbers with the prefix M. The Minot Collection is named after Charles S. Minot (1852-1914), who collected and prepared over 1900 embryos of different animal species, and approximately 100 human embryos, close to a century ago.

C. Photography and Computer Processing All specimens were photographed using either an Olympus photomicroscope or a Wild photomakroskop. The magnification varied for each specimen according to the size of the head; the section with the largest area that could be accommodated within the field of view set the magnification for all sections of a particular specimen. All photographs were taken with a green filter to increase the contrast of the black and white film (Kodak technical pan #TP442). The film was developed at 20°C for 6 to 7 min in Kodak HC110 developer (dilution F), followed by Kodak stop bath for 30 s, Kodak fixer for 5 min, Kodak hypo clearing agent for 1 min, running water rinse for 10 min, and a brief rinse in Kodak photoflo before drying. The negatives were scanned at 2700 dots-per-inch (dpi) with a Nikon Coolscan-1000 35-mm film scanner, which was interfaced to a PowerPC G3 Macintosh computer running Adobe Photoshop with a plug-in Nikon driver. To capture the subtle shades of gray, the negatives were scanned as color positives, inverted, and converted to grayscale. Using the enhancement features built into Adobe Photoshop and the additional features of Extensis Intellihance, adjustments were made to increase contrast and sharpness. When the image resolution was set to 300 dpi, a full-size photographic file printed at approximately 12 to 10 in. Most images are shown at slightly reduced full size on separate pages. Adobe Illustrator was used to superimpose labels and to outline structural details on low contrast copies of the Adobe Photoshop files. The plates were placed into a book-form layout using Adobe InDesign. Finally, camera-ready files were provided to Taylor & Francis in Adobe portable document format (pdf).

The entire brain and upper cervical spinal cord of each transversely cut specimen was three-dimensionally reconstructed in five steps. First, photographs of serial sections were made throughout the entire brain; the negatives were scanned and converted to computer files as described in the preceding paragraph. Second, all the files of sections selected for the reconstruction were placed into one large Photoshop file that contained a separate photograph in each layer. By altering the visibility and transparency of these layers the sections were aligned to each other as they were before sectioning. Then each layer was saved as a separate file. Third, Adobe Illustrator was used to outline the brain surface of each aligned section, and these contours were saved in separate Adobe Illustrator encapsulated postscript (eps) files. Fourth, the eps files were imported into 3D space (x, y, and z coordinates) using Cinema 4DXL (C4D, Maxon Computer, Inc.). For each section, points on the contours have unique x-y coordinates and the same z coordinate. By calculating the distance between sections, the entire array of contours was stretched out in the z axis. The C4D loft tool builds a “skin” of the brain as a spline mesh of polygons. The polygons start from the x-y points on the first contour with the most anterior z coordinate, to the x-y points on the next contour behind it, and finish with the x-y points on the last contour with the most posterior z coordinate. The spline meshes of the entire brain surface were rendered at various camera angles as completely opaque using the C4D ray-tracing engine. These reconstructions are shown in Figure 1 to Figure 14. Fifth, spline meshes of the brain surface posterior to a specific section (coronal brains) or ventral to a specific section (horizontal brain) were rendered with a copy of the photograph of the particular section texture-mapped as a cap on the model. These reconstructions are shown as insets in Part A of each low magnification plate of the coronal and horizontal specimens.

D. Identification of Transient and Immature Brain Regions With the exception of the rhombomeres in the pons and medulla that are visible prior to and including GW5.5, the identification of most structures in early first trimester human brain—in particular, the various neuroepithelial (NEP) compartments—have received little attention in the past. Most identifications are based on our previous 3H-thymidine autoradiographic work with rats. There is a great similarity between the rat brain and human brain in the sequential order of neurogenesis and early neuronal differentiation, especially in the brainstem. Our experimental studies in the rat and the rationale for most of the proven or putative identifications we make are in the following publications. Amygdala: Bayer (1980c). Basal Ganglia: Bayer (1984, 1985b, 1987). Cerebellum: Altman and Bayer (1978a, 1982a, 1985a,

3 1985b, 1985c, 1997). Cerebral Cortex: Altman and Bayer (1990a, 1990b); Bayer and Altman (1990, 1991a). Cranial Nerve Nuclei: Altman and Bayer (1980a, 1980b, 1980c, 1982b). Hippocampus: Altman (1963); Altman and Das (1965a); Altman and Bayer (1975, 1990c, 1990d, 1990e); Bayer (1980a, 1980b). Hypothalamus: Altman and Bayer (1978c, 1978d, 1978e, 1986). Medulla: Altman and Bayer (1978b, 1980a, 1980b, 1980c, 1982b). Midbrain: Altman and Bayer (1981a, 1981b, 1981c). Olfactory Bulb: Altman (1969); Bayer (1983). Pontine Area: Altman and Bayer (1978b, 1980d, 1987a, 1987b, 1987c, 1987d). Precerebellar Nuclei: Altman and Bayer (1978b, 1987a, 1987b, 1987c, 1987d, 1997). Preoptic Area: Altman and Bayer (1986); Bayer and Altman (1987). Rhinencephalon: Bayer (1985a, 1986a, 1986b); Bayer and Altman (1991b). Septal Area: Bayer (1979a, 1979b). Spinal Cord: Altman and Bayer (1984, 2001). Thalamus: Altman and Bayer (1979a, 1979b, 1979c, 1988a, 1988b, 1988c, 1989a, 1989b, 1989c).

E. Major Developmental Features of the First Trimester Brain In Part XVI, Concluding Essay, we summarize the landmark events that characterize the development of the human CNS during the first trimester. Briefly reviewed, they are the following. (i) For several weeks after closure of the neural tube (the future spinal cord) and the neural vesicles (the future rhombencephalon, mesencephalon, diencephalon, and telencephalon), the CNS consists of a single proliferative tissue, the stockbuilding neuroepithelium (NEP). These NEP cells do not produce neurons and neuroglia but rather the growing stock of pluripotent progenitor cells that will later give rise to the differentiating cells of the CNS. (ii) The proliferating NEP cells undergo mitosis near the lumen of the ventricles, hence the growth of the stockbuilding NEP matrix is associated with the expansion of the narrow protoventricles to produce the large rhombencephalic, mesencephalic, diencephalic, and telencephalic superventricles.

(iii) The rate of stockbuilding cell mitosis varies in different components of the NEP matrix in relation to the sizes of the neuronal populations being generated for different brain structures. This results in a variegated ventricular shoreline (rhombomeres, evaginations, invaginations, eminences). We refer to these distinguishable NEP matrix shorelines as NEP cell mosaics. (iv) When NEP cell proliferation shifts from stockbuilding progenitor cells to unloading postmitotic neurons and neuroglia, these cells migrate outward and accumulate in the brain parenchyma, the space situated between the NEP and the pia. We present evidence that the formation of a hitherto unidentified meningeal structure, the superarachnoid reticulum, is related to this parenchymal expansion. The superarachnoid reticulum is a broad, fluid-rich meningeal tissue sandwiched between the early-developing pia and the formative dura. The initial expansion of the superarachnoid reticulum antedates the appearance of the brain parenchyma. While the parenchyma continually expands as more and more neurons migrate into it and differentiate, the superarachnoid reticulum continually shrinks until it is a thin meninx. We postulate that the transient hypertophy of the superarachnoid reticulum serves as a parenchymal expansion field for the developing brain. (v) The shrinkage of the NEP matrix is coupled with cell migration. A small complement of migrating cells produce fate-restricted secondary germinal matrices away from the ventricle, such as the external germinal layer of the cerebellum and the subgranular zone of the hippocampus. The bulk of migrating cells are young neurons that may sojourn in transitional fields but eventually settle in their final locations throughout the parenchyma. (vi) Peripheral and central inductive and signaling mechanisms play a major role in producing fate-restricted NEP cell mosaics, guiding migrating neurons, and directing axons to grow to their targets. Interactions between the NEP and the cephalic and branchial placodes (peripherocentral signaling) influence the diversification of NEP mosaics. Centro-central signaling between CNS structures is responsible for the coordinated development of different brain regions not directly connected with the periphery. (vii) An attempt is made to relate the morphological evidence for NEP matrix diversification, cell-fate restriction, neuronal migration, and axonal guidance in the human CNS with the underlying genetic and molecular mechanisms revealed by current research in animals.

4

PART PARTII: II: GW7 GW7 CORONAL CORONAL This specimen is embryo #2155 in the Minot Collection, designated here as M2155. The crown-rump length (CR) is 17.5 mm estimated to be at gestational week (GW) 7. Most of M2155’s brain sections are cut (10 µm) in the coronal plane, but the plane shifts to predominantly horizontal in the posterior medulla. We photographed 71 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and cerebellum. Seventeen of these sections are illustrated in Plates 1AB to 17AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of M2155’s brain (Figure 1), and to show each illustrated section in situ (insets, Plates 1A-17A). A prominent developmental strategy during the early first trimester is that many developing brain structures interact with primordial structures in the head and neck. Consequently, each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify nonneural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. Plates 18-20 show high-magnification views of the cerebral cortex, diencephalon, and mesencephalon. Some highmagnification plates are rotated 90˚ (landscape orientation) to more efficiently use page space. The brain of M2155 has considerable variation in the thickness of the neuroepithelium and in the number of migrating neurons in various parts of the brain parenchyma. Throughout the telencephalon, the neuroepithelium is the most prominent structure surrounding the enlarging telencephalic superventricle. A cell-sparse primordial plexiform layer is adjacent to the cerebral cortical neuroepithelium. A few pioneer Cajal-Retzius neurons have migrated into this layer, but most cortical neurons have not yet been generated. The cerebral cortical neuroepithelium is growing by adding more neuonal stem cells that will produce neurons during the late first trimester (Volume 4, Bayer and Altman, 2006) and early second trimester (Volume 3, Bayer and Altman, 2005). In contrast to the cerebral cortical neuroepithelium, the basal ganglionic and basal telencephalic neuroepithelia do have adjacent migrating neurons. In some areas, these neurons appear to migrate together in early (outermost and less dense) to late (innermost and most dense) waves. In accordance with the peripheral interaction theme, there is only the slightest indication of an olfactory bulb evagination in spite of the fact that a fully invaginated olfactory epithelium is in the nasal cavity and olfactory nerve fibers already contact the brain just anterior to the basal telencephalon. We hypothesize that olfactory nerve fibers have the capacity to induce the cortical neuroepithelium to proliferate and evaginate into an olfactory bulb later on. There is an olfactory evagi-

nation by GW7.5 (See Volume 4, Bayer and Altman, 2006, Plates 188A and B, pp. 464-465). The diencephalic neuroepithelium surrounds a slit-like superventricle. It is thinnest in the hypothalamic and subthalamic areas, where it is surrounded by densely packed waves of migrating neurons. It is postulated that these areas of the superventricle have shrinking shorelines as the neuroepithelia “unload” their stock of neuronal precursors. In contrast, the superventricle shoreline is still expanding as the thalamic neuroepithelium continues to add more neuronal precursors than to unload postmitotic neurons. The few neurons outside the thalamic neuroepithelium are postulated to be the oldest neurons in the ventral complex, posterior complex, and the reticular nucleus. The mesencephalon contains a stockbuilding neuroepithelium in the pretectum and tectum (relatively few adjacent migrating neurons). On the other hand, the tegmental and isthmal neuroepithelia are much thinner because most of their neuronal progeny has migrated out. These cells accumulate as inner dense clumps and outer sparse arrays interspersed among the thick accumulations of subpial fiber bands in the tegmental and isthmal parenchyma. Both the pons and medulla have neuroepithelia that are shrinking as they have already unloaded their neuronal precursors into an expanding parenchyma. Cells are migrating and settling in longitudinal arrays at the pontine flexure. A few cells are settling in the superior olive complex and many are settling in the reticular formation throughout the pons and medulla. Facial motor neurons are migrating from medial to lateral, leaving behind their axons in the genu of the facial nerve. Migrating cochlear nuclear neurons are outside the neuroepithelium in the anterior part of the lower rhombic lip, while migrating inferior olive neurons are in the posterior intramural migratory stream outside the precerebellar neuroepithelium in the posterior lower rhombic lip; some neurons have already settled in the inferior olive. Many neurons have settled in the solitary nucleus, surrounding a definite solitary tract. The hypoglossal nucleus is also distinguishable in the lower medulla. The cerebellar neuroepithelium is exceptional in the rhombencephalon because it is the only neuroepithelium still in the stockbuilding phase, mainly adding precursors of Purkinje cells. Many deep nuclear neurons have already been generated and are migrating in the cellular layers of the cerebellar transitional field. The fibrous layers probably contain afferents from the spinal cord and the vestibular ganglion.

5

M2155 Computer-aided 3-D Brain Reconstructions Pineal evagination

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Inferior colliculus

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Infundibulum

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Upper rhombic lip

Invagination of choroid plexus into fourth ventricle

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Lower rhombic lip

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1. Medullary

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Medullary velum (covers 4th ventricle)

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BRAINSTEM FLEXURES

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Optic evagination

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Optic evagination

Lower rhombic lip

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Upper rhombic lip Medullary velum

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Olfactory evagination

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Pretectum

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Superior colliculus

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Pineal evagination

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Spinal cord

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Figure 1. A, The left side of the 3-D model viewed from the front at a 45º heading; this view is used to "peel away" sections of each level in the following o r Plates. l u s B, a straight view of the left side. C, a straight down view of the top. D, an upward view of the bottom, angled (120º) to look into the mesencephalic and diencephalic flexures.

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PLATE 1A GW7 Coronal CR 17.5 mm M2155 Level 1: Section 50 Non-neural structures labeled Branches of anterior cerebral artery in pial vascular network

Superior sagittal sinus (in interhemispheric fissure) Dural vascular network

The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.

Future skin and skull (cell dense) Future dura (cell dense internal border of skull) Superarachnoid reticulum (cell sparse) Pia

Branches of middle cerebral artery Superarachnoid reticulum (cell sparse) Future frontal bone Frontonasal process Nasal septum Lateral nasal process

Nostril

Level 1: Computer-aided 3-D Brain Reconstruction

Medial nasal process

The GW7 Face and Neck

Figure 247E modified (Patten, 1953, p. 429.)

Nasal septum Medial nasal process

Frontal prominence Frontonasal process Eye

Lateral nasal process Nostril Mouth

Maxilla Mandible External ear Hyoid bone Laryngeal cartilages

7

PLATE 1B

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

NEP - Neuroepithelium

Neural structures labeled Interhemispheric fissure

TELENCEPHALON CEREBRAL CORTEX

Brain surface (heavier line)

Neocortical NEP

Dural outline is external border of superarachnoid reticulum Cortical primordial plexiform layer

telencephalic superventricle

anterodorsal pool

Limbic cortical NEP

(future lateral ventricle)

Migrating Cajal-Retzius cells and subplate neurons

Cingulate

Limbic cortical (insular) NEP

Prefrontal? anteroventral pool

Basal ganglionic (anterolateral) NEP?

Migrating basal ganglionic neurons?

BASAL GANGLIA Migrating Cajal-Retzius cells and subplate neurons

Earlier maturation is indicated by the presence of migrating neurons outside the basal ganglionic, insular, and prefrontal NEPs. Later maturation is indicated by the absence of migrating neurons outside the neocortical and cingulate NEPs.

Major Arteries at Base of Brain

Figure 394D modified (Patten, 1953, p. 625.) Anterior cerebral Anterior communicating Middle cerebral Opthalmic Internal carotid

Circle of Willis Posterior communicating Posterior cerebral

Major inputs from aortic arch

Superior cerebellar Pontine Basilar Anterior inferior cerebellar Posterior inferior cerebellar

Vertebral

Major branches of input arteries

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

8

PLATE 2A GW7 Coronal CR 17.5 mm M2155 Level 2: Section 116 Peripheral neural and non-neural structures labeled Superior sagittal sinus (in interhemispheric fissure) Dural vascular network Branches of anterior cerebral artery in pial vascular network

Superarachnoid reticulum (cell sparse)


Future skin and skull (cell dense) Future dura (cell dense internal border of skull)

Branches of middle cerebral artery

Pia

Superarachnoid reticulum (cell sparse) Hypothetical olfactory induction field

Nerve I (olfactory) Olfactory epithelium Eyelid

Nasal cavity Nasal epithelium

Zygomatic bone?

Maxilla

Oral cavity

Nasal septum

Mandible


PLATE 2B

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium

Central neural structures labeled Interhemispheric fissure

TELENCEPHALON

Dural outline is external border of superarachnoid reticulum

CEREBRAL CORTEX

Neocortical NEP


Cingulate dorsal pool



Cortical primordial plexiform layer

Limbic cortical NEP


Hippocampal


Corticoganglionic NEP

Fornical GEP

Migrating neurons originating in corticoganglionic NEP

Choroid plexus stem cells

Migrating basal ganglionic neurons

Anterolateral ganglionic NEP

ventral pool

Basal telencephalic NEP BASAL GANGLIA/ BASAL TELENCEPHALON

Septal NEP

Settling basal ganglionic neurons

Migrating basal telencephalic neurons Settling basal telencephalic neurons

Migrating and settling septal neurons


9

10

PLATE 3A GW7 Coronal CR 17.5 mm M2155 Level 3: Section 164

Peripheral neural and non-neural structures labeled Superior sagittal sinus (in interhemispheric fissure) Dural vascular network

Branches of anterior cerebral artery in pial vascular network The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.

Anterior cerebral artery Superarachnoid reticulum (cell sparse)

Future skin and skull (cell dense)

Vascular bed of choroid plexus

Future dura (cell dense internal border of skull) Pia

Branches of middle cerebral artery Orbito-sphenoid process


Hypothetical olfactory induction field

Nerve I (olfactory)

Eye

Eyelid Earliest ganglion cells Sclera Intraretinal space Cornea Lens Vitreous body Retinal NEP Pigment epithelium

Olfactory epithelium

Nasal cavity Nasal epithelium Nasal septum

Tongue

Zygomatic bone? Maxilla

Palatal process Oral cavity Mandible


Lingual epithelium

11

PLATE 3B


Central neural structures labeled Interhemispheric fissure

TELENCEPHALON


CEREBRAL CORTEX

Neocortical NEP Cingulate Hippocampal




dorsal pool

Fornical GEP Telencephalic


Choroid plexus stem cells Diencephalic


third ventricle foramen of monro


ventral pool

Anteromedial ganglionic NEP BASAL GANGLIA/ BASAL TELENCEPHALON


Limbic cortical NEP

Basal telencephalic NEP Septal NEP

Migrating Cajal-Retzius cells and subplate neurons Migrating neurons originating in corticoganglionic NEP? Migrating basal ganglionic neurons Pioneer internal capsule axons Settling basal ganglionic neurons Migrating basal telencephalic neurons

Settling basal telencephalic neurons Migrating and settling septal neurons


12


See a high magnification view of the thalamus and cerebral cortex in Plates 18A and B. Peripheral neural and non-neural structures labeled Superior sagittal sinus


Dural vascular network Anterior cerebral artery




Future parietal bone?

Pia

Branches of middle cerebral artery in pial vascular network Superarachnoid reticulum (cell sparse) Orbito-sphenoid process

Eye

Nerve II (optic) Eyelid Sclera Intraretinal space Vitreous body Retinal NEP Ganglion cells Pigment epithelium

Lingual epithelium

Ethmoid bone? Tongue

Maxilla

Palatal process Oral cavity

Meckel's cartilage


PLATE 4B


Neocortical NEP

Cingulate Hippocampal

Limbic cortical NEP



posterior pool Choroid plexus stem cells

for of mamen onr o

Posterior ganglionic NEP Basal telencephalic NEP

(PREOPTIC AREA)

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.


Migrating Cajal-Retzius cells and subplate neurons Migrating neurons originating in corticoganglionic NEP? Pioneer internal capsule axons Migrating basal ganglionic neurons Settling basal ganglionic (globus pallidus?) neurons Settling basal telencephalic neurons Migrating basal telencephalic neurons Settling basal telencephalic neurons

preoptic preoptic pool pool

BASAL GANGLIA/ BASAL TELENCEPHALON

DIENCEPHALON


Fornical GEP

(future lateral ventricle) Limbic cortical (insular) NEP


thalamic thalamic pool pool

CEREBRAL CORTEX

(future third ventricle)

(THALAMUS)

TELENCEPHALON

Dorsal thalamic NEP Migrating thalamic Reticular NEP neurons Anterior thalamic NEP

diencephalic superventricle

DIENCEPHALON

Central neural structures labeled

Preoptic NEP Migrating preoptic neurons

Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

13

14


See a high magnification view of the thalamus and cerebral cortex from Section 236 in Plates 19A and B. Peripheral neural and non-neural structures labeled Superior sagittal sinus


Dural vascular network

Anterior cerebral artery The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.

Branches of anterior cerebral artery Future skin and skull (cell dense) Future dura (cell dense internal border of skull)


Pia

Future parietal bone? Branches of middle cerebral artery in pial vascular network Superarachnoid reticulum (cell sparse)

Orbito-sphenoid process Nerve II (optic)? Future ethmoid/ sphenoid bones Lingual epithelium

Maxilla Palatal process

Meckel's cartilage

Tongue

Salivary gland?


Oral cavity

PLATE 5B



ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium

(EPITHALAMUS AND THALAMUS)

Migrating thalamic neurons

Dorsal

Thalamic NEP

Ventral

TELENCEPHALON


Reticular

CEREBRAL CORTEX Limbic cortical NEP

Migrating epithalamic neurons

Epithalamic NEP

DIENCEPHALON

Anterior

Retrosplenial? Hippocampal

thalamic pool




Fornical GEP


posterior pool





Neocortical NEP

foramen of monro

Posterior ganglionic NEP Strionuclear NEP

Preoptic NEP

(HYPOTHALAMUS)


Pioneer internal capsule axons

Settling basal telencephalic neurons

Lateral preoptic pool

Medial

optic recess

Optic nerve GEP Chiasmatic GEP

DIENCEPHALON

Migrating neurons originating in corticoganglionic NEP?

Migrating bed nucleus of the stria terminalis neurons


(PREOPTIC AREA)



Basal telencephalic NEP

DIENCEPHALON


(intermingled with anterobasal hypothalamic NEP?)


Migrating basal telencephalic neurons Medial forebrain bundle? Migrating lateral preoptic neurons Anterobasal nuclear neurons?


15


Peripheral neural and non-neural structures labeled Superior sagittal sinus



Branches of anterior cerebral artery in pial vascular network Future skin and skull (cell dense)


Future dura (cell dense internal border of skull)

Pia Future parietal bone Middle cerebral artery Circle of Willis artery


Carotid artery

Car otid arte ry

16

Trigeminal ganglion (V) Ali-sphenoid process? Pituitary gland (anterior lobe, adenohypophysis) Palatal process Lingual epithelium

Sphenoid bone (sella turcica) Maxilla Future temporomandibular joint Meckel's cartilage

Tongue Laryngo-tracheal groove Larynx Oral cavity

External auditory meatus (in petrous temporal bone) Petrous temporal bone Salivary gland?


PLATE 6B



DIENCEPHALON (EPITHALAMUS, THALAMUS, AND SUBTHALAMUS)


Migrating epithalamic neurons Epithalamic NEP

Thalamic NEP

Migrating thalamic neurons Dural outline is external border of superarachnoid reticulum

Dorsal Ventral thalamic pool

Reticular

Subthalamic NEP TELENCEPHALON




CEREBRAL CORTEX


Limbic cortical

(retrosplenial?) NEP

Neocortical NEP posterior pool


Migrating CajalRetzius cells and subplate neurons


Amygdaloid NEP

subthalamic pool

Strionuclear NEP? BASAL GANGLIA (AMYGDALA)

hypothalamic pool

Lateral

Hypothalamic NEP

infundibular recess

Anterior

Migrating bed nucleus of the stria terminalis neurons? Migrating subthalamic neurons (zona incerta, Forel's fields) Migrating lateral hypothalamic neurons Migrating anterior hypothalamic neurons

DIENCEPHALON (HYPOTHALAMUS)


Migrating amygdaloid neurons

Medial forebrain bundle?



17

18

PLATE 7A GW7 Coronal CR 17.5 mm M2155 Level 7: Section 325 Superarachnoid

Peripheral neural and non-neural structures labeled Pineal gland

Superior sagittal sinus Dural vascular network

reticulum (cell sparse)

Pial vascular network


Future skin and skull (cell dense) Future dura (cell dense internal border of skull) Pia

Future parietal bone


Middle cerebral artery and branches Circle of Willis artery Carotid artery

Future squamous temporal bone?

Ali-sphenoid process Pituitary gland

Trigeminal ganglion (V)

Anterior lobe, adenohypophysis

Nerve V (trigeminal) Sphenoid bone (sella turcica)

Posterior lobe, neurohypophysis

Facial ganglion (VII)?

Temporal bone labyrinth

External auditory meatus

Inferior glossopharyngeal ganglion (IX)?

Petrous temporal bone

Esophagus

Level 7: Computer-aided 3-D Reconstruction

19

PLATE 7B


Pineal GEP

DIENCEPHALON


Epithalamic NEP

(EPITHALAMUS, AND THALAMUS)

Posterior (dorsal lateral geniculate)

Thalamic NEP




Ventral

thalamic pool

Reticular




Dorsal lateral geniculate

TELENCEPHALON CEREBRAL CORTEX Cortical primordial plexiform layer

Migrating CajalRetzius cells and subplate neurons Neocortical NEP

Ventrobasal complex Reticular nucleus Brain surface (heavier line)

Migrating subthalamic neurons (zona incerta, Forel's fields) Migrating lateral hypothalamic neurons

subthalamic pool

hypothalamic pool

Subthalamic NEP Lateral


Hypothalamic NEP DIENCEPHALON

(SUBTHALAMUS AND HYPOTHALAMUS)

Dorsomedial/ventromedial complex? Arcuate nucleus?

Middle

Migrating middle hypothalamic neurons

infundibular recess

Median eminence/ neurohypophysis (pituicyte) GEP




20

PLATE 8A Peripheral neural and non-neural structures labeled

GW7 Coronal CR 17.5 mm M2155 Level 8: Section 375 The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.

Superior sagittal sinus Dural vascular network





Pia


Middle cerebral artery and branches

Circle of Willis arteries? Basilar artery Trigeminal

Nerve V (trigeminal)

* boundary cap (V)

Posterior cerebral artery

Vestibulocochlear * boundary cap (VIII)

Future squamous temporal bone? Nerve VIII (vestibulocochlear)

Facial

Temporal bone labyrinth (otic vesicle)

* boundary

cap (VII)?

Vestibular ganglion (VIII)


Vertebral artery? Spiral ganglion (VIII)?

Immature cochlea and semicircular canals

Superarachnoid reticulum (cell sparse) Basal occipital bone Basilar artery


* Boundary caps are

Schwann cell GEPs?

PLATE 8B


MESENCEPHALON

Posterior commissural GEP

(PRETECTUM)



Posterior commissure Migrating pretectal neurons

Pretectal NEP mesencephalic superventricle


(future aqueduct)


DIENCEPHALON Posterior (dorsal lateral geniculate) thalamic pool Posterior (medial geniculate)

Dorsal lateral geniculate



Thalamic NEP


Reticular

Medial geniculate

Reticular nucleus Migrating subthalamic neurons (zona incerta, Forel's fields) Settling subthalamic nuclear neurons?

subthalamic pool

Subthalamic NEP

hypothalamic pool


Hypothalamic NEP

Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)

(middle) Medial lemniscus?

RHOMBENCEPHALON

Central trigeminal tract?

(PONS)

Longitudinal domains of migrating and settling pontine neurons

Principal sensory nucleus (V)? Caudal extension of trigeminal nuclear complex (V)? Lateral lemniscus?

Lateral Intermediate Medial

Pontine reticular formation Migrating raphe nuclear complex neurons Midline raphe glial structure




21

22


See a high magnification view of the mesencephalon and diencephalon Peripheral neural and from Section 390 in non-neural structures labeled Plates 20A and B. Superior sagittal sinus







Future parietal bone Middle cerebral artery

Superarachnoid reticulum (cell sparse) Circle of Willis arteries? Basilar artery Trigeminal

Posterior cerebral artery and branches

* boundary cap (V)?

Future squamous temporal bone Vestibulocochlear * boundary cap (VIII)


Temporal bone labyrinth (immature cochlea and semicircular canals) Mastoid air cells? Nerve and ganglion IX (glossopharyngeal) Basal occipital bone

Foramen magnum Basilar artery


* Boundary caps are


Schwann cell GEPs?

PLATE 9B


MESENCEPHALON (PRETECTUM)


Central neural structures labeled Posterior commissure


Migrating pretectal neurons Brain surface (heavier line)

Pretectal NEP


mesencephalic superventricle

(future aqueduct)

DIENCEPHALON


Posterior (medial geniculate)

Medial geniculate? thalamic pool

Thalamic NEP

Reticular nucleus

Reticular



Migrating subthalamic neurons (Forel's fields and zona incerta?) Medial forebrain bundle?

Subthalamic NEP subthalamic pool

hypothalamic pool

Hypothalamic NEP

Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)

(middle)

PONS

Midline raphe glial structure Midline raphe glial structure GEP Medial pontine NEP Lateral pontine NEP

CEREBELLUM

Cerebellar NEP

Auditory (cochlear) NEP

Nerve VII (genu)

Medial longitudinal fasciculus

Lateral medullary NEP

Medial lemniscus? Migrating raphe nuclear complex neurons Central trigeminal tract? Migrating trigeminal nuclear complex neurons?

Pontine reticular formation

Migrating cerebellar deep nuclear neurons?

Facial motor neurons

Premigratory Migrating

Sojourning Purkinje cells?

Settling?

Migrating cochlear nuclear neurons?

Medial medullary NEP

Ventral nucleus of the lateral lemniscus?

Midline raphe glial structure GEP

UPPER MEDULLA

Lateral lemniscus? Superior olivary complex

RHOMBENCEPHALON

Medullary reticular formation Migrating raphe nuclear complex neurons? Midline raphe glial structure




23

24

PLATE 10A GW7 Coronal CR 17.5 mm M2155 Level 10: Section 424 The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.




Future skin and skull (cell dense) Pial vascular network Future dura (cell dense internal border of skull)


Pia Superarachnoid reticulum (cell sparse)

Middle cerebral artery

Circle of Willis arteries? Basilar artery


Future squamous temporal bone Vestibulocochlear boundary cap * (VIII)?

* Boundary caps are

Schwann cell GEPs?

Temporal bone labyrinth Petrous temporal bone Nerve and ganglion IX (glossopharyngeal)? Nerve and ganglion X (vagus)? Basilar artery

Foramen magnum

Mastoid air cell? Basal occipital bone Superarachnoid reticulum (cell sparse)



MESENCEPHALON

PLATE 10B


ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium

Posterior commissure



Pretectal NEP

Migrating pretectal neurons mesencephalic superventricle


(future aqueduct)

Migrating central gray neurons? Mesencephalic reticular formation

Mesencephalic tegmental NEP

Migrating red nuclear neurons? Migrating interpeduncular nuclear neurons?

DIENCEPHALON Subthalamic NEP

Migrating subthalamic neurons

subthalamic pool




Hypothalamic NEP

(mammillary)

PONS

Midline raphe glial structure GEP Medial pontine NEP Lateral pontine NEP


CEREBELLUM Cerebellar NEP

Migrating mammillary neurons

hypothalamic pool

Lateral lemniscus? Medial lemniscus?

Dorsal nucleus of the lateral lemniscus?

CTF1 (fibers) CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons) CTF6 (cells-Purkinje cells)


Upper rhombic lip

Nerve VII (genu)

Auditory (cochlear)

NEP Lateral medullary NEP

Abducens nucleus (VI)? Medial longitudinal fasciculus

rhombencephalic superventricle

(future fourth ventricle)

Premigratory facial motor nucleus (VII) neurons?

Lower rhombic lip Migrating vestibular nuclear complex neurons?


UPPER MEDULLA

Layers of the cerebellar transitional field

Vestibular nuclear complex?


Migrating cochlear nuclear neurons? Spinal nucleus and tract (V)? Settling facial motor nucleus (VII) neurons?

RHOMBENCEPHALON

Superior olivary complex Medullary reticular formation Migrating raphe nuclear complex neurons? Midline raphe glial structure




25

26





Future skin and skull (cell dense) Pial vascular network Future dura (cell dense internal border of skull) Pia

Future parietal bone Middle cerebral artery

Circle of Willis arteries?


Basilar artery


Future squamous temporal bone



Foramen magnum Basal occipital bone

Superarachnoid reticulum (cell sparse) Basilar artery

Vagal ganglion (X)? Mastoid air cells?


PLATE 11B




MESENCEPHALON




Migrating pretectal neurons

Pretectal NEP



(future aqueduct)



Migrating red nuclear neurons? Migrating oculomotor (III) nuclear neurons Medial forebrain bundle?

DIENCEPHALON diencephalic superventricle


(future third ventricle, mammillary recess)

Hypothalamic NEP (mammillary) PONS

Midline raphe glial structure Midline raphe glial structure GEP Medial longitudinal fasciculus Medial pontine NEP Lateral pontine NEP

Lateral lemniscus? Medial lemniscus?



Layers of the cerebellar transitional field CTF1 (fibers) CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons)? CTF6 (cells-Purkinje cells)


CEREBELLUM Cerebellar NEP

Medial cerebellar notch


Precerebellar NEP Lateral medullary NEP

Abducens nucleus (VI)? Medial longitudinal fasciculus

Lower rhombic lip Medullary reticular formation

Posterior intramural migratory stream (inferior olivary neurons)?


MEDULLA

Solitary nucleus and tract?


Vestibular nuclei? Settling facial motor nuclear (VII) neurons

RHOMBENCEPHALON

Inferior olive? Migrating raphe nuclear complex neurons? Midline raphe glial structure




27

28





Pial vascular network Pia


Future skin and skull (cell dense) Nerve III (oculomotor)



Future squamous temporal bone


Glossopharyngeal

* boundary cap (IX)? Vagal boundary * cap (X)? Vertebral artery?

Future squamous occipital bone Superarachnoid reticulum (cell sparse)

Traces of the vagal (X) and glossopharyngeal (IX) nerve sheaths?

* Boundary caps are

Schwann cell GEPs?


29 FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

PLATE 12B




MESENCEPHALON

Migrating neurons and glia invade posterior commissure


Pretectal NEP

Migrating pretectal neurons mesencephalic superventricle


(future aqueduct)

Migrating central gray neurons?


Mesencephalic reticular formation Migrating red nuclear neurons? Migrating oculomotor (III) nuclear neurons Medial forebrain bundle? Migrating and settling substantia nigra neurons? Migrating and settling ventral tegmental area neurons?

PONS


Midline raphe glial structure Midline raphe glial structure GEP

Medial lemniscus?


Lateral lemniscus?

Medial pontine NEP

Layers of the cerebellar transitional field CTF1 (fibers)


Lateral pontine NEP

CTF2 (cells-deep neurons) CTF3 (fibers)

CEREBELLUM

CTF4-5 (cells-deep neurons)? CTF6 (cells-Purkinje cells)

Cerebellar NEP

Upper rhombic lip

metencephalic pool

Lateral

Medial

rhombencephalic superventricle (future fourth ventricle)

Cerebellar notches

MEDULLA

Precerebellar NEP

myelencephalic pool


Vestibular nuclei?

Medullary reticular formation

Medullary velum

Lower rhombic lip

Lateral medullary NEP Migrating precerebellar nuclear neurons


Solitary nucleus and tract

Midline raphe glial system GEP


RHOMBENCEPHALON

Migrating raphe nuclear complex neurons? Midline raphe glial structure Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.



30


Superior sagittal sinus

Peripheral neural and non-neural structures labeled

Pia



Interpeduncular fossa




Future squamous temporal bone?


Vagal boundary * cap (X)? Superarachnoid reticulum (cell sparse)

* Boundary caps are

Schwann cell GEPs?

Traces of the vagal (X) nerve sheath?

Future squamous occipital bone?

Traces of the hypoglossal (XII) nerve sheath?

PLATE 13B


Posterior commissural GEP Posterior commissure Migrating glia invade posterior commissure

MESENCEPHALON TECTUM


Pretectal NEP



(future aqueduct)

TEGMENTUM



Migrating oculomotor (III) nuclear neurons Settling red nuclear neurons? Medial forebrain bundle? Migrating and settling substantia nigra neurons?

ISTHMUS

Migrating and settling ventral tegmental area neurons? Midline raphe glial structure



isthmal canal

Lateral lemniscus? Layers of the cerebellar transitional field

Isthmal NEP

CTF1 (fibers)

Mesencephalic reticular formation

CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons)?

CEREBELLUM

CTF6 (cells-Purkinje cells)

Cerebellar NEP Upper rhombic lip

metencephalic pool

Medial


Lateral


Cerebellar notches

myelencephalic pool


Vestibular nuclei

MEDULLA

Medullary velum

Precerebellar NEP Lateral medullary NEP

Medial longitudinal fasciculus Midline raphe glial structure GEP

SPINAL CORD

Migrating precerebellar nuclear neurons Solitary nucleus and tract


RHOMBENCEPHALON

Lower rhombic lip


Floor plate Ventral spinal G/EP

Posterior intramural migratory stream (inferior olivary neurons)? Migrating raphe nuclear complex neurons? Midline raphe glial structure Ventral commissure Ventral funiculus Ventral gray Segregating ventral horn motoneuron columns

Ventral horn interneurons

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: CTF - Cerebellar transitional field G/EP - Glioepithelium/ependyma GEP - Glioepithelium NEP - Neuroepithelium

Lateral funiculus Intermediate spinal NEP Dorsal spinal NEP Roof plate

Intermediate gray central canal Dorsal funiculus Dorsal gray

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

31

32




Peripheral neural and non-neural structures labeled Future skin and skull (cell dense)




Level 14: Computer-aided 3-D Brain Reconstruction Future squamous occipital bone?

Superarachnoid reticulum (cell sparse, fluid-filled spaces)

PLATE 14B


Posterior commissural GEP Posterior commissure Migrating glia invade posterior commissure


Migrating superior collicular neurons Tectal (superior collicular) NEP Brain surface (heavier line)

mesencephalic superventricle (future aqueduct)


Tectal (inferior collicular) NEP Migrating inferior collicular neurons

TEGMENTUM

Migrating central gray neurons?


Migrating trochlear (IV) nuclear neurons Lateral lemniscus? Mesencephalic reticular formation

ISTHMUS

isthmal canal

Medial longitudinal fasciculus Migrating isthmal neurons

Isthmal NEP

Layers of the cerebellar transitional field

CEREBELLUM

CTF1 (fibers) CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons)? CTF6 (cells-Purkinje cells)

Cerebellar NEP

Upper rhombic lip Medial

metencephalic pool

Lateral Cerebellar notches


Medullary velum


MEDULLA

Vestibular nuclei

Lower rhombic lip

myelencephalic pool

Precerebellar NEP

Migrating precerebellar nuclear neurons

Lateral medullary NEP Medial medullary NEP Midline raphe glial structure GEP Medial longitudinal fasciculus

RHOMBENCEPHALON

Ventral commissure

SPINAL CORD

Floor plate Ventral spinal G/EP

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: CTF - Cerebellar transitional field G/EP - Glioepithelium/ependyma GEP - Glioepithelium NEP - Neuroepithelium

Intermediate spinal NEP Dorsal spinal NEP Roof plate


Solitary nucleus and tract Posterior intramural migratory stream (inferior olivary neurons) Settling inferior olive neurons Migrating raphe nuclear complex neurons? Midline raphe glial structure Ventral funiculus Ventral gray Segregating ventral horn motoneuron columns Ventral horn interneurons

Lateral funiculus Intermediate gray central canal Dorsal funiculus Dorsal gray


33

34

PLATE 15A


GW7 Coronal CR 17.5 mm M2155 Level 15: Section 588 The large cell-sparse superarachnoid reticulum defines various brain parenchymal expansion zones. It is sandwiched between external (dural) and internal (pial) vascular networks.

Peripheral neural and non-neural structures labeled Future skin and skull (cell dense) Future dura (cell dense internal border of skull)

Pia

Pial vascular network Superarachnoid reticulum (cell sparse)





PLATE 15B

Central neural structures labeled MESENCEPHALON Brain surface (heavier line)

TECTUM

Migrating superior collicular neurons

Tectal (superior collicular) NEP



(future aqueduct)

Tectal (inferior collicular) NEP

Migrating inferior collicular neurons Migrating trochlear (IV) nuclear neurons? isthmal canal

Trochlear nuclear NEP?

ISTHMUS Isthmal NEP

CEREBELLUM

Lateral lemniscus Migrating isthmal neurons Layers of the cerebellar transitional field CTF1 (fibers) CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons)? CTF6 (cells-Purkinje cells)

Cerebellar NEP


Upper rhombic lip metencephalic pool Lateral cerebellar notch


MEDULLA

myelencephalic pool

Medullary velum

Vestibular nuclei

Lower rhombic lip Migrating precerebellar nuclear neurons

Precerebellar NEP


Lateral medullary NEP Medullary reticular formation

Medial medullary NEP Prepositus, vagal (X), and hypoglossal (XII) nuclei?



Migrating raphe nuclear complex neurons?

Midline raphe glial system GEP Midline raphe glial structure

RHOMBENCEPHALON SPINAL CORD FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: CTF - Cerebellar transitional field G/EP - Glioepithelium/ependyma GEP - Glioepithelium NEP - Neuroepithelium

Floor plate Ventral spinal G/EP Intermediate spinal NEP

Settling inferior olive neurons Ventral horn interneurons Lateral funiculus Intermediate gray central canal

Dorsal spinal NEP Roof plate

Dorsal funiculus Dorsal gray


35

36





Pial vascular network Superarachnoid reticulum (cell sparse)





PLATE 16B

Central neural structures labeled MESENCEPHALON TECTUM


Migrating superior collicular neurons Tectal (superior collicular) NEP



isthmal canal


Migrating inferior collicular neurons Lateral lemniscus

Isthmal NEP

ISTHMUS

Vermis

CEREBELLUM

Layers of the cerebellar transitional field CTF1 (fibers)

Intermediate hemisphere Lateral hemisphere

Cerebellar NEP

CTF2 (cells-deep neurons) CTF3 (fibers) CTF4 (cells-deep neurons)


CTF5 (fibers) CTF6 (cells-Purkinje cells)

Upper rhombic lip metencephalic pool

Lateral cerebellar notch

Medullary velum


Rhombencephalic choroid plexus

MEDULLA Choroid plexus stem cells Lower rhombic lip

Vestibular nuclei myelencephalic pool

Precerebellar NEP

Migrating precerebellar nuclear neurons


Solitary nucleus and tract lar reticu llary n Meduformatio


RHOMBENCEPHALON FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium

Migrating raphe nuclear complex neurons? Cuneate nucleus? Hypoglossal nucleus (XII)?

Dorsal gray Dorsal spinal NEP

Roof plate

SPINAL CORD

Dorsal funiculus central canal


37

38 Peripheral neural and non-neural structures labeled










39 Central neural structures labeled

PLATE 17B



Migrating superior collicular neurons Tectal (superior collicular) NEP



Tectal (inferior collicular) NEP Migrating inferior collicular neurons Lateral lemniscus/ brachium of the inferior colliculus

Layers of the cerebellar transitional field is

CTF1 (fibers)

Ve m

CEREBELLUM

CTF3 (fibers)

EP

CTF4 (cells-deep neurons)

rN

CTF5 (fibers) CTF6 (cells-Purkinje cells)

lla

r he

p mis He e


C ere

be

metencephalic pool

Upper rhombic lip



Medullary velum

Rhombencephalic choroid plexus Choroid plexus stem cells

myelencephalic pool

MEDULLA

Medullary velum

Lower rhombic lip

Medullary (gracile?) NEP Migrating gracile nuclear neurons?


RHOMBENCEPHALON FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: CTF - Cerebellar transitional field NEP - Neuroepithelium

Medullary velum

Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

40

PLATE 18A GW7 Coronal, CR 17.5 mm, M2155 Near Level 4: Section 203

See Level 4 in Plates 4A and B.

CEREBRAL CORTEX AND THALAMUS

PLATE 18B

RAL B RE E C


CORT EX

Brain surface (pia, heavier line)

Superarachnoid reticulum

Limbic cortical (cingulate) NEP



An

NEP thal ami c

Dor sal

H A L A M

T m ic NE P t Reticu hala mic lar NEP ?

Fornical glioepithelium

s neuron lamic g t ha

(future lateral ventricle posterior pool)

thalamic pool

r a ti n


t m ig

Limbic cortical (hippocampal) NEP

Scattered pioneer Cajal-Retzius cells and subplate neurons migrate into the primordial plexiform layer.

li e s Ear

U S

Neocortical neuroepithelium (NEP)


Primordial plexiform layer absent (marks border of fornical glioepithelium)

Thalamic primordial plexiform layer


ala terior th

The superarachnoid reticulum is continuous with the vascular bed of the choroid plexus.



Primordial plexiform layer absent (marks border of choroid plexus stem cells)

41

42

PLATE 19A GW7 Coronal CR 17.5 mm, M2155 Near Level 5: Section 236


CEREBRAL CORTEX AND THALAMUS

PLATE 19B Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. rc r io nte

e

ery a rt l ra reb


Limbic cortical (insular) NEP foramen of monro

l co

An


T

Migrating and settling anterior thalamic neurons

Migrating Cajal-Retzius cells and subplate neurons Migrating hippocampal neurons

Primordial plexiform layer absent (marks border of fornical glioepithelium) The superarachnoid reticulum is continuous with the vascular bed of the choroid plexus.


Primordial plexiform layer absent (marks border of choroid plexus stem cells) Migrating neurons originating in corticoganglionic NEP?

te ri o

r



T

(future lateral ventricle posterior pool)


H

Fornical glioepithelium


A L A M U S helium h o e p iVtem (NE c o a l a m i c n e u r lex tral comp P mple lex Do Reticular c o mp x rsa )

i ep

o

Limbic cortical (hippocampal) NEP

ne

ur

(retrosplenial?) NEP

thalamic pool

ettling thalamic neuro ns and s

e


ting

th

u m Limbic cortical


gra

(N

li


Mi

CEREBRAL CORTEX

) EP

Future dura (internal border of skull)

Epithalamic NEP?

mp lex

A

Neocortical



foramen of monro Posterior ganglionic NEP

Strionuclear NEP

BASAL GANGLIA

Migrating bed nucleus of the stria terminalis neurons


Pioneer internal capsule axons

43

44

PLATE ?A 20A GW7 Coronal CR 17.5 mm M2155 Near Level 9: Section 390


DIENCEPHALON AND MESENCEPHALON

45 Posterior commissure

Posterior commissural glioepithelium

Pretectal neuroepithelium (NEP)


PRETECTUM



Thalamic NEP

(future aqueduct)


e) (do Pos te rsa l la rior N t e ra E l ge P nic ula t

Clumps of pioneer thalamic axons

Dorsal lateral geniculate?

thalamic pool


lar P icu R e t ar N E le nu c

(

SUBTHALAMUS


Medial geniculate?

ior NEP ster Po l geniculate) ia ed m

THALAMUS

PLATE 20B

Reticular nucleus

Settling zona incerta neurons? Migrating subthalamic neurons (zona incerta?)

subthalamic pool

Subthalamic NEP SUBTHALAMUS


Medial forebrain bundle? Migrating and settling lateral hypothalamic neurons?

HYPOTHALAMUS

Middle/posterior hypothalamic NEP Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

Settling subthalamic nuclear neurons?

Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP) hypothalamic pool


46

PART PARTIII: III: GW7 GW7 SAGITTAL SAGITTAL Carnegie Collection specimen #1390 (designated here as C1390) was collected in 1916 from a tubal pregnancy. The crown-rump length (CR) is 18 mm estimated to be at gestational week (GW) 7. C1390 was fixed in formalin, embedded in paraffin, and was cut in 20-µm sagittal sections that were stained with aluminum cochineal. Various orientations of the computer-aided 3-D reconstruction of M2155’s brain is used to show the gross external features of a GW7 brain (Figure 2). Like most sagittally cut specimens, C1390’s sections are not parallel to the midline; Figure 2 shows the approximate rotations in top (B) and back views (C). We photographed 62 sections at low magnification from the left to right sides of the brain. Seven of the sections, mainly from the left side of the brain, are illustrated in Plates 21AB to 27AB. Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) indicate the approximate location of the midline and identify non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (low-contrast images) identify central neural structures. Plates 28AB show three highmagnification sections in the region of the facial nerve genu. The brain of C1390 is at a similar stage of development as M2155, the previous GW7 specimen. Throughout the telencephalon, the neuroepithelium is the most prominent structure surrounding the enlarging telencephalic superventricle. All parts of the neuroepithelium are in “stockbuilding” stage, increasing the shorelines of the expanding telencephalic superventricles as more neuronal stem cells are added. Very few pioneer Cajal-Retzius neurons have migrated into the primordial plexiform layer adjacent to the cortical neuroepithelium. In contrast, the basal ganglionic and basal telencephalic neuroepithelia are adjacent to migrating neurons that form distinctive mounds in the floor of the telencephalon. The sagittal plane is ideal to show the slight evagination of the olfactory neuroepithelium in exactly the same region that is contacted by olfactory nerve fibers. The diencephalic neuroepithelium surrounds a large superventricle. It is shrinking in the hypothalamic and subthalamic areas where stem cells are depleted as they generate neurons. Many migrating and settling young neurons are in the parenchyma surrounding these neuroepithelia. In contrast, the superventricular shoreline is expanding in tha-

lamic areas as the thalamic neuroepithelium continues to add more neuronal precursors than to unload postmitotic neurons. The few neurons outside the thalamic neuroepithelium are best seen in sections grazing the dorsolateral part of the diencephalon. The roof (tectum and pretectum) of the mesencephalon contains a stockbuilding neuroepithelium adjacent to a very thin layer of pioneer migrating neurons. However, bundles of fibers in the posterior commissure are very distinct, and spike-like arrays of cells extend between these bundles. Possibly premigratory neurons produce axons while they are sequestered in the pretectal neuroepithelium. The tegmental and isthmal neuroepithelia are much thinner because most of their neuronal progeny has migrated out. These cells accumulate as inner dense clumps and outer sparse arrays interspersed among the thick subpial fiber band in the tegmental and isthmal parenchyma. Both the pons and medulla have neuroepithelia that are shrinking as stem cells unload their neuronal and glial progeny into an expanding parenchyma. The longitudinal arrays at the pontine flexure are easy to see in the sagittal plane. The genu of the facial motor nerve forms fascicles adjacent to the neuroepithelium in medial sections; these fascicles are adjacent to the pial surface in lateral sections. What is presumed to be the solitary tract is the most prominent internal fiber tract in the medulla. Both the pons and medulla have a thick subpial fibrous layer. Lateral sections show large peripheral sensory nerves contacting the brain. No doubt, many of the superficial fibers are the afferent axons of these ganglia along with ascending fiber tracts from the the spinal cord. All of the peripheral nerves (most clearly shown in the trigeminal nerve) have dense glia (Schwann cells), while central fiber tracts are clear. Thus, peripheral gliogenesis precedes the generation of oligodendrocytes in central fiber tracts. The exceptionally thick cerebellar neuroepithelium, in comparison with the thin adjacent pontine neuroepithelium, is most easily seen in lateral sections where it sharply juts into the rhombencephalic superventricle at the medial cerebellar notch. All parts of the cerebellar neuroepithelium are stockbuilding Purkinje stem cells. Most deep nuclear neurons are migrating in the cellular layers of the cerebellar transitional field.

47

EXTERNAL FEATURES OF THE GW7 BRAIN A.

Pineal evagination

Ep

T h a

mus ala ith

Side view l

A perfect sagittal cut through the brain is parallel to the entire midline. Based on Level 1, the sections of C1390's brain rotate an estimated 8.1º from the midine, 4.3º counterclockwise from the anterior horizontal midline (B, top view), and 3.8º clockwise from the posterior vertical midline (C, back view). In the sections illustrated on the following pages, the anterior brain (top and left) is tilted away from the observer, while the posterior brain (bottom and right) is tilted toward the observer.

Pretectum Superior colliculus

a

t

T e

s

o

C e

b

r e

l

e

l

u

m

Upper rhombic lip

BRAINSTEM FLEXURES 1. Medullary

2

n

p

2. Pontine

o

y

Mammillary body

m u s

l

a t In h fu nd ib ul um

e

tel

Optic evagination

H

s

3

a

h a l a m u s

r

C e r e b B

b

Inferior colliculus

u

Olfactory evagination Preoptic area

u

S

4

Isthm

sal B ae p h a l o n nc

m

anglia l g

u

u s

a as

g m e n t

m

c o r t e

l

x

a

P


M

e

d

3. Mesencephalic 4. Diencephalic

u

l

Figure 2. A, The lateral view of the left side of a computer-aided 3-D reconstruction of the brain and upper cervical spinal cord in M2155, the preceding GW7 specimen, which has a similar crown-rump length to C1390 (18 mm and 17.5 mm, respectively). External features are identified as in Figure 1B. The heavy numbered lines refer to brainstem flexures (boxed key). B, Top view of the brain in A shows how C1390's sections rotate right (arrow) from the anterior horizontal midline. C, Back view of the brain in A shows how C1390's sections rotate left (arrow) from the posterior vertical midline.

Medullary velum Lower rhombic lip

l a

1

C.

Pineal evagination

Back view

Superior colliculus

Cerebral cortex (occipital pole)

Spinal cord

Inferior colliculus Cerebellum

Isthmus

Vermis

Hemisphere

B.

Top view

s i d e

Medullary velum

-4.3º Horizontal midline 3.8º L e f t Scale bars = 1 mm

s i d e

Left side

Vertical midline

R i g h t


Lower medulla

Right side Spinal cord

PLATE 21A

GW7 Sagittal, CR 18 mm, C1390 Level 1: Slide 13, Section 5

Cell-dense future dura

Ce l l

-den

s e fu

tu

ll d sku in an re sk Cell-sparse superarachnoid reticulum

th (f ph al ut a ur li am etc s ic hi u po r p su d e ol bt ve rv ha nt e la ri n mi t c le r cp ) ic oo le l

ce di

en

ve nt ra l

po ol

te

hypothalamic pool mammillary

recesses

infundibular

optic Nostril Nasal cavity

C


Maxilla

l c a

T

se

Pituitary (anterior lobe)

vi

ty

o

n

Mandible

ar

metencephalic pool

Sella turcica

Medullary velum

g u e

Sphenoid bone


Larynx Arytenoid swellings

Epiglottis

Opening to trachea

Invaginating rhombencephalic choroid plexus

Basal occipital bone

myelencephalic pool Pharynx/esophagus

Vertebral body Cell-sparse superarachnoid reticulum

Labeled on this page: Peripheral neural and non-neural structures, brain ventricular divisions

Medullary velum

Cell-dense future skin and skull Cell-dense future dura

LEFT S IDE


M IDLINE

Ora

sp

isthmal narrows canal

IN

Sphenoid bone

lel

d oi hn ac r ra pe su

um ul tic re

RIGHT SIDE OF BRA

I (olfa rve Ne

Frontonasal process

e icl tr en rv pe t) su educ lic aqu ha ure cep(fut

Future frontal bone

foramen of monro

s en

"Budding" telencephalic choroid plexus

pretectal narrows

me

le nc do (f ep rs ut h al ur a e li po la c ol te s ra up l e ve r nt ve ri n cl t e) ri cl e

Pia

ry) cto

48

49

PLATE 21B TELENCEPHALON

i

m

a

Is

NEP

urons

l ne ma ist h

at

al

l

mic NE P Subt hal a in ial longitu g Med din po al fa sc

in

g

t

lp

r di a

o

a

Preop ti c

NE

P

M

i gr

Migrating cerebellar deep nuclear neurons

Pontine NEP

at Migr

Nerve IV (trochlear)

Upper rhombic lip

ic

Medial lemniscus? (intermingled with midline raphe glial system)

Inferior collicular

m Cerebellar NEP (vermis)

th

( t e c t a

s on ur e n ic lam a th ns po uro hy ne g e n n ati ti n ? igr us M ul

l i c h a e p

Future optic tract?

Anterior

c

EP c N Posterior i m (mammillary) ala th o p Middle Hy

P

Migrating preoptic neurons


E

Migrating septal neurons

Superior collicular

tal) N

tel M en igr ce at ph ing ali b c n as eu al ro ns

? ea ar

Septal NEP


tral teg me Ven nt al

tal en

Migrating olfactory neurons

Olfactory cortical NEP? Olfactory bulb NEP

n

Reticular

e e

h

T

cephali en es Migrating tegm M

M

Pretectal NEP

s

Dorsal complex

Anterior complex Brain surface (heavier line)


men eg ro ns (t neu

N e o c

Epithalamic NEP

c

Primo

Limbic cortical NEP


Posterior complex

c

Hippocampal Fornical GEP


E P

N

N E P

lex ifo r r

Cingulate/ retrosplenial

)

i

c

a

P N E

l

MESENCEPHALON

l

m

er lay

DIENCEPHALON Migrating thalamic neurons

Pontomedullary trench

( st

Midline raphe GEP

t

s) tem xure sys fle a l s t em g li a in he r b r rap t fo ne por dli up Mi ural s

ruc

t ing me d

u

eu

ro

ns


lus l funicu Ventra Ventral gray

n


Lower medullary NEP

ry


Posterior intramural migratory stream (inferior olive neurons?)

lla

The preoptic NEP, hypothalamic NEP, tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

Upper medullary NEP Migra

The telencephalic NEP, thalamic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase.

Lower rhombic lip

Intermediate gray Dorsal gray Dorsal funiculus

SPINAL CORD

RHOMBENCEPHALON

Labeled on this page: Central neural structures

PLATE 22A llCe


d


skull and kin re s u t e fu Cell-sparse superarachnoid reticulum ens

c li e le) ha icltric p r e t en nc en v le rv eral e t pe lat suure

e

ol po

Future frontal bone Frontonasal process

ph

ut

ce

foramen of monro

ol

recesses

mammillary

Sphenoid bone

optic infundibular

Maxilla

Pituitary (posterior lobe)

metencephalic pool

RAIN

Sella turcica

it

Pituitary (anterior Or lobe) al ca v Sphenoid bone

se ar sp lle C


m lu cu eti r id no ch ra a er up

LEFT S IDE OF B

hypothalamic pool

M ID L I N E

po

E

l

S ID

ra

T GH RI

nt

e icl tr en rv pe t) su educ lic aqu ha ure cep(fut

ve

s en

"Budding" telencephalic choroid plexus*

pretectal narrows

me

ut (f

(f

al rs do

th al ural am i e c ic th s po ir up d e ol su ve r bt nt v ha ri en la cl t mi cp e) r ic oo l l

Pia

di en

50

y

T g u e o n

Mandible


Medullary velum

Larynx

Incipient rhombencephalic choroid plexus


Epiglottis Pharynx

Arytenoid swellings myelencephalic pool Laryngotracheal groove

Medullary velum

Cell-sparse superarachnoid reticulum n um col ral ies) b e t d Ver (bo

*At GW7, the telencephalic choroid plexus is beginning to "bud," while the rhombencephalic choroid plexus is marked only by an incipient indentation in the medullary velum. Compare with GW7.5, C6202, Plates 207A and B in Volume 4, pages 514-515.


Cell-sparse superarachnoid reticulum

Cell-dense future skin and skull Cell-dense future dura


DIENCEPHALON

MESENCEPHALON


Cingulate/ retrosplenial Hippocampal

c

Posterior

a

Limbic cortical NEP

Dorsal

h

T

e e

n

Ventral tegmental area?

Septal NEP

ig

l N

Posterior (mammillary)

la



th

ha ot

Is

H

yp

Middle

Inferior colliculus collicular

P

Anterior

Migrating hypothalamic neurons

E

Preoptic NEP

ic

E

ma

m

ic

N

EP

Interpeduncular nucleus?

N

ra t an ing d ba se sa pt l al te ne len ur ce on ph s al

Superior colliculus collicular

(tectal)

M

l) NE t a urons P e n l ne ta

M e s enc M

m


c ali ph ce

ic (te al g h p ting teg me e ra n g i

Anterior

Olfactory bulb NEP

Migrating olfactory neurons

M

Pretectal NEP

Reticular Cingulate/ prefrontal?



Epithalamic NEP

l

a

c


P N E

m

i

Fornical GEP

N e o

r di a

i c

N E P

s

Primo

t

a l

P

lp

r

lex ifo r o

m

er lay


Cerebellar NEP (vermis)

Future optic tract?

Pontine NEP Midline raphe glial system

(structural support for brainstem flexures)

Dorsal Upper rhombic lip

Midline raphe glial structure GEP Pontomedullary trench Midline raphe glial structure GEP

M

Upper medullary NEP

ig ra

ti ng

an d


se ttl Solitary nuclear complex?

N LO HA BE

NC

EP

s


SPINAL CORD

on

s niculu ral fu Late

s niculu ral fu Vent y al gra Ventr gray te edia Interm Dorsal gray Dorsal funiculus

Lower Ventral rhombic lip

OM

edullary n g m eu r



RH

in


Lower medullary NEP

Solitary tract

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium


51

52

PLATE 23A

RIGHT

ll- d Ce

en

u tu se f

d in an re sk

skull

SID E

MI

Cell-sparse Cell sparse superarachnoid reticulum


GW7 Sagittal, CR 18 mm, C1390 Level 3: Slide 11, Section 5 DL INE

LE FT

Pia

SI D

thalamic pool


nc

pretectal narrows

ep (f

a

tu

r

l

ic

e

u

a

Diencephalic/telencephalic choroid plexus

h

u

ed

t)

ntricle

ol po

Nerve III (oculomotor) mammillary


um ul tic re d i no ch ra ra e p su se ar p s llCe

hypothalamic pool Nasal septum

optic infundibular Sphenoid bone

Or

al

Pituitary (anterior lobe) ca v it

y

Sphenoid bone

Sella turcica

ve

uc

l ra nt ve

foramen of monro

recesses

Maxilla

er

u


Frontonasal process

p

q


Future frontal bone

N

s

al rs o d

mese


ol po

EO FB RA I

metencephalic pool

T g u e o n

Mandible


Medullary velum

Incipient rhombencephalic choroid plexus

Pharynx

Arytenoid swellings


Laryngotracheal groove

lic ha ep nc l ele p o o

Epiglottis

my

Larynx

Medullary velum Vertebral column (bodies)

Cell-sparse superarachnoid reticulum Cell-dense future skin and skull




DIENCEPHALON Migrating thalamic neurons NEP ic m a l


r aye EP ml r l N o a if t i c x e te l or la

h

a

Epithalamic NEP

Pretectal NEP

M

n

sen M e Oc c

E

ma

l

N ic la ha ot

yp

Middle


t

h

H

Migrating preoptic neurons Lamina terminalis Migrating hypothalamic neurons Future optic tract?

N

Preoptic NEP

Inferior collicular

Anterior

Is

s ro n

and s

Pontine NEP

Migrating

Sprouting facial nerve (VII)

Upper rhombic lip


Migrating facial motor neurons (VII)?


Cerebellar NEP

et t

g lin

neu tine pon

E

Migrating isthmal neurons

N

m


P

EP

Interpeduncular nucleus?

(tectal )

Septal NEP (right) (left)

Superior collicular

al) NEP

(right) Septal NEP (left) Migrating septal neurons


c ali ph ce

lic (t ha e e pomotor comple g l x u


t en? m (I I I )


P

o

Ne

l

e

b ic

ica


u

s

Lim

rt co

g cin P( NE

e

Primordia lp

c


Posterior complex

Dorsal complex

)

T

MESENCEPHALON

Upper medullary NEP M

g in at igr d an



Spinal nucleus (V)?

Solitary nuclear complex?

LO N HA EP BE NC

Lower rhombic lip Solitary tract

OM


y neurons dullar me


Lower medullary NEP

RH

g in ttl se


Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium


53

PLATE 24A


nterior al and a medi o s r Do ll

d sku n an e ski u tur f e Cell sparse Cell-sparse ens ll-d superarachnoid reticulum Ce

LEF TS IDE OF BR A IN

Pia


me

tri

t

(f

c t)

en

du

cle

se ar sp lle C

m lu cu eti r id no ch ra a r pe su

l and posterior olatera Ventr

ue

Nerve III (oculomotor)

Sella turcica metencephalic pool


n




g

e ?

Pharynx

u

Otic vesicle

Pa la of tal pr m a oc e x O r a illa ss l c av it T y o

Meckel's cartilage

rv

aq

Mandible

c pe

(o rve I Ne

hypothalamic pool optic recess

Nasal cavity

Maxilla

li

(fu tu

subthalamic pool

Sphenoid bone


ha

re

ventral pool

Frontonasal process

ep


"Budding" telencephalic choroid plexus foramen of monro

Future frontal bone

nc

pretectal narrows

u


se

s

do ut su el rs ur p en al e er ce la v p po te e h ol ra nt a l r lic ve ic nt le ri cl e)

thalamic pool

lfactory)

54

Larynx?

Medullary myelencephalic velum pool Incipient rhombencephalic choroid plexus

Spiral ganglion (VIII)

Inferior glossopharyngeal ganglion (IX)? Basal occipital bone

Inferior vagal ganglion (X)?

Vertebral column Dorsal root ganglia




Cell-dense future skin and skull

See a high magnification view of the pons and medulla in Plates 28A and B.


DIENCEPHALON

MESENCEPHALON

Migrating thalamic neurons Migrating Cajal-Retzius cells and subplate neurons

N

Posterior complex

a

Epithalamic NEP

h

EP

Pretectal NEP

s

(tegmen

ta

e c

n

e s

e M P

NE

Pre o

La te ra l

p ti c An NE ter ior P

P

Facial nerve (VII) genu

nd

Mig rat in g

a


pon ng i l t t se

CTF1 (fibers)

ons neur ti n e

CTF2 (deep nuclear cells) CTF3 (cells and fibers)


Pontine NEP

CTF4 (cells)

Cerebellar NEP



Pontine NEP

Migrating facial motor neurons (VII)?

E

su ne

g c tin i ra am ig thal ns M o ro p u hy ne

Trigeminal nuclear complex (V)?

Upper rhombic lip

Upper medullary NEP

Superior olive complex?

Lower medullary NEP?


Migrating and settling medullary neurons

Inferior collicular

N

Future optic tract?

P


l NE

Migrating tegmental neurons Migrating isthmal neurons

Hypothalamic ic NEP a m atingmic thal r b ig la s M btha ron Su u

(tectal)

Substantia nigra?

Is t h m a

Migrating basal telencephalic neurons Migrating septal neurons

EP


Superior collicular

alic

N

Olfactory bulb NEP

ph

p

ic

ce

h

al

en

Anterior complex?

Limbic cortical NEP Cingulate/prefrontal?


M e

Dorsal complex

Subicular/ hippocampal

Pioneer migrating pretectal and tectal neurons


T

l

ic

a

er lay

a

i c

l)

Primo rdia l pl Neo exi co fo rt r

m

l

m


P N E

Solitary nuclear complex Posterior intramural migratory stream (inferior olive neurons?)

So

lit

ar

y

Lower rhombic lip

tr

ac

t

Precerebellar NEP?

Spinal nucleus (V)?

RHOMBENCEPHALON

The telencephalic NEP, thalamic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase. The preoptic NEP, hypothalamic NEP, tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.


Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

55

56

PLATE 25A


nterior al and a medi o s r Do


kull and s skin t u re u f Cell sparse Cell-sparse e ens superarachnoid reticulum ll-d e C Pia



future aqueduct

m su es pe en rv ce en ph tr a l ic ic le


subthalamic pool


(f

foramen of monro

ventral pool

hypothalamic pool

Nerve III (oculomotor)

Future frontal bone

m lu cu eti r id no ch ra a r pe su rse a sp llCe

Nerve I (olfactory)

Frontonasal process

optic recess Olfactory epithelium Nasal cavity

Sphenoid bone

Nerve V (trigeminal)? Maxilla


Nerve VIII (vestibulocochlear)


i


Meckel's cartilage

Otic vesicle

Mandible


metencephalic pool

Nerve VII (boundary cap*)

Pa la of tal pr m a oc Ora e l c xilla ss av ty


do t rs ut s u e l al e ur p n po e er ce ol la v p te e h ra n t a l r l ic ve i c nt l e ri cl e)

thalamic pool

Medullary velum

Pharynx


myelencephalic pool

Incipient rhombencephalic choroid plexus Nerve IX and X (boundary caps*) Nerve X (vagus)

Inferior glossopharyngeal ganglion (IX)? Nerve X (vagus)

Occipital bone

Nerve IX (glossopharyngeal) Superior glossopharyngeal ganglion (IX)?

Superior vagal ganglion (X)?

Dorsal root ganglion

* Boundary caps are

Schwann cell GEPs?

Cell sparse superarachnoid reticulum

Cell dense future dura Cell dense future skin and skull


See a high magnification view of the pons and medulla in Plates 28A and B.

57


DIENCEPHALON

MESENCEPHALON

Migrating thalamic neurons Migrating Cajal-Retzius cells and subplate neurons

m

a l a

Dorsal complex

EP N c

La te ra l

Pre o

An ptic ter ior NEP

t on gp n li


CTF1 (fibers) CTF2 (deep nuclear cells) CTF3 (cells and fibers)

Pontine NEP

CTF4 (cells)

Cerebellar NEP (hemisphere)



Pontine NEP? Superior olive complex?

Spinal nucleus (V)?




Upper rhombic lip

Upper medullary NEP



cus?


nd Migra tin ga

nis

s ron neu

se tt

in e

lem al ter

al) NE P

La

Facial nerve (VII)

(tect

Migrating inferior collicular neurons

g c tin i ra am ig al s M oth ron p u hy ne


lic

ra m ig la s M btha ron su neu

ha

i am g tin ic

Future optic tract?


Superior collicular

lar


e

l li c u

Sub

l tha

es

Pioneer migrating pretectal and tectal neurons

co rior Infe


M

lic (tegm cepha en sen tal Me )N g tegmenta n i t EP a r g Mi

Reticular Anterior complex?

Migrating basal telencephalic and olfactory neurons

Pretectal NEP


ep

Limbic cortical NEP

Olfactory cortical Basal telencephalic NEP NEP? Anteromedial ganglionic NEP HypoOlfactory bulb thalamic NEP NEP


Epithalamic NEP

nc

co

i Cingulate/ retrosplenial Hippocampal Fornical GEP

h

l

EP

T

i

a

rt

c

N


Posterior complex

s ron eu ln

Primord ial p lex ifo rm Ne o

l

er ay

P N E

c

Lower medullary NEP Precerebellar NEP?

Lower rhombic lip

RHOMBENCEPHALON


PLATE 26A


d anterior ial an med o s r skull Do and skin

re s futu n se e d llCe Cell-dense future dura



Pia

thalamic pool do t rs ut s u e l al e ur p nc po e e r ol la v e p te e h ra n t a l r l ic ve i c nt l e ri cl e)

diencephalic superventricle (future third ventricle)



(f

58

ventral pool

Future frontal bone Frontonasal process se ar sp lle C

Nasal epithelium

um ul tic re d oi hn ac ar r pe su


Sphenoid bone Nerves VII+VIII (facial +vestibulocochlear) Maxilla

metencephalic pool



Oral cavity


Mandible

Medullary velum

Meckel's cartilage

Pharynx


Otic vesicle

myelencephalic pool


NerveX (vagus)

Superior vagal ganglion (X)? Basal occipital bone






DIENCEPHALON

MESENCEPHALON

Migrating thalamic/epithalamic neurons GEP (posterior commissure)

i

m

l

E

P

a

on

m

ur

ala

ne

th

ic

am

S

al

us

ti n g

hyp al er at

ot ha

la

su

a mic

bt

h

Brachium of the inferior colliculus

Successive waves of migrating and settling basal ganglionic neurons

CTF1 (fibers)

Medial forebrain bundle? Migrating preoptic neurons

Preoptic NEP


eurons ine n ont p g lin

CTF2 (deep nuclear cells) CTF3 (cells and fibers)

ett and s

Migratin g

Lateral lemniscus?

CTF4 (cells)


Pontine NEP

Cerebellar NEP (hemisphere) Lateral cerebellar notch



l


rea

Future optic tract?

ha

al

collicular

ub

nt

EP al) N ent ons ur

of the stria terminalis?

T

h

N e o c o

ep

al) NE ect (t Inferior

me

c li

s

nc

a lar u lic ol

teg

h

ng

ne

L

ic

se

gm

le B ed nuc

Migrating basal telencephalic and olfactory neurons

N

Migrat i

(te

ra

ig

Olfactory bulb NEP Basal telencephalic NEP

rio rc

Central complex and reticular

ic

Olfactory cortical NEP?

Strionuclear NEP Anteromedial/ anterolateral ganglionic NEP

Anterior complex?

Mese nc Su e pe e

Limbic cortical NEP

Posterior complex Dorsal complex Epithalamic NEP

M


Cingulate/ retrosplenial Hippocampal Fornical GEP


p

r

t

i

l

a

lay e

a

c

M

Primordial p lex ifo rm

c

E P

P

P

r

N

E

N

Nucleus of the lateral lemniscus ?


Upper rhombic lip

Medullary NEP Auditory (cochlear) NEP

Lower rhombic lip

RHOMBENCEPHALON

The telencephalic NEP, thalamic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase. The preoptic NEP, hypothalamic NEP, tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.



59

PLATE 27A

GW7 Sagittal, CR 18 mm, C1390 Level 7: Slide 6, Section 11 LEF T SID EO FB RA IN


do

r

ri

cl

sa t l ut s u e l po ur p e n ol e er ce la v te e p h ra n t a l r l ic ve i c nt l e


skin u re fu t se Pia n -de ell

e)

C

erior nd ant ial a d e m l rso sku l and Do

osterior and p teral trola Ven


(f ventral pool

se ar sp llCe

Future frontal bone

um ul tic re d oi hn ac ar r pe su

Cell sparse Cell-sparse superarachnoid reticulum

Sphenoid bone

Frontonasal process


Trigeminal boundary cap*

Trigeminal ganglion (V) metencephalic pool

Facial ganglion and nerve (VII)

Maxilla



Oral cavity Mandible

Medullary velum

Ot


ic



l b on e

Pharynx

le s ic ve

Meckel's cartilage


oc

ci

pi

ta



u

s

60

Squamo

* Boundary caps are

Schwann cell GEPs?



Strionuclear NEP Olfactory Anterolateral cortical NEP? ganglionic NEP

Ventral lateral geniculate?

Me

dia

l genic ul a t e b

o




Thalamic NEP (posterior complex) ?

Insular cortical Insular? NEP?

Fornical GEP

dy

o

Limbic cortical NEP

late body? icu en

Cingulate/ retrosplenial Hippocampal

g

l

al l Dors ateral

a

Migrating posterior complex thalamic neurons

P N E

r

t

ic

c

N e o

Primordial

plex ifo rm

lay

er

DIENCEPHALON

Bed nucleus of the stria terminalis?

Migrating basal telencephalic and olfactory neurons Successive waves of migrating and settling basal ganglionic neurons

Globus pallidus? CTF1 (fibers) CTF2 (deep nuclear cells)

Central trigeminal tract (devoid of glia)

CTF3 (cells and fibers)

Plentiful glia in peripheral trigeminal nerve and boundary cap*


Medial cerebellar notch Trigeminal Cochlear?

Pontine NEP


Upper rhombic lip


Lower rhombic lip

RHOMBENCEPHALON

The telencephalic NEP, thalamic NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase. The pontine NEP forms a shrinking shoreline of the superventricle as stockbuilding NEP cells decrease. Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium



61

62

PLATE 28A Level 4: Slide 9, Section 5

See Level 4 in Plates 24A and B. Between Levels 4 and 5: Slide 9, Section 2

Level 5: Slide 8, Section 8


GW7 Sagittal, CR 18 mm, C1390

PONS/MEDULLA

PLATE 28B

Facial (VII) and abducens (VI) motor neurons sequestered in superficial Pontine NEP?

t Pon


in e

ne

Nerve VI (abducens)?



u oe r

ro Medullary neu

elium p it h

ep

m i t h e li u

Solitary tract?

Medial

Migrating facial (VII) motor neurons?

Lateral

Medullary reticular formation interspersed with other migrating medullary neurons

Nerve VII (facial nerve genu axons are growing laterally)

Predomina


Nerve VII (facial motor axons are turning ventrally)

ial fibe l superfic n tl y l o n g it u d i n a

r tr a cts

Between Levels 4 and 5: Slide 9, Section 2

ract?

ary t

Migrating facial (VII) motor neurons?

Solit

Migratingand settling superior olive complex and central auditory neurons? Afferent axons from ganglion IX (glossopharyngeal)?

Predomina

ntly longitudinal superficial fiber tracts

le x ? om p ear c l c u rn ibula Vest

Nerve VI (abducens)? Migrating facial (VII) motor neurons?

Solitary tract?

Afferent axons from ganglion IX (glossopharyngeal)?

Nerve VII (facial motor axons exit brain)

Predominantly longit

erficia u din al su p

Nerve VII (Sensory axons from facial ganglion enter brain) Facial nerve boundary cap* * Boundary caps are Schwann cell glioepithelia?


l fiber

tract

s

Vagal nerve boundary cap*

These arrows indicate the direction of neuronal migration.

Glossopharyngeal nerve boundary cap*

Nerve IX (glossopharyngeal)

Nerve X (vagus)

These arrows indicate the direction of axon growth.

63

64

PART PARTIV: IV: GW7 GW7 HORIZONTAL HORIZONTAL

Carnegie Collection specimen #492 (designated here as C492) was obtained in 1911 after a miscarriage. It has a crown-rump length (CR) of 16.8 mm and is estimated to be at gestational week (GW) 7. C492 was preserved in Zenker’s fixative, embedded in paraffin, and was cut in 40-µm sections that were stained with aluminum cochineal. The dorsal diencephalon and mesencephalic tectum are cut in the horizontal section plane. The plane shifts to predominantly coronal in the anterior telencephalon, ventral diencephalon, pons, and medulla. In general, C492’s sections are cut perpendicular to M2155’s sections (Specimen 1, Part II). We photographed 101 sections at low magnification from the uppermost tip of the pretectum through the spinal cord. Seventeen of these sections are illustrated in Plates 29AB to 45AB. All photographs containing the brain were used to produce computer-aided 3-D reconstructions of the external features of C492’s brain (Figure 3), and to show each illustrated section in situ (insets, Plates 29A to 45A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normalcontrast images) identify non-neural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. A “stockbuilding” telencephalic neuroepithelium surrounds the enlarging telencephalic superventricle. As in other GW7 specimens, few migrating neurons are adjacent to the cortical neuroepithelium while there are many adjacent to the basal ganglionic and basal telencephalic neuroepithelia. The plane of C492’s sections are ideal to show two features of the telencephalic/diencephalic junction that are not seen as clearly in the other GW7 specimens. First, the telencephalic and diencephalic superventricles are continuous at the very wide foramen of Monro. Second, the posterior basal ganglionic neuroepithelium forms a continuum with the ventral diencephalic neuroepithelium, making it difficult to distinguish telencephalic from diencephalic structures.

The “stockbuilding” thalamic neuroepithelium surrounds an expanding thalamic pool in the superventricle, while “shrinking” subthalamic and hypothalamic neuroepithelia surround subthalamic and hypothalamic pools. Many migrating and settling young neurons are in the parenchyma of the future subthalamus and hypothalamus. There are few neurons outside the thalamic neuroepithelium, except in ventral areas where pioneer reticular nuclear neurons are migrating. The roof (tectum and pretectum) of the mesencephalon contains a stockbuilding neuroepithelium adjacent to a very thin layer of pioneer migrating neurons. The bundles of fibers in the posterior commissure are very distinct in the uppermost sections of the pretectum. The tegmental and isthmal neuroepithelia are thinning as their neuronal progeny migrate out, but that thinning is less obvious in C492 compared to other GW7 specimens. Similar to the other GW7 specimens, there is a very thick subpial fiber band; no doubt, these are fibers from sources outside the mesencephalon and isthmus. As in other GW7 specimens, the pons and medulla have neuroepithelia that are shrinking as stem cells unload their neuronal and glial progeny into an expanding parenchyma. For the most part, nuclear subdivisions are indistinct. The superior olivary complex, facial motor nucleus, inferior olivary complex, and solitary nucleus can be tentatively identified. The subpial fiber band is thick and prominent. The “stockbuilding” cerebellar neuroepithelium sharply juts into the lateral part of the rhombencephalic superventricle at the cerebellar notches. The bands of cells and fibers in the cerebellar transitional fields are similar to other GW7 specimens. These bands are prominent in the future hemisphere and are indistinct in the future vermis.

65

C492 Computer-aided 3-D Brain Reconstructions B.

Pretectum

Side view

l l l

a Lower rhombic lip

4. Diencephalic H e m i s p h

Top view

e

r

s

b

r

a

l

S u p e r i o r c o l l i c u l u s

P ons

C o r t e x

Isthmus

C

e

r

e

b

e

l

Figure 3. A, The left side of the 3-D model viewed from the front at a 45º heading; this view is used to "peel away" sections of each level in the following Plates. B, A straight view of the left side. C, A straight down view of the top. D, An upward view of the bottom, angled (120º) to look into the mesencephalic and diencephalic flexures.

Medullary velum

D.

Occipital pole

n

a li g a n g

H

y

l

u

p

lo

a

P

e

B a sal B a s a l

ph

Optic evagination

Lower rhombic lip

e

p

r p e

C ce tele n

a

o

n

m o

t h a l a

Preoptic area

m

p t

u

Olfactory bulb

Scale bars = 1 mm

e

l

w

b

e r

s

S

u


r h o mbi c l ip

m e d

x

Upper

Tegmentum

r

e

a

t

m e d u l l

r

s

o

U

a

c

o

r

l

L

Bottom view

Frontal pole

s V e r m i

u

m

m

u

a

l

e

l

P

r

a

E

C

e

h

p

T

u i t h a l a m


r e t e c t u m

s

e

Frontal pole

Upper rhombic lip

u

3. Mesencephalic

Occipital pole

l

Medullary velum

2. Pontine

C.

e

d

BRAINSTEM FLEXURES

Lower rhombic lip

m

mus

th e

C

e

b

e

e d u l l a

2

r

M

Mammillary body

M

3

Is

o

Infundibulum

Medullary velum

Inferior colliculus

P

Optic evagination

P o n s

t

u

T

m u s

H

Preoptic area

n

n

u

S

me

la

h

a

Olfactory bulb

e

C

li

t

Basal gang

eg

4

y

m b e l l u

e

Upper rhombic lip

r

Olfactory bulb Preoptic area Optic evagination

a l a m u s

b

Subthalamus

C e r e

T

Ba sa l Basa l te

rebral co Ce

Ist hmus

t

um

nglia ga cephalon len

c o r

x

tum

Inferior colliculus

a l

r

u s a m

egm

e

x te

en

s


r

T

u

h

T hal

us am

Occipital pole

lamus tha

Epitha l a l a m

Superior colliculus

Epi

s

Superior colliculus

Pretectum

b

Angled front view

p o t h a

A.

66

PLATE 29A


GW7 Horizontal CR 16.8 mm C492 Level 1: Section 9

Non-neural structures labeled

Superior sagittal sinus?

Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Superarachnoid reticulum (brain parenchymal expansion zone) Pia and pial blood vessels

PLATE 29B


Neural structures labeled Light zone at ventricular border is tangentially cut end feet of NEP cells

Superior collicular primordial plexiform layer Successive waves of migrating superior collicular neurons


Wave 1 (oldest neurons) Wave 2 (next oldest neurons)

Columns of migrating pretectal neurons (and glia?)

Uncrossed commissure of the superior colliculus?


Posterior commissural GEP?

(future aqueduct)

Roof plate

(future GEP of tectal commissural tracts)

Posterior

Pretectal NEP

(nuclei of the posterior commissure and optic tract?)

(thin, depleted of waves 1 and 2 neuronal precursors)

Anterior

(thick, stockbuilding precursors of neurons) Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.


MESENCEPHALIC TECTUM

PRETECTUM

(SUPERIOR COLLICULUS)

MESENCEPHALON

67

68

PLATE 30A




Dural blood vessels Middle cerebral artery and branches


Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Pia and pial blood vessels

Dark stain in some blood vessels is injected ink.

Superarachnoid reticulum (brain parenchymal expansion zone)

PLATE 30B


Neural structures labeled Migrating pretectal neurons


Nucleus of the optic tract?


Superior collicular primordial plexiform layer

Earliest migration wave of thalamic neurons




thalamic pool

Roof plate

(diencephalic)

T

al

Posterior complex (medial geniculate)?

am

Roof plate

i c NEP

(tectal commissural GEP)

Pretectal NEP Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

Commissure of the superior colliculus?

pretectal narrows

Posterior complex (dorsal lateral geniculate and pulvinar)?

h

(future aqueduct)

THALAMUS

DIENCEPHALON

PRETECTUM


TECTUM (SUPERIOR COLLICULUS)

MESENCEPHALON 69

70

PLATE 31A GW7 Horizontal CR 16.8 mm C492 Level 3: Section 36 Middle cerebral artery and branches

Anterior cerebral artery and branches


Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case)


Level 3: Computer-aided 3-D Brain Reconstruction Non-neural structures labeled Dural blood vessels Superarachnoid reticulum (brain parenchymal expansion zone) Pia and pial blood vessels

PLATE 31B


Neural structures labeled Successive waves of migrating subthalamic neurons Earliest migration wave of thalamic neurons

Migrating oculomotor nuclear (III) neurons?


Migrating red nuclear neurons? Superior collicular primordial plexiform layer

Mesencephalic reticular formation?





diencephalic superventricle thalamic pool

Roof plate

(diencephalic)


subthalamic pool

Dorsal/central complexes?

Th

Commissure of the superior colliculus

(future aqueduct)


Tegmental NEP Interpeduncular NEP? Oculomotor (III) NEP?

al

Reticular

am

Ventral complex?

ic

NE

P ( t h i ck

Rubral NEP?

) Roof plate

(mesencephalic commissural GEP) Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

Subthalamic NEP (thin) THALAMUS

SUBTHALAMUS

DIENCEPHALON

Tectal (superior collicular) NEP TEGMENTUM

TECTUM

MESENCEPHALON 71

72

PLATE 32A GW7 Horizontal CR 16.8 mm C492 Level 4: Section 39 Middle cerebral artery and branches



Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case)


Level 4: Computer-aided 3-D Brain Reconstruction Non-neural structures labeled Dural blood vessels Superarachnoid reticulum (brain parenchymal expansion zone) Pia and pial blood vessels

PLATE 32B

Neural structures labeled


Migrating mammillary neurons Migrating subthalamic neurons Earliest migration wave of thalamic neurons Thalamic primordial plexiform layer

Migrating interpeduncular nuclear neurons Mesencephalic reticular formation? Migrating oculomotor nuclear (III) neurons? Migrating red nuclear nucleus neurons?


Migrating substantia nigra neurons? neurons Superior collicular primordial plexiform layer



Medial lemniscus and medial forebrain bundle?

Commissure of the superior colliculus? mesencephalic superventricle


(future aqueduct)


Roof plate (diencephalic) telencephalic superventricle

Tegmental NEP

Interpeduncular? Oculomotor (III)?

Central complex?

(very thick due to tangential cut)

am

Ventral complex?

ic

NE

P ( thic k)

Roof plate


THALAMUS

SUBTHALAMUS

CEREBRAL CORTEX

TELENCEPHALON

Substantia nigra and ventral tegmental area?

DIENCEPHALON

(mesencephalic commissural GEP)

TEGMENTUM

Neocortical NEP

al

Hypothalamic (mammillary) NEP (thick) Subthalamic NEP (thin)

(mammillary body)

Th

Rubral? Reticular

HYPOTHALAMUS

(future lateral ventricle, posterodorsal pool)


(thick and thin areas)

subthalamic pool and mammillary recess

thalamic pool

Tectal (superior collicular) NEP (thick) TECTUM

MESENCEPHALON 73

74

PLATE 33A GW7 Horizontal CR 16.8 mm C492 Level 5: Section 45

Level 5: Computer-aided 3-D Brain Reconstruction Non-neural structures labeled Dural blood vessels Pia and pial blood vessels





Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Interpeduncular fossa


PLATE 33B




Migrating ventral tegmental area neurons Pioneer exiting oculomotor nerve (III) fibers

Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)?

Migrating oculomotor nucleus (III) neurons

Medial lemniscus and medial forebrain bundle

Sprouting oculomotor nerve (III) fibers



Earliest migration wave of thalamic neurons

Migrating substantia nigra and ventral tegmental area neurons?



Migrating inferior collicular neurons? Tectal primordial plexiform layer


Migrating superior collicular neurons Commissure of the superior colliculus?

subthalamic pool


Roof plate (diencephalic)

(future third ventricle, Ante thalamic pool) rio


mammillary recess

(future aqueduct)

Roof plate

(mesencephalic)

r

Cingulate

or

al

po ol )

h

Ce

a

Retrosplenial

nt

r al

la m

ic

Ve nt

r al

NE P

ar

T

Re ti c

NEP


Hypothalamic (mammillary) NEP Subthalamic NEP

THALAMUS

DIENCEPHALON

HYPOTHALAMUS (mammillary body)

ic

bi

SUBTHALAMUS

co

rt

m

ul

(CEREBRAL CORTEX)

Fo rn Hi ic pp al oc G cc te EP a s l m ve up en o pa n t (f e r c t r e u ri t v p ic a l cl u e h e, r e n a lN po la tr lic EP st te ic er ra le i l

Li

Neo

TELENCEPHALON

Raphe complex Oculomotor (III)?

Superior collicular

Substantia nigra/ ventral tegmental area?

Tegmental NEP

Inferior collicular Tectal NEP

TEGMENTUM

TECTUM

MESENCEPHALON

75

76


Level 6: Computer-aided 3-D Brain Reconstruction Non-neural structures labeled Dural blood vessels Pia and pial blood vessels





Primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Circle of Willis arteries? Dark stain in some blood vessels is injected ink.

PLATE 34B



Medial forebrain bundle? Migrating subthalamic neurons (zona incerta, Forel's fields) Settling subthalamic nuclear neurons? Migratory waves of thalamic neurons Thalamic primordial plexiform layer


Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)? Migrating mammillary neurons Migrating raphe, ventral tegmental area, substantia nigra, and oculomotor (III) neurons Medial lemniscus? Mesencephalic reticular formation? Sprouting oculomotor nerve (III) fibers


Migrating inferior collicular neurons?


Tectal primordial plexiform layer Migrating superior collicular neurons

Fornical GEP Telencephalic stem cells of choroid plexus


Roof plate (diencephalic,

subhypothalamic pools

Fornical GEP


stem cells of choroid plexus)

T

Ce

Li e pp m oc bi t am (f s el c pa ut u e c l or ur pe nc t e r e Re i c po la v p a st te en ha l N tros er ra t l E P p le n io l ri ic ia l r ve c ? po nt le ol ri ) cl e,

o

h

n tr

r

ti c a l

Re t

N E P

NEP

Subthalamic NEP

THALAMUS Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

(mammillary?)

i

N E P


al

Ven al t r al am ic

(mesencephalic)


Hypothalamic Substantia nigra/ ventral tegmental area?

Tegmental NEP HYPOTHALAMUS

Hi

SUBTHALAMUS

at

la r

ul

c

(CEREBRAL CORTEX)

o N e

TELENCEPHALON

ng

Oculomotor (III)?

cu

Ci

Roof plate

Raphe complex?

or

(future third ventricle, thalamic pool) Anteri

DIENCEPHALON

Superior collicular

Inferior collicular

Tectal NEP

TEGMENTUM

TECTUM

MESENCEPHALON

77

78


Level 7: Computer-aided 3-D Brain Reconstruction Non-neural and peripheral neural structures labeled Dural blood vessels


Pia and pial blood vessels

Pia and pial blood vessels Anterior cerebral artery and branches Superior sagittal sinus?


Primordial mesenchymal brain case (skin/bone) Nerve III (oculomotor) Dark stain in some blood vessels is injected ink.

Future dura (internal border of brain case)


PLATE 35B



Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)?

Medial forebrain bundle? Migratory waves of subthalamic neurons

Migrating raphe, ventral tegmental area, substantia nigra, and oculomotor (III) neurons Mesencephalic reticular formation?

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Cortical primordial plexiform layer

Sprouting oculomotor nerve (III) fibers? Migrating inferior collicular neurons?


Tectal primordial plexiform layer

Fornical GEP


(future lateral ventricle, posterior pool)


Telencephalic stem cells of choroid plexus

TH ng

Li

N e

bi

Hi

e

cc

or

o

c

(CEREBRAL CORTEX)

m

at

o

TELENCEPHALON

ul

r

t i c a l

pp

tic

AL

AM

oc

am p Pa al ra hi pp

al

NE

oc

P

am

pa

SU Subthalamic BT HA NEP LA MU S

(future aqueduct)

Oculomotor (III)?

Hypothalamic NEP PO HY

Substantia nigra/ ventral tegmental area? Superior collicular

Tegmental NEP Inferior collicular

N E P



(mesencephalic)

Premammillary Middle

US

l?

Roof plate

Raphe complex?

A M US

Ci


(future third ventricle, thalamic pool)

AL

Anterior thalamic NEP

stem cells of choroid plexus)

TH

Roof plate (diencephalic,

hypothalamic pool

Fornical GEP

Tectal NEP

TEGMENTUM Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

DIENCEPHALON

TECTUM

MESENCEPHALON

79

80

PLATE 36A

Level 8: Computer-aided 3-D Brain Reconstruction Non-neural and peripheral neural structures labeled

GW7 Horizontal CR 16.8 mm C492, Level 8: Section 65

Middle cerebral artery Dural blood vessels



Branches of anterior cerebral artery Superior sagittal sinus?


Cerebellar artery?


Nerve III (oculomotor) Circle of Willis arteries?

Primordial mesenchymal brain case (skin/bone) Dark stain in some blood vessels is injected ink.



PLATE 36B


Migrating middle hypothalamic neurons


Migrating subthalamic neurons?


Sprouting oculomotor (III) nerve? mesencephalic superventricle (future aqueduct, isthmal canal)




Lateral lemniscus and brachium of inferior colliculus

Migrating bed nucleus of the stria terminalis neurons?



(future lateral ventricle, posterior pool) Stem cells of choroid plexus

Raphe glial structure GEP in mesencephalic floor plate

Telencephalic

Limbic cortical NEP

L

Amygdaloid/posterior ganglionic NEP

N

C

e

O R

T

E

o X

c

o

r t i c a l


N E P

(mesencephalic) Diencephalic floor plate?

Su bt h NE ala SU P? mic BA BT SA LG HA AN LA GL MU S? IA

C E R E B R A L

ic lam a h ot yp EP h le N dd TH Mi PO HY

US

Strionuclear NEP

Roof plate

M

Hippocampal Cingulate


(future third ventricle, hypothalamic pool)

Isthmal NEP

LA

A B R C E R E

Fornical GEP

fo ra m mo of en nr o

Diencephalic

A

Junction of telencephalic and diencephalic roof plates


X T E C O R

DIENCEPHALON Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

ISTHMUS

TECTUM

MESENCEPHALON

TELENCEPHALON

81

82

PLATE 37A GW7 Horizontal CR 16.8 mm C492, Level 9: Section 71

Level 9: Computer-aided 3-D Brain Reconstruction Non-neural and peripheral neural structures labeled Pia and pial blood vessels Middle cerebral artery

Dural blood vessels

Superior sagittal sinus? Superarachnoid reticulum (brain parenchymal expansion zone) Cerebellar artery?


Primordial mesenchymal brain case (skin/bone) Dark stain in some blood vessels is injected ink.

Basilar artery? Nerve III (oculomotor) Future dura (internal border of brain case)



PLATE 37B


Migrating arcuate nuclear neurons? Migrating and settling posterior hypothalamic neurons


Migrating and settling anterior hypothalamic neurons




(future aqueduct, isthmal canal)

Sprouting trochlear nerve (IV) fibers?

posterior pool




Migrating trochlear (IV) neurons?

Migrating amygdaloid/ posterior ganglionic neurons?

anterior pool

Stem cells of choroid plexus

Migrating bed nucleus of the stria terminalis neurons?

Telencephalic

Junction of telencephalic and diencephalic roof plates

fo ra me n

L

C

O

R

of mo nr o

Amygdaloid/posterior ganglionic NEP

N

A

e

o T

Anterior hypothalamic NEP G AN GL IA

o r t i c a l X

Middle hypothalamic NEP

HYPOTHALAMUS

DIENCEPHALON

c E

(mesencephalic)


Strionuclear NEP

Limbic cortical NEP

Roof plate

Diencephalic floor plate?

BA S A L

BR C E R E

Cingulate

Lateral lemniscus


Diencephalic

Fornical GEP Hippocampal


Raphe glial structure GEP in mesencephalic floor plate

N E P C E R E B R A L

TELENCEPHALON

RT C O

E

Isthmal NEP

ISTHMUS

Tectal (inferior collicular?) NEP

TECTUM?

MESENCEPHALON

X




83

84

PLATE 38A

Non-neural structures labeled Pia and pial blood vessels Posterior cerebral artery Internal carotid artery Dural blood vessels Middle cerebral artery

GW7 Horizontal CR 16.8 mm C492, Level 10: Section 87


Cerebellar artery?



Dural blood vessels Primordial mesenchymal brain case (skin/bone)




Basilar artery?

PLATE 38B


Migrating arcuate nuclear neurons?

Successive migratory waves of basal ganglionic neurons

2 Future putamen?

3

Layers of the cerebellar transitional field (CTF)

Central trigeminal tract?


Migrating trigeminal (V) neurons? Migrating raphe and abducens (VI) neurons?


Sprouting abducens nerve (VI)?

(future lateral ventricle) anterior pool

CT F2: Mig rati CTF ng d 1 eep nu c l e a CT F3

ons ur

rn e

1


Lateral hypothalamic neuron migration?

Future pallidum?


Migrating middle hypothalamic neurons?

Po n fo tine rm r at etic io u n? la r

Migrating neurons originating in corticoganglionic NEP



Damaged area in section

cor tic

Insular

al

NE

P

CEREBRAL CORTEX

Limbic cortical NEP

Anteromedial

Cortico ganglionic AnteroNEP lateral

Posterior Lateral?

Middle

H y pot h

TELENCEPHALON

ar

In f u n d

ic N E P a l am

Raphe glial structure


(future fourth ventricle) Metencephalic roof plate splits into paired upper rhombic lips

Raphe complex?

Medullary velum

Abducens (VI)? Reticular formation? Trigeminal (V)

G

BASAL GANGLIA

ul

EP

Diencephalic floor plate? Raphe glial structural GEP in pontine floor plate

ib

(future third ventricle) hypothalamic pool

N

Neo



f of ora mo me Fornical nr n o GEP

an g l i onic

Subfornical organ primordium in telencephalic roof plate

infundibular recess

Medial lemniscus?

Pontine NEP

HYPOTHALAMUS

PONS

DIENCEPHALON Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

Cerebellar NEP Medial cerebellar notch

CEREBELLUM

RHOMBENCEPHALON Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.


85

86

PLATE 39A GW7 Horizontal CR 16.8 mm C492 Level 11: Section 94 Superior sagittal sinus?

Non-neural structures labeled Internal carotid artery Middle cerebral artery

Maxillary process

Circle of Willis arteries? Basilar artery? Posterior cerebral artery


Frontonasal process?


Primordial mesenchymal brain case (skin/bone)




Dural blood vessels


PLATE 39B


Successive migratory waves of basal ganglionic/basal telencephalic neurons




(future lateral ventricle) Migrating basal telencephalic neurons originating in limbic cortical NEP



1

CTF4-5 (cells-deep neurons) Migrating abducens (VI) and premigratory facial (VII) motor neurons?

2

ca

l

EP

G

Cortical (olfactory) NEP

al

u lar

infundibular recess

In f u n d

hal pot a m i c N E Hy

P

te r

(future fourth ventricle) Metencephalic roof plate splits into paired upper rhombic lips

Raphe glial Raphe structural GEP in pontine complex? floor plate

Medullary velum

Abducens (VI)?

enc

Facial motor (VII)? Reticular formation?

s ba

Fu

t ur

e

BASAL GANGLIA AND BASAL TELENCEPHALON

TELENCEPHALON

La

le Midd

ep h

F tu re pallu idum ? alo n

P



te l

An

c o r ti

Anterolateral

NE


Sprouting abducens (VI) and facial (VII) nerve fibers?

al

bic

Insular

en o r amon r fo f m o

tero m an g l i o edial nic Future putamen?

Li

m Cingulate/ prefrontal


r r io s te Po

Septal NEP

Diencephalic floor plate?

3

anterior pool

Subfornical organ primordium in telencephalic roof plate

Medial lemniscus?

ib

ol f re act ce or ss y

Premigratory Purkinje neurons sequestered in superficial cerebellar NEP


Ret icu lar for ma tion

Lateral hypothalamic neuron migration?

CEREBRAL CORTEX


Migrating middle hypothalamic neurons?


N

CTF1 (fibers)

Migrating arcuate nuclear neurons?

Pontine NEP Cerebellar NEP

HYPOTHALAMUS

DIENCEPHALON


PONS

Lateral cerebellar notch?

CEREBELLUM

RHOMBENCEPHALON Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.



87

88

PLATE 40A

Peripheral neural and non-neural structures labeled Nerve V (trigeminal, boundary cap*) Nerve V (trigeminal, sensory) Nerve V (trigeminal, motor) Circle of Willis arteries?

GW7 Horizontal CR 16.8 mm C492 Middle cerebral/carotid artery Level 12: Section 112


* Boundary caps are Schwann cell GEPs?


Nerve II (optic) Zygomatic bone? Nerve I (olfactory) Olfactory epithelium Nasal septum Medullary velum


Superarachnoid reticulum (brain parenchymal expansion zone) Hypothetical olfactory induction field


Eye

Pigment epithelium Retinal NEP Vitreous body Intraretinal space Sclera Dark stain in some blood vessels is injected ink.

Sella turcica in sphenoid bone


Future dura and dural blood vessels (internal border of brain case)

PLATE 40B





CTF1 (fibers)


CTF2 (cells-deep neurons) CTF4-5 (cells-deep neurons)


CTF6 (cells-Purkinje cells) Migrating trigeminal (V) sensory neurons? Central trigeminal fibers Migrating arcuate nucleus neurons? Settling trigeminal motor (V) neurons? Migrating anterior hypothalamic neurons?

Metencephalic roof plate splits into paired upper rhombic lips

optic recess

Medial

infundibular recess preoptic recess

Preoptic area Germinal zones

PREOPTIC AREA

Migrating median preoptic nuclear neurons?

Preoptic NEP Chiasmal GEP

Infundibular NEP Posterior pituitary GEP in diencephalic floor plate

Anterior NEP


Ret formicular ation


(future third ventricle, hypothalamic/ preoptic pool)

Raphe glial lemniscus? structure

Lateral hypothalamic neurons?




Raphe glial structure GEP in pontine floor plate Raphe complex? Abducens (VI) and facial motor (VII)?

Sprouting abducens (VI) and facial (VII) nerve fibers?

Medullary velum

Pontine NEPs

Reticular formation?

Migrating abducens (VI) and premigratory facial (VII) motor neurons?

Trigeminal (V)?

Hypothalamic Germinal zones HYPOTHALAMUS

Cerebellar NEP Metencephalic roof plate splits into paired upper rhombic lips

Medial

DIENCEPHALON

Lateral

PONS

Cerebellar notches

CEREBELLUM

RHOMBENCEPHALON


89

90

PLATE 41A

Peripheral neural and non-neural structures labeled Nerve V (trigeminal, sensory) Nerve V (trigeminal, motor) Trigeminal (V) ganglion


Nerve V (trigeminal, boundary cap*) Future dura and dural blood vessels (internal border of brain case) Posterior cerebral artery Pia and pial blood vessels


Olfactory epithelium Nerve I (olfactory) Nasal cavity

Nasal septum

Ethmoid bone?

Sphenoid bone?

Frontonasal process?

C ar aro te tid ry ?

Superarachnoid reticulum (brain parenchymal expansion zone) ica Sella turc

Or bit o-s ph en oid pr oc ess ?

Neurohypophysis Adenohypophysis

Pituitary gland


Nerve VII (boundary cap?*)

Eye

Sprouting optic nerve (II) fibers Retinal NEP Pigment epithalium Vitreous body Cornea Lens Pioneer retinal ganglion cells Intraretinal space Sclera

* Boundary caps are

Schwann cell GEPs?



PLATE 41B

Central neural structures labeled Metencephalic roof plate splits into paired upper rhombic lips



Migrating trigeminal (V) sensory neurons?


Settling trigeminal motor (V) neurons?

Peripheral trigeminal (V) axons have more interstitial glia than central axons.

Trigeminal nerve (V) motor axons have fewer interstitial glia than sensory axons. Longitudinal domains of migrating and settling pontine reticular formation neurons Migrating arcuate nuclear neurons? Medial Raphe glial lemniscus? structure

Pioneer optic tract fibers?

Migrating suprachiasmatic nucleus neurons?


(future third ventricle, hypothalamic pool, infundibular recess)

Chiasmal GEP

Infundibular NEP Posterior pituitary GEP in diencephalic floor plate

Hypothalamic Germinal zones


Superior olive? Migrating abducens (VI) and premigratory facial motor (VII) neurons? Raphe glial structure GEP in pontine floor plate Raphe complex?

Sprouting abducens (VI) nerve fibers?

Abducens (VI), facial motor (VII), and reticular formation?

Trigeminal (V)? Auditory (cochlear)

DIENCEPHALON Central trigeminal fibers Lateral\ lemniscus?

PONS

Medullary velum

Pontine NEPs

Sprouting facial (VII) nerve fibers?

HYPOTHALAMUS



Facial motor nucleus?

Metencephalic roof plate splits into paired upper rhombic lips

Cerebellar NEP Medial Lateral Cerebellar notches


CEREBELLUM

RHOMBENCEPHALON


91

92


Peripheral neural and non-neural structures labeled Nerve VIII (vestibulocochlear, boundary cap*) Nerve VIII (vestibulocochlear) Vestibular (VIII) ganglion

Squamous temporal bone


Future dura and dural blood vessels (internal border of brain case)


avit al c y Or


Tongue

M an dib le

Basilar artery

a

le

Sphenoid bone?

M

ib nd

Palatal process


Maxillary sinus?

Maxilla

tid ro y? Ca rter a

Maxillary sinus?

* Boundary caps are

Schwann cell GEPs?



PLATE 42B



Myelencephalic roof plate splits into paired lower rhombic lips Migrating cochlear nuclear neurons? Lateral lemniscus?

Migrating vestibular neurons?

mati on

Settling facial motor nucleus (VII) neurons?


ar fo r

Longitudinal domains of migrating and settling pontomedullary neurons

ticu l

Superior olive?

Re

Medial lemniscus?


Prepositus nucleus?


Raphe complex NEP?

Medullary velum

Raphe glial structure GEP in floor plate

Medial longitudinal fasciculus?

Prepositus nuclear and reticular formation NEP

Spinal nucleus and tract (V)? Vestibular NEP

Pontomedullary germinal zones

Auditory (cochlear) NEP Myelencephalic roof plate splits into paired lower rhombic lips


LOWER PONS/UPPER MEDULLA

RHOMBENCEPHALON


93

94

PLATE 43A





Squamous temporal bone Future dura and dural blood vessels (internal border of brain case)

Basilar artery

c avity

Basal occipital bone?

a Or l

Future hyoid bone?

Tongue

Sublingual salivary gland? Meckel's cartilage (mandibular process)

tid ro y? Ca rter a

Pia and pial blood vessels Superarachnoid reticulum (brain parenchymal expansion zone)


Temporal bone labyrinth (future cochlea, utricule, saccule, and semicircular canals)

Primordial mesenchymal brain case (skin/bone) Vestibulocochlear nerve (VIII) boundary cap (Schwann cell GEP?)


PLATE 43B


Myelencephalic roof plate splits into paired lower rhombic lips


Premigratory precerebellar nuclear neurons?

Posterior intramural migratory stream (inferior olive neurons)? Migrating vestibular neurons?

Longitudinal domains of migrating and settling medullary neurons Inferior olive?

Medial lemniscus? Medial longitudinal fasciculus? Raphe glial structure

Hypoglossal (XII) nucleus? Vagal motor (X) nucleus?



Re

Hypoglossal nuclear (XII), vagal motor (X), and reticular formation NEP

tic ar ul n io at rm fo

Spinal nucleus and tract (V)?

Raphe complex NEP

Medullary velum

Medullary germinal zones

Raphe glial structure GEP in medullary floor plate Vestibular NEP? Precerebellar NEP

Myelencephalic roof plate splits into paired lower rhombic lips Lateral lemniscus?

MEDULLA

RHOMBENCEPHALON

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

95

96

PLATE 44A




Basilar artery

cavit y


ral

Tongue

tid ro y? Ca rter a

Internal auditory meatus?



O

Primordium of larynx?


Squamous temporal bone

Ve r

teb

r al

arte

ry?


External ear?

Vagal nerve (X) boundary cap (Schwann cell GEP?) Nerve X (vagal) Dark stain in some blood vessels is injected ink.


Superior vagal (X) ganglion

PLATE 44B





Densely packed premigratory precerebellar nuclear neurons? Few interstitial glia in central fiber tracts Abundant interstitial glia in peripheral nerve



Migrating vestibular neurons? Posterior intramural migratory stream (inferior olive neurons)?

Hypoglossal (XII) nucleus?

Medullary velum

Vagal motor (X) nucleus?

Inferior olive? Medial longitudinal fasciculus? Raphe glial structure

Hypoglossal nucleus (XII), vagal motor (X), and reticular formation NEP

Medial lemniscus decussation? Medial lemniscus? Spinocerebellar tracts in inferior cerebellar peduncle?

Reticular formation


Raphe complex NEP Raphe glial structure GEP in medullary floor plate Solitary nuclear NEP


Visceral sensory afferent fiber tracts? Precerebellar NEP Myelencephalic roof plate splits into paired lower rhombic lips

LOWER MEDULLA

RHOMBENCEPHALON

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

97

98

PLATE 45A



Dorsal root ganglia

Squamous occipital bone


Traces of nerve XII (hypoglossal)? Pia and pial blood vessels Future dura (internal border of brain case)

Spinal nerve (dorsal roots) Dorsal root boundary caps

Vertebral column

Primordial mesenchymal brain case (skin/bone) Superior vagal (X) ganglion Superarachnoid reticulum (brain parenchymal expansion zone) Dark stain in some blood vessels is injected ink.


PLATE 45B


Abundant interstitial glia in peripheral nerve

Densely packed premigratory precerebellar nuclear neurons?

Dorsal sensory nucleus (X)?


Dorsal motor nucleus (X)? Hypoglossal nucleus (XII)? Posterior intramural migratory stream (inferior olive neurons)?


Dorsal funiculus Few interstitial glia in central fiber tracts

Dorsal gray


Inferior olive?

Medullary velum

Medial longitudinal fasciculus? Raphe glial structure

Hypoglossal nuclear (XII), vagal motor (X), and reticular formation NEP Raphe complex NEP

Reticular formation Solitary nucleus and tract Spinocerebellar tracts in inferior cerebellar peduncle?

SPINAL CORD

Raphe glial structure GEP in medullary floor plate Solitary nuclear and vagal sensory (X) NEP Precerebellar NEP

Medullary germinal zones Myelencephalic roof plate splits into paired lower rhombic lips

LOWER MEDULLA

RHOMBENCEPHALON Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons.

99

100

PART PARTV: V: GW6.5 GW6.5 CORONAL CORONAL

This specimen is embryo #2051 in the Minot Collection, designated here as M2051. The crown-rump length (CR) is 15 mm estimated to be at gestational week (GW) 6.5. Most of M2051’s brain sections are cut (10 µm) in the coronal plane, but the plane shifts to predominantly horizontal in the posterior medulla. We photographed 87 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and medulla. Seventeen of these sections are illustrated in Plates 46AB to 62AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of M2051’s brain (Figure 4), and to show each illustrated section in situ (insets, Plates 46A to 62A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. Plates 63AB show a high-magnification view of the neocortical neuroepithelium. All parts of the telencephalic neuroepithelium are rapidly increasing their pool of neuronal and glial stem cells as they expand the shorelines of the enlarging telencephalic superventricle. Only very few pioneer Cajal-Retzius neurons have migrated into the cell-sparse primordial plexiform layer adjacent to the cerebral cortical neuroepithelium. The rest of the cortical neuronal population has yet to be generated. The basal ganglionic and basal telencephalic neuroepithelia, although increasing their stock of stem cells, do have adjacent migrating neurons. There are many fewer neurons in these areas than in the GW7 specimens (Parts II, III, and IV). As in the GW7 specimens, these neurons appear to migrate together in early (outermost and less dense) to late (innermost and most dense) waves. The diencephalic neuroepithelium surrounds a narrowing superventricle. It is thinnest in the hypothalamic and subthalamic areas, where it is surrounded by denselypacked waves of migrating neurons. It is postulated that these areas of the superventricle have shrinking shorelines

as the neuroepithelia “unload” their stock of neuronal precursors. In contrast, the superventricle shoreline is still expanding as the thalamic neuroepithelium continues to add more neuronal precursors than to unload postmitotic neurons. There are only a few neurons migrating outside the thalamic neuroepithelium. The mesencephalon contains a stockbuilding neuroepithelium in the pretectum and tectum (relatively few adjacent migrating neurons). On the other hand, the tegmental and isthmal neuroepithelia are thinner. Migrating neurons are leaving in large numbers and accumulate in clumps so dense that the superficial border of the neuroepithelium is indistinct. Some cells lie farther out in the tegmental and isthmal parenchyma and are more sparsely scattered adjacent to the subpial fiber band. Both the pons and medulla have neuroepithelia that are thicker than at GW7, but are nevertheless shrinking as they unloaded their neuronal and glial progeny into an expanding parenchyma. Cells are migrating and settling in longitudinal arrays at the pontine flexure. A few cells are settling in the faintly discernable superior olivary complex and many are settling in the reticular formation throughout the pons and medulla. Some facial motor neurons are migrating from medial to lateral, leaving behind their axons in a small, but definite genu of the facial nerve. Migrating inferior olive neurons are in the posterior intramural migratory stream outside the precerebellar neuroepithelium in the posterior lower rhombic lip, but very few neurons have settled in the inferior olive. Many neurons have settled in the solitary nucleus, surrounding a definite solitary tract. The cerebellar neuroepithelium is in the stockbuilding phase, mainly adding precursors of Purkinje cells and some late-generated deep nuclear neurons. Many deep nuclear neurons are migrating in the cellular layers of the cerebellar transitional field, but these and the fibrous layers are thinner and less definite than at GW7.

101

M2051 Computer-aided 3-D Brain Reconstructions B.

Side view

b

C e r e

m

b e l l u

Ba

B

Upper rhombic lip

as

sal ganglia on al al t e l e nce p h

H

e

Preoptic area

r

C

e

Optic evagination

Preoptic area Optic evagination

3 C

e

hm r

e

b

e

u l l

2

Mammillary body

Lower rhombic lip

M e d u

Ist

Upper rhombic lip

Medullary velum

Infundibulum

Medullary velum

P o n s

Inferior colliculus

y

T Subthalamus

Ba Ba s a l s al t el e

rebral co Ce

x t e

Ist hmus

4

o

m tu

c

eg me n

a l

r

Inferior colliculus

ia gl a n alon g eph nc


us am

r

t u m e n

u s

r

x te

T

u s

h a l a

e g m

m

T

us am

Occipital pole

T

m

Epitha l

Superior colliculus

m

Angled front view

Pretectum Epit ha l l a ha

s

Superior colliculus

u

Pretectum

Su bthalam us p o t h a l a n s m u s P o

A.

M

a l l

BRAINSTEM FLEXURES

e

d u l l a

1

1. Medullary

Lower rhombic lip

2. Pontine Spinal cord

3. Mesencephalic Spinal cord

4. Diencephalic

C.

Top view

H e m i s p h e r

e

m

u

s

e

C o r t e x

Isthmus

ns

r a l


Po

b

V e r m i s

a

P

r

l

p

e

a

E

C

T h

m


u

Frontal pole

i t h a l a m

u

r e t e c t u m

s

Occipital pole


C

e

b

e

r

e

l

l

Upper rhombic lip

D.

Medullary velum

Scale bars = 1 mm

Optic evagination

lip

a

l

l

m e d u

r al Co

in

p

S

w

m e d u l l a

ic

o

a

li

mb

e r

p

P o n s

y

U

n ga

g

H

rho

e r p p

lo

n

B a s a l

ha

t h a l a

C B asal telencep

o

e

S

Preoptic area

p t u

s

e r e

m


er

L

Occipital pole Tegmentum

u

Frontal pole

l

m

b

r

a

Low

d

Bottom view

r t e x c o

102

PLATE 46A GW6.5 Coronal CR 15.0 mm M2051 Level 1: Section 66

Non-neural structures labeled Interhemispheric fissure Cell-dense primordial mesenchymal brain case (skin/bone) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)


Pia


Frontonasal process Naso-optic furrow Placodal epithelium Medial nasal process

PLATE 46B


TELENCEPHALON

The dorsomedial neocortical primordial plexiform layer contains even fewer cells.

EP Nanterodorsal (future lateral ventricle)

Few migrating Cajal-Retzius cells are outside the lateral neocortical NEP.

ic

co

r tical NE

P

Cingulate

anteroventral pool

Insular


pool


Neocorti

ca

l

CEREBRAL CORTEX

Lim

b

Prefrontal

More migrating Cajal-Retzius cells are in the primordial plexiform layer outside the ventrolateral limbic cortical NEP. Few migrating Cajal-Retzius cells outside ventromedial limbic cortical NEP

The density of migrating cells in the primordial plexiform layer indicates ventrolateral-todorsomedial and ventrolateral-to-ventromedial maturation gradients in the cerebral cortex.


NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

103

104


Non-neural and peripheral neural structures labeled Interhemispheric fissure Cell-dense primordial mesenchymal brain case (skin/bone) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)


Pia

Frontonasal process


Naso-optic furrow Olfactory epithelium Nasal epithelium Placodal epithelium Nasal septum

Medial nasal process Nostril opening to nasal cavity Lateral nasal process

105

PLATE 47B




Limbic cortical NEP

Brain surface (heavier line) Cortical primordial plexiform layer

anterodorsal pool

Neocortical NEP

Fornical GEP Limbic cortical (insular) NEP

Migrating Cajal-Retzius cells More migrating cells adjacent to insular NEP indicate earlier maturation.


Corticoganglionic NEP?



anteroventral pool

Roof plate

Migrating neurons originating in the corticoganglionic NEP?

(telencephalic stem cells of choroid plexus)

Migrating pioneer basal ganglionic neurons



Migrating pioneer basal telencephalic neurons

Migrating medial septal neurons FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

106


Non-neural and peripheral neural structures labeled Interhemispheric fissure Cell-dense primordial mesenchymal brain case (skin/bone) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)

See a high-magnification view of the cerebral cortex in Section 122 in Plates 63A and B.


Pia

Frontal sinus? Hypothetical olfactory induction field Nerve I (olfactory)

Naso-optic furrow Frontonasal process Olfactory epithelium Nasal epithelium

Lateral nasal process Nostril opening to nasal cavity


Placodal epithelium Medial Nasal septum nasal process


Figure 247D modified (Patten, 1953, p. 429.)

Nasal septum

Frontal prominence

Medial nasal process Naso-optic furrow Lateral nasal process Nostril Mouth Hyoid arch (II)

Hyoid bone

Eye Maxillary process Mandibular arch (I) Hyo-mandibular cleft Laryngeal cartilages Mandible

107

PLATE 48B Central neural structures labeled Roof plate

(telencephalicdiencephalic junction)


Limbic cortical NEP



Corticoganglionic NEP?


anterodorsal pool Telencephalic

Migrating Cajal-Retzius cells More migrating cells adjacent to insular NEP indicate earlier maturation.

Diencephalic future third ventricle (diencephalic roof)

e (futur


Fornical GEP

c ephali telenc entricle le) v ventric supelr ateral

Neocortical NEP

anteroventral pool

Stem cells of choroid plexus


foramen of monro

Migrating pioneer basal ganglionic neurons

Basal telencephalic (olfactory?) NEP Septal NEP


Migrating pioneer basal telencephalic neurons

Migrating medial septal neurons

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

108


Non-neural and peripheral neural structures labeled

Cell-dense primordial mesenchymal brain case (skin/bone) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)



Pia

Orbito-sphenoid process? Naso-optic furrow Hypothetical olfactory induction field Sclera of eye Nerve I (olfactory)

Olfactory epithelium Nasal cavity Maxillary process Placodal epithelium

Nasal septum Nasal epithelium Medial nasal process Lateral nasal process


109

PLATE 49B


DIENCEPHALON THALAMUS

Roof plate

Dorsal complex

Thalamic NEP

(diencephalic stem cells of choroid plexus?)

Reticular nucleus

Migrating pioneer thalamic neurons

Anterior complex


TELENCEPHALON CEREBRAL CORTEX Hippocampal Cingulate


(thalamic pool)

Neocortical NEP


Limbic cortical NEP

Fornical GEP dorsal pool


tu

l ic e e ) ha cl ic l ep ri t r nc nt v e n le ve l te per t e r a a sure l

(f u

Migrating Cajal-Retzius cells

Limbic cortical (insular) NEP Corticoganglionic NEP?

Anterolateral ganglionic NEP Basal telencephalic (olfactory?) NEP Septal NEP

ventral pool

Roof plate

More migrating cells adjacent to insular NEP indicate earlier maturation.



foramen of monro

Successive waves of migrating basal ganglionic neurons

Anteromedial ganglionic NEP

BASAL GANGLIA/ BASAL TELENCEPHALON Migrating pioneer basal telencephalic neurons

Migrating medial septal neurons

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

110






Pia

Nerve I (olfactory) Naso-optic furrow

Eye Pigment epithelium Intraretinal space Vitreous body Retinal NEP Sclera

Lens of eye

Olfactory epithelium Nasal cavity

Nasal septum

Maxillary process Nasal epithelium Placodal epithelium Oral cavity Medial nasal process Lateral nasal process


Mandibular process

111

EPITHALAMUS THALAMUS

PLATE 50B


DIENCEPHALON

Roof plate

Epithalamic (habenular) NEP?

(primordium of pineal gland)

Migrating pioneer epithalamic/ thalamic neurons

Posterior complex?


Ventral complex?

Thalamic NEP

Reticular nuclear Anterior complex epithalamic/ thalamic pool

Limbic cortical NEP

Hippocampal Cingulate/ retrosplenial


telencephali c superv

Neocortical NEP

(future late entricle ral ventricl e) posterior pool

Fornical GEP

Anterolateral ganglionic NEP Anteromedial ganglionic NEP

foramen of monro



CEREBRAL CORTEX


TELENCEPHALON

preoptic pool



Roof plate


bus Glolidus? l a p

Migrating Cajal-Retzius cells More migrating cells adjacent to lateral cortical NEP indicate earlier maturation.

Migrating neurons originating in the corticoganglionic NEP? Successive waves of migrating basal ganglionic neurons


Successive waves of migrating basal telencephalic neurons

DIENCEPHALON PREOPTIC AREA

Preoptic NEP Migrating preoptic area neurons



Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Arrows indicate the regionally shrinking shoreline of the superventricle as NEP cells are depleted while generating neurons. Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.

112



Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)

Cell-dense primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Pia

Orbito-sphenoid process? Pial blood vessels

Eye

Intraretinal space Pigment epithelium Retinal NEP Pioneer retinal ganglion cells Vitreous body Sclera

Naso-optic furrow Nerve II (optic)

Ethmoid bone? Oral cavity

optic recess

Tongue

Anterior cardinal vein? Maxillary process Palatal process

Mandibular process


113

PLATE 51B

Central neural structures labeled MESENCEPHALON

Roof plate

PRETECTUM?

(posterior commissural GEP?)

Migrating pretectal neurons?

Pretectal NEP?


DIENCEPHALON

mesencephalic superventricle?

THALAMUS

(future aqueduct)

Posterior complex?

Thalamic NEP


Ventral complex?



SUBTHALAMUS

Subthalamic NEP TELENCEPHALON

CEREBRAL CORTEX

Neocortical NEP telencephalic superventricle


thalamic pool

Reticular nuclear

posterior pool


Migrating reticular nuclear neurons? Migrating subthalamic (Forel's fields, zona incerta) neurons? Cortical primordial plexiform layer Migrating Cajal-Retzius cells

subthalamic pool

Limbic cortical NEP Amygdaloid NEP

AMYGDALA

Strionuclear NEP?

DIENCEPHALON

Lateral area

PREOPTIC AREA Preoptic NEP

preoptic pool

Migrating amygdaloid neurons Migrating bed nucleus of the stria terminalis neurons? Migrating lateral preoptic neurons Migrating medial preoptic neurons

Medial area

Chiasmal channel? Optic nerve (II) GEP Preoptic NEP bordering optic recess Chiasmal GEP




114



Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Cell-dense primordial mesenchymal brain case (skin/bone) Dural blood vessels

Future dura (internal border of brain case) Pia

Pial blood vessels

Orbito-sphenoid process? Naso-optic furrow

Eye Pigment epithelium Intraretinal space Retinal NEP Sclera

Sphenoid bone? Oral cavity Tongue

Anterior cardinal vein? Maxillary process Palatal process

Meckel's cartilage

Mandibular process Hypoglossal nerve (XII)?


115

PLATE 52B

Central neural structures labeled MESENCEPHALON PRETECTUM

Roof plate



Pretectal NEP

Migrating pretectal neurons mesencephalic superventricle?

DIENCEPHALON

(future aqueduct)

Posterior complex (lateral geniculate and pulvinar)?

Thalamic NEP

thalamic pool


Ventral complex?


THALAMUS

Reticular nuclear

TELENCEPHALON

CEREBRAL CORTEX

Neocortical NEP telencephalic superventricle

(future lateral ventricle, posterior pool)



Migrating reticular nuclear neurons? Cortical primordial plexiform layer Migrating Cajal-Retzius cells

subthalamic pool

Migrating subthalamic (Forel's fields, zona incerta) neurons?

DIENCEPHALON SUBTHALAMUS

Subthalamic NEP

Migrating lateral preoptic neurons Lateral area

PREOPTIC AREA


preoptic pool

Preoptic NEP Medial area

Trajectory of future optic nerve (II)

optic recess

Optic nerve (II) and chiasmal GEP

Suprachiasmatic NEP?

HYPOTHALAMUS

Anterobasal NEP?

Migrating suprachiasmatic neurons? Migrating anterobasal neurons?




116




Cell-dense primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case)

Dural blood vessels

Pia

Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Pial blood vessels

Ali-sphenoid process? Anterior pituitary gland Sphenoid bone? Maxillary process

Oral cavity Tongue

Anterior cardinal vein? Palatal process Meckel's cartilage

Mandibular process Hypoglossal nerve (XII)?


PLATE 53B


MESENCEPHALON PRETECTUM

Roof plate



Pretectal NEP


(future aqueduct)

Posterior complex (lateral geniculate and pulvinar)?


thalamic pool

Ventral complex?


Thalamic NEP

Migrating pretectal neurons mesencephalic superventricle?

Posterior complex (medial geniculate)? Reticular nuclear

SUBTHALAMUS

Subthalamic NEP

subthalamic pool

HYPOTHALAMUS

Lateral

Dorsal

Hypothalamic NEP




Migrating reticular nuclear neurons?

Migrating subthalamic neurons (Forel's fields, zona incerta)?

Migrating lateral hypothalamic neurons? hypothalamic pool

Migrating dorsal hypothalamic neurons?

Anterior

infundibular recess

Migrating anterior hypothalamic neurons? Medial forebrain bundle?




117

118

PLATE 54A


GW6.5 Coronal CR 15.0 mm M2051 Level 9: Section 330


Cell-dense primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) Dural blood vessels

Pia Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)

Pituitary gland

Ali-sphenoid process?

Posterior (neurohypophysis) Anterior (adenohypophysis)

Sphenoid bone (sella turcica)

Trigeminal ganglion (V) Nerve V (trigeminal) Eustachian tube? Oral cavity Tongue

Palatal bone?

Lingual epithelium Palatal process


Anterior cardinal vein? Maxillary process

Mandibular process Maxillary-mandibular placodal epithelium

119

PLATE 54B

Central neural structures labeled Roof plate

MESENCEPHALON


PRETECTUM

Pretectal NEP mesencephalic superventricle?


(future aqueduct)

Posterior complex (lateral geniculate and pulvinar)? thalamic pool


Ventral complex?


Thalamic NEP


SUBTHALAMUS

Subthalamic NEP

subthalamic pool

HYPOTHALAMUS

Hypothalamic NEP

Lateral

Anterior/dorsal

Posterior commissure Migrating pretectal neurons Light areas are pockets of sprouting pretectal and thalamic axons Thalamic primordial plexiform layer Successive waves of migrating thalamic neurons separated by sprouting axons

Migrating reticular nuclear neurons? Brain surface (heavier line)

Migrating subthalamic neurons (Forel's fields, zona incerta)? Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)?

Migrating lateral hypothalamic neurons? hypothalamic pool infundibular recess

Middle/infundibular

Medial forebrain bundle? Migrating dorsal and anterior hypothalamic neurons? Migrating middle hypothalamic neurons? Migrating arcuate nuclear neurons?

Median eminence/ neurohypophysis (pituicyte) GEP




120



Pia

Cell-dense primordial mesenchymal brain case (skin/bone) Dural blood vessels



Middle cerebral artery? Circle of Willis artery? Pontine artery? Sphenoid bone Posterior cerebral artery? Nerve V (trigeminal) Trigeminal ganglion (V) Anterior cardinal vein?

Basilar artery




Otic vesicle

Facial ganglion (VII)? Petrous temporal bone

Nerve VII (facial)?

121

PLATE 55B

Central neural structures labeled Roof plate

MESENCEPHALON


PRETECTUM


Pretectal NEP

Pretectal primordial plexiform layer Successive waves of migrating pretectal neurons separated by sprouting axons


TEGMENTUM


Rubral?

Tegmental NEP

Migrating red nuclear neurons? Oculomotor (III)?

Migrating oculomotor (III) nuclear neurons?

Interpeduncular?


DIENCEPHALON SUBTHALAMUS

subthalamic pool

Lateral

Hypothalamic NEP Middle


HYPOTHALAMUS



Subthalamic NEP

Migrating subthalamic neurons (Forel's fields, zona incerta)? Migrating lateral hypothalamic neurons? Luysian migration (subthalamic nuclear neurons originating in hypothalamic NEP)?

hypothalamic pool

Migrating arcuate nuclear neurons? Floor plate (diencephalic)

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium

Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.


122



Pia

Cell-dense primordial mesenchymal brain case (skin/bone) Dural blood vessels

Future dura (internal border of brain case) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)

Middle cerebral artery? Circle of Willis artery?

* Boundary caps are

Schwann cell GEPs? Trigeminal boundary cap* Facial boundary cap* Facial ganglion (VII)?

Nerve VII (facial)? Vestibular ganglion (VIII)

Temporal bone labyrinth (otic vesicle) Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone)

Basilar artery Basal occipital bone



Spiral ganglion (VIII) budding from otic vesicle epithelium?

Anterior cardinal vein?

123

TECTUM

PLATE 56B


MESENCEPHALON

Roof plate

Posterior commissure?



Superior collicular NEP mesencephalic superventricle

Successive waves of migrating superior collicular neurons separated by sprouting axons

(future aqueduct)

Migrating red nuclear neurons?

TEGMENTUM Rubral?

Tegmental NEP


Oculomotor?

Migrating oculomotor (III) nuclear neurons? Migrating interpeduncular nucleus neurons?

Interpeduncular?

DIENCEPHALON

HYPOTHALAMUS

diencephalic superventricle (mammillary recess)


Hypothalamic NEP (posterior, mammillary)

Luysian migration (subthalamic nucleus neurons originating in hypothalamic NEP)?

Floor plates

PONS Central trigeminal tract?

Diencephalic Pontine (raphe glial structure GEP)


Trigeminal (V) nuclear complex


Migrating raphe nuclear complex neurons Caudal extension of trigeminal nuclear complex?

RHOMBENCEPHALON Midline raphe glial structure Medial lemniscus?




124

PLATE 57A



Pia

Cell-dense primordial mesenchymal brain case (skin/bone)



Circle of Willis arteries? Basilar artery

Vestibolocochlear boundary cap (Schwann cell GEP?)



Basilar artery


Superior glossopharyngeal ganglion (IX)? Basal occipital bone?


125

PLATE 57B



Roof plate

(commissural GEP?)

TECTUM


Superior collicular NEP

Migrating superior collicular neurons Brain surface (heavier line)


TEGMENTUM Mesencephalic reticular formation?

Substantia nigra?

Tegmental NEP

Migrating substantia nigra neurons?

Ventral tegmental area? Oculomotor (III)?

Migrating oculomotor (III) nuclear neurons? Migrating ventral tegmental area neurons?

DIENCEPHALON

HYPOTHALAMUS (mammillary body) PONS

Pontine floor plate

(raphe glial structure GEP)



Medial pontine NEP

Midline raphe glial structure Medial lemniscus? Migrating raphe nuclear complex neurons Pontine reticular formation

Lateral pontine NEP

Trigeminal (V) nuclear complex?

CEREBELLUM

Migrating deep nuclear neurons?

Cerebellar NEP Migrating cochlear nuclear neurons? Nucleus of the lateral lemniscus?

Auditory (cochlear?) NEP Lateral medullary NEP

Lateral lemniscus?

Nerve VII (genu)?


Settling facial motor nuclear neurons (VII)? Medial medullary NEP

Medial longitudinal fasciculus? Medullary floor plate


Spinal nucleus (V)? Premigratory facial motor nuclear neurons (VII)? Abducens nucleus (VI)? Superior olivary complex? Migrating raphe nuclear complex neurons Medial lemniscus? Midline raphe glial structure

MEDULLA

RHOMBENCEPHALON Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium



126

PLATE 58A



Pia

Cell-dense primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) with blood vessels

Nerve III sheath (oculomotor) Basilar artery


Vestibolocochlear boundary cap (Schwann cell GEP?) Temporal bone labyrinth (otic vesicle)



Basilar artery


Glossopharyngeal ganglion (IX) Vagal ganglion (X) Basal occipital bone? Foramen magnum?

127


PLATE 58B


Roof plate

(commissural GEP?)






(future aqueduct)

Inferior collicular NEP?

Migrating inferior collicular neurons Brain surface (heavier line)

TEGMENTUM

Tegmental NEP

Substantia nigra?

Migrating substantia nigra neurons?


Migrating oculomotor (III) nuclear neurons?

Oculomotor (III)?

Migrating ventral tegmental area neurons?

Midline raphe glial structure Medial lemniscus?

PONS

Pontine floor plate

Migrating raphe nuclear complex neurons


Medial pontine NEP

(raphe nuclei and reticular formation?)

CTF1 (fibers)




CEREBELLUM

CTF4-5 (cells-deep neurons?)


metencephalic pool


Upper (pontine roof plate)


Lower (medullary roof plate)

myelencephalic pool

Auditory NEP

Segregating rhombic lips

Migrating cochlear nuclear neurons? Nucleus of the lateral lemniscus?

(cochlear?)

Lateral medullary NEP (vestibular nuclear complex?)



(reticular formation, raphe nuclei and prepositus nucleus?)

Lateral lemniscus? Spinal nucleus (V)? Migrating vestibular nuclear neurons? Prepositus nucleus?

Medial longitudinal fasciculus? Medullary floor plate

MEDULLA


Raphe nuclear complex? Medial lemniscus? Midline raphe glial structure

RHOMBENCEPHALON




128

PLATE 59A



Pia




Squamous temporal bone Petrous temporal bone


Trajectory of nerve XII (hypoglossal)?


Superior vagal ganglion (X)? Squamous occipital bone?

129

Central neural structures labeled Roof plate (commissural GEP?) MESENCEPHALON

PLATE 59B



TECTUM

"Cave in" here is a processing artifact.





(future aqueduct)

Inferior collicular NEP?


TEGMENTUM

Tegmental NEP

Substantia nigra?

Migrating substantia nigra and ventral tegmental area neurons?

Ventral tegmental area? Oculomotor (III)?


isthmal canal

ISTHMUS


Isthmal NEP

Layers of the cerebellar transitional field (CTF) CTF1 (fibers)

CEREBELLUM

CTF2 (cells-deep neurons) CTF3 (fibers) CTF4-5 (cells-deep neurons?)

isthmal canal

Cerebellar NEP metencephalic pool

Metencephalic roof plate (upper rhombic lip)


Medial cerebellar notch rhombencephalic superventricle (future fourth ventricle)

Myelencephalic roof plate

Medullary velum

myelencephalic pool

(lower rhombic lip)

Posterior intramural migratory stream (inferior olive neurons)?

Precerebellar nuclear NEP

Migrating vestibular nuclear neurons?



(vestibular nuclear complex?)

Solitary nucleus and tract?


(reticular formation, raphe nuclei and prepositus nucleus?)


MEDULLA

Medullary floor plate


Settling inferior olive neurons? Raphe nuclear complex? Medial lemniscus? Midline raphe glial structure

RHOMBENCEPHALON


ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.


130

PLATE 60A



Pia



Level 15: Computer-aided 3-D Brain Reconstruction Squamous occipital/ temporal bones?

Trajectory of nerve X (vagus)?


131


Roof plate

MESENCEPHALON

PLATE 60B


(commissural GEP?)


TECTUM



Tectal primordial plexiform layer Brain surface (heavier line)


Inferior collicular NEP


ISTHMUS

Isthmal NEP


isthmal canal

CTF1 (fibers)

CEREBELLUM


Cerebellar NEP



metencephalic pool

Metencephalic roof plate


(dorsal rhombic lip)

Medullary velum rhombencephalic superventricle (future fourth ventricle)

These folds are processing artifacts.

MEDULLA


myelencephalic pool

(ventral rhombic lip)

Migrating precerebellar nuclear neurons? Precerebellar nuclear NEP

Migrating vestibular nuclear neurons?



(vestibular/solitary nuclear complexes?)


(reticular formation, raphe complex, hypoglossal [XII] and vagal motor [X] nuclei?)


Medial longitudinal fasciculus? Medullary floor plate (raphe glial structure GEP)

Migrating hypoglossal and vagal motor nuclear neurons? Raphe nuclear complex Medial lemniscus decussation?

RHOMBENCEPHALON SPINAL CORD

Spinal floor plate



Posteior intramural migratory stream (inferior olive neurons)? Settling inferior olive neurons?

Midline raphe glial structure Ventral funiculus Ventral gray

Ventral spinal NEP

FONT KEY: ventricular divisions - capitals Intermediate spinal NEP Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Dorsal spinal NEP Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons. Spinal roof plate

Intermediate gray Lateral funiculus Dorsal gray Dorsal funiculus


132



Pia





133

PLATE 61B

Neural structures labeled MESENCEPHALON

Migrating superior colliculus neurons

Roof plate

TECTUM

(commissural GEP?)







isthmal canal

ISTHMUS

Trochlear (IV) nucleus Nerve IV (trochlear)

Trochlear nuclear NEP

CEREBELLUM


Vermis

Cerebellar NEP

CTF1 (fibers) CTF2 (cells-deep neurons)

Intermediate hemisphere


Lateral hemisphere


metencephalic pool




Medullary velum

These folds are processing artifacts.


myelencephalic pool

(lower rhombic lip)

Migrating precerebellar nuclear neurons?

Precerebellar nuclear NEP Lateral medullary NEP


(solitary nuclear complex and sensory vagal [X] nuclei?)


(reticular formation, raphe nuclear complex, hypoglossal (XII) and vagal motor (X) nuclei?)


MEDULLA

RHOMBENCEPHALON ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.

Posteior intramural migratory stream (inferior olive neurons)? Migrating hypoglossal (XII) and vagal motor (X) nuclear neurons? Raphe nuclear complex

SPINAL CORD Intermediate spinal NEP

Dorsal spinal NEP Spinal roof plate

Lateral funiculus Intermediate gray Dorsal funiculus Dorsal gray


134








Pia

135

PLATE 62B Neural structures labeled MESENCEPHALON POSTERIOR TIP OF TECTUM Brain surface (heavier line)

RHOMBENCEPHALON CEREBELLUM

Layers of the cerebellar transitional field (CTF) CTF1 (fibers)


CTF2 (cells-deep neurons) metencephalic pool



Medullary velum The infolding of the medullary velum and overlying brain case primordium is an artifact of shrinkage during histological processing.



MEDULLA

Myelencephalic roof plate (lower rhombic lip)

Migrating precerebellar nuclear neurons?

Precerebellar nuclear NEP? Migrating cuneate nuclear neurons?

Lateral medullary NEP (cuneate nucleus?)

myelencephalic pool

Migrating gracile nuclear neurons? Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

Posterior medullary NEP (gracile nucleus?)

Medullary roof plate

RHOMBENCEPHALON FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

NEP - Neuroepithelium Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.



CEREBRAL CORTEX FUTURE PARACENTRAL LOBULE

136

PLATE 63A GW6.5 Coronal, CR 15.0 mm, M2051 Near Level 3: Section 122

PLATE 63B Pia Brain surface (heavier line)

Neocortical primordial plexiform layer Earliest migrating Cajal-Retzius cells Synthetic zone of pseudostratified NEP

Neocortical NEP

(in "stockbuilding" stage when more NEP cells are being added and few neurons are being generated)

Mitotic NEP cells

NEP cell end feet (clear space near ventricle)

telencephalic superventricle (future lateral ventricle)

Mitotic zone of pseudostratified NEP

137

138

PART PARTVI: VI: GW6.5 GW6.5 SAGITTAL SAGITTAL Carnegie Collection specimen #9247 (designated here as C9247) was collected in 1954 from a tubal pregnancy. The crown-rump length (CR) is 15 mm estimated to be at gestational week (GW) 6.5. C9247 was fixed in formalin, embedded in a celloidin/paraffin mix, and was cut in 8-µm sagittal sections that were stained with azan. Various orientations of the computer-aided 3-D reconstruction of M2051’s brain are used to show the gross external features of a GW6.5 brain (Figure 5). C9247’s sections are perfectly aligned in the sagittal plane. Considering all of the specimens in every volume of the Atlas, this is one of the best for quality of histological preservation and adherance to a section plane. Indeed, nearly an entire volume could be dedicated to the analysis of this brain. We photographed 64 sections at low magnification from the left to the right sides of the brain. Seven sections from the left side of the brain are illustrated in Plates 64AB to 70AB. Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (lowcontrast images) identify central neural structures. Plates 71AB to 84AB show high-magnification views of all parts of the developing brain. The sagittal plane is ideal to show the relative sizes of major brain subdivisions and the entry zones of sensory nerve fibers. The telencephalon is the smallest overall brain structure, composed mainly of a “stockbuilding” neuroepithelium surrounding an expanding telencephalic superventricle. The primordial plexiform layer adjacent to the cortical neuroepithelium has only scattered Cajal-Retzius cells. Migrating neurons are adjacent to the basal ganglionic and basal telencephalic neuroepithelia forming mounds in the floor of the telencephalon. The olfactory neuroepithelium is indistinct; a few pioneer olfactory nerve fibers get near the brain surface but do not appear to contact it. The diencephalon is the larger forebrain structure. The “stockbuilding” neuroepithelium surrounds a dorsally expanding superventricle in the future thalamic area. The neuroepithelium is shrinking in the hypothalamic and subthalamic areas where stem cells are depleted as they gener-

ate neurons. Migrating and settling young neurons accumulate in a thick band outside the neuroepithelium in the ventral diencephalic parenchyma adjacent to a thin subpial fibrous band. The mesencephalon is a prominent arch between the mesencephalic and diencephalic flexures. The roof (tectum and pretectum) of the mesencephalon contains a stockbuilding neuroepithelium adjacent to a very thin layer of pioneer migrating neurons. In comparison to the GW7 specimens, bundles of fibers in the posterior commissure are distinct but smaller. The tegmental and isthmal neuroepithelia are rapidly unloading their neuronal progeny in dense bands in the adjacent parenchyma. The outermost clumps of young neurons appear to interact with axons in the thick subpial fiber band. The rhombencephalon is the largest brain structure. Both the pons and medulla have neuroepithelia that are still relatively thick as stem cells unload their neuronal and glial progeny into an expanding parenchyma at a faster rate than the addition of new stem cells. The pons and medulla contain longitudinal bands of migrating cells, more dense just outside the neuroepithelium, less dense in the core, and again more dense adjacent to the subpial fiber band. The genu of the facial motor nerve forms fascicles adjacent to the neuroepithelium in both medial and lateral sections; these fascicles never reach the pial surface. What is presumed to be the solitary tract is the most prominent internal fiber tract in the medulla. Lateral sections show large peripheral sensory nerves contacting the brain. The mesencephalic nuclear neurons associated with the trigeminal nerve are migrating into the brain. The subpial fiber band is thicker where the axons from sensory ganglia enter the brain and appear to mingle with migrating neurons at the entry zones. As in the GW7 specimens, peripheral nerves have dense glia (Schwann cells), while central fiber tracts are clear. The cerebellum stands out as the most immature part of the rhombencephalon. All parts of the cerebellar neuroepithelium are stockbuilding neuronal and glial stem cells. Relatively indistinct layers are in the cerebellar transitional field.

139

EXTERNAL FEATURES OF THE GW6.5 BRAIN

4

Sub t h alamus ha lam us

m m ill a r bo d y

Ma

ot nd

ib

u

r

e

b

u l l

Upper rhombic lip

m

2

BRAINSTEM FLEXURES 1. Medullary 2. Pontine 3. Mesencephalic

Medullary velum

n

I n fu

C

e

4. Diencephalic

(with artifactual infoldings)

P

o

Preoptic area Optic evagination

s

yp

H

Basal telencephalon

3

m

al ga n g l i a

e

lu

as

A perfect sagittal cut through the brain is parallel to the midline in both horizontal and vertical directions. C9247 is a rare perfectly cut sagittal specimen.

Inferior colliculus

Isthmus

B

egment

m

c o r t

T

Superior colliculus

u

al

h a l a

ex

r e b C e r

T

Pretectum

mus ala u s ith m

Side view

Ep

y

Pineal evagination

A.

M e d

1

Front view

l

Lower rhombic lip

Spinal Spinal cord cord

Superior colliculus

B.

u

l a

Figure 5. A, The lateral view of the left side of a computer-aided 3-D reconstruction of the brain and upper cervical spinal cord in M2051, the preceding GW6.5 specimen, which has the same crown-rump length as C9247 (15 mm). External features are identified as in Figure 4B. The heavy numbered lines refer to brainstem flexures (boxed key). B, Front view of the brain in A. C, Back view of the brain in A.

C.

Superior colliculus

Back view

Pretectum

Inferior colliculus

Pineal evagination Epithalamus Isthmus

Vermis He

Pons Thalamus

Right side

Cerebellum

Left side Cerebral cortex

Medulla

Basal ganglia

Left side

m

isp

he

Cerebral cortex (occipital pole) re

Right side

Medullary velum

(with artifactual infoldings)

Pons

Rhombic lip border

Medulla

Spinal cord Spinal cord

Scale bars = 1 mm

PLATE 64A GW6.5 Sagittal, CR 15.0 mm C9247, Level 1: Slide 27 ll Section 14 sku and n ski Future dura?

enc

ephalic (future

su

aq

u

e t)

th al am ic

c

tr

Pia

p

ed

en

po ol

m

es

SID EO FB RA IN

u


Pineal gland primordium

E FT

v

Ce

se

re

ENTIRE SECTION IS FR OM L

r


en ll- d

u f ut

e


(future third ventricle) Incipient telencephalic/ diencephalic choroid plexus

icl


dors al po ol

Basilar artery

hy su po bt th ha al la am m ic ic/ po ol

140


mammillary

foramen of monro Frontonasal process

ve nt ra lp oo l

recesses Pituitary (posterior lobe)

metencephalic pool

infundibular


Medial Medial nasal process

optic

Nasal septum

Sphenoid bone

Oral c av

Placodal Placodalepithelium epithelium

ity

Pituitary (anterior lobe)

Mandibular Mandibular arch arch (I)

o u e n g

Larynx

Sphenoid bone

T



Maxillary process Maxillary

Epiglottis?

Ph yn x

Basal occipital bone Basilar artery

c ol u m n Body

myelencephalic pool

ar

Arytenoid swellings

Ve r t e b r a l

Medullary velum


Intervertebral disks Medullary velum

central canal

See the following for higher magnification views of this and nearby sections. Plates 72A and B: hippocampus and thalamus Plates 73A and B: hypothalamus Plates 74A and B: mesencephalic tegmentum Plates 76A and B: cerebellum

141

E

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold C

N N

E

IE D T h al am

ic m bic cortica lN E

Li

l plexiform la rdia o im Cingula te Pr

TELENCEPHALON

Mesencephalic (tegmental) NEP

Posterior complex

ing grat Mi

Dorsal complex

P

H

tegmental

Superior colliculus

ne

Mesencephalic (tectal) NEP

Isthmal neurons (sequestered in isthmal NEP?)

Inferior colliculus

Anterior complex Fornical GEP?


Isthmal NEP


Prefrontal

Hypothalamic NEP


Basal telencephalic neurons (sequestered in NEP?)

P

N

r ye

Pinealocyte GEP Epithalamic NEP

E

O


Pretectal NEP and posterior commissure GEP

s on ur

Hippocampal

P

ON

L

Migrating Cajal Retzius cells

AL

PLATE 64B C

A


PH

M E S E N

Cerebellar NEP (vermis) Migrating cerebellar deep nuclear neurons

Middle

Mi

g ra t aning b Pituicyte d s as ept al GEP a l te le ne nc u ro ep Preoptic ns hal ic NEP Migrating preoptic neurons Anterior Lamina terminalis Migrating anterior hypothalamic neurons

Pontine germinal zone

(predominantly midline raphe GEP)

R

Radially migrating cells

Upper rhombic lip

H


O N

Fibrous processes

Upper

C E

P

H

Midline raphe glial structure

(predominantly midline raphe GEP)

E


Tangentially migrating cells

B



M

Cell body layer

A

Radially migrating cells

L

O N

Fibrous processes

Ventr al fun iculus Ventral gray

Lower

Intermediate gray Dorsa l funic ulus

Ventral

Spinal NEP Dorsal gray

S P I N A L

C O R D

Intermediate Dorsal

Lower rhombic lip

Indentations in the mesencephalic tectum are artifacts of histological processing. The telencephalic NEP, thalamic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. The preoptic NEP, hypothalamic NEP, tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricles as stockbuilding NEP cells decrease.

142

PLATE 65A GW6.5 Sagittal,CR 15.0 mm C9247, Level 2: Slide 25 Section 9 ll sku po ol th al am ic

c

t)

le )

icl

e isthmal narrows canal

l

dorsal pool

tr

Incipient telencephalic choroid plexus


foramen of monro

ventral pool


optic recess

Sphenoid bone

Maxillary Maxillary process Placodal Placodalepithelium epithelium

T

Or

Mandibular arch (I) Mandibular arch

u e n g

avity al c

o

ar x

m yel

yn

en ce

Medullary velum

phal


ol ic po

Sympathetic chain ganglia?

Spinal nerves

Dorsal root ganglia

Ph


Medullary velum


Lateral nasal Lateral process

Na sal cav ity

metencephalic pool


Nerve I (olfactory) Frontonasal process Olfactory Olfactory epithelium epithelium Medial Medial nasal process Nostril

en

u

Pia

ed

(f su die ut p n ur e c hy su po bt e rv ep th e h th ha ir n a al la d tr li am m ve i c i c ic / nt cl po ri e o c

f

N AI

n se

nd na ski

v

-de

IS FR OM LEF TS ID EO F BR ephalic c n e s s u t (fu ure e p aq m e u r

ll Ce

Future dura?

re u tu

ENTIRE SECTION


See a higher magnification view of the hippocampus and thalamus from a nearby section in Plates 72A and B.

143

C


n M e s e c e p

N

Reticular complex

Hippocampal

N

Fornical GEP Anterior complex

ub

al

te r

Midd le?

t

An te rio r

P o n

e o p tic NEP

Pr

n

H

i

R


N e

Medial lemniscus?


E

Sprouting abducens (VI) and facial (VII) nerves?

N C

P

r

M

H

e

A

d

u O

N

Hypoglossal (XII) and vagal motor (X) nuclei?

Lower


Indentations in the mesencephalic tectum are artifacts of histological processing.

P N E

Prepositus nucleus?

r y l a

Raphe nuclear complex?

L


l


E


pe



Up


Upper rhombic lip

Migrating pontine neurons

B



M


Superficial fibrous layer

O

Brain surface (heavier line) Migrating olfactory neurons Migrating basal telencephalic neurons Migrating basal ganglionic and septal neurons Migrating preoptic neurons


S

Hypothalamic NEP


Inferior collicular


La

Insular? Olfactory cortical NEP



th

Septal and medial ganglionic NEP

A

Isthmal NEP

mic


P

ala

Limbic cortical NEP

E

ur

H

P

Retrosplenial?

i c ( t e g m a l tegmental n e h e ting gra Mi

P

E

N

c

E i

DI

T h a l a m

A

H al

tic

Dorsal complex

E

N

P

C

Superior collicular

N E P

Neocor

Epithalamic NEP

t a l )

T E L E N C E P

Posterior complex

n

N

E

E

s on

L

P

N

Pretectal NEP and posterior commissural GEP

O

Primordial plexiform layer

O

N

ON

PLATE 65B

L


E

AL

PH

M E S E

Gracile and cuneate nuclei?

Lower rhombic lip Ventral funiculus

Ventral gray La Intermediate te gray ra lf un icu l gray lu Dorsa s niculus Dorsal fu

S P I N A L

C O R D


144

ure

na ski

thalamic pool

m

es

e

halic ncep (future

su

aq

p

ed

e c

t)


en

u


u

BR

v

Ce

e ll- d

ns

ut ef

IS FR OM LEF TS ID EO F

r

Future dura?

ENTIRE SECTION

N AI

PLATE 66A GW6.5 Sagittal, CR 15.0 mm C9247, Level 3: Slide 23 ll Section 8 sku nd

tr

icl

subthalamic pool

e

Pia

Incipient telencephalic choroid plexus dorsal pool Cell-sparse superarachnoid reticulum


ventral pool

Nerve I (olfactory) Frontonasal process Olfactory epithelium

metencephalic pool

Sphenoid bone

Nasal cavity



Carotid artery? Maxillary process Otic vesicle O

ra

lic ha ep nc le ye m

Plates 71A and B: neocortex Plates 75A and B: tectum, isthmus, and cerebellum

ty

g u e

avi l c

n

I) h( a rc la r ibu

o

nd

T

Ma

See the following for higher magnification views of this and nearby sections.

Arterial trunk

ol po

Petrous temporal bone Pharynx Inferior vagal ganglion (X)


optic recess

Medullary velum

Medullary velum

Basal occipital bone Cell-sparse superarachnoid reticulum

Sympathetic chain ganglia?

Nerve X (vagus)

Dorsal root ganglia


145


M

N

N

E

l a m i c a

DI

N EP

a

ic

rt

co

l a m i c a

A H




i

eb

e

ch ot

e (h

is m

ph

M

CTF1 (fibers)

e) er

l EP el eb rN er la c l l be ra te re La Ce

e

e

n ar

ch ot

Upper Dorsal rhombic lip

H

Medial lemniscus? Sprouting facial (VII) nerve?

O


M

B N

M d

u

Reticular formation

l l a

r y

we Lo


r

N

Spinal nucleus V?

E

P

Solitary nucleus? Posterior intramural migratory stream (inferior olive neurons)?

O N A L P H C E

e


E

Reticular formation

r pe


t

er

rn

Up


n

n

di

c al

lla

R

Preoptic NEP Anterior hypothalamic NEP

E

N

P

P o

P

lay er

m

ifor

P

a l pl e x

h

Neo

T

T E L E N C E ri m o r d i

CTF2 (cells)

Migrating pontine neurons






Migrating olfactory neurons Migrating basal telencephalic neurons

Inferior collicular

Isthmal NEP

Migrating subthalamic neurons Trochlear (IV) nucleus?

Bed nucleus of the stria terminalis


t

N

Substantia nigra? Oculomotor (III) nuclear complex? Interpeduncular nucleus?

t h

Globus pallidus?

(

P

Olfactory Basal cortical telenNEP cephalic NEP

A

N E

Insular?

l i c

a l ) c t

Sub

Strionuclear NEP

Medial ganglionic NEP

on s

H

O


Limbic cortical NEP

e p h a Superior

ig

P

c

collicular

Reticular complex

Fornical GEP?

E

cephalic (te e n egme gm n t al gt es neu M ratin r

Dorsal complex

Hippocampal

n

e

N

P Retrosplenial?

Posterior complex Epithalamic NEP

C

L

l

E

e


EP l) N

N

E

P

M e

ta

L

E

ON

en

O

C

P

L HA

PLATE 66B

N

s


M E S E


Ventralrhombic Lower rhombiclip lip

Solitary tract?

Gracile and cuneate nuclei?



146

PLATE 67A GW6.5 Sagittal, CR 15.0 mm C9247, Level 4: Slide 21 l Section 8 kul ds an

u

N AI

thalamic pool

e

u

ct

)

en


tr

icle

subthalamic pool

Nerve III (oculomotor) Pia

v


d

ns

r

Future dura?

-de

kin es ur

IS FR OM LEF TS ID EO F p h BR e a c l n i c se su (futur me e p aq e

ll Ce

ut ef

ENTIRE SECTION

dorsal pool



ventral pool Nerve III (oculomotor) Nerve I (olfactory)

metencephalic pool

Olfactory Olfactory epithelium epithelium


Frontonasal process optic recess


Lateral Lateral nasal process

Sphenoid bone

Nasal cavity Carotid artery?

Ciliary and otic ganglia? Otic vesicle

Maxillary Maxillary process

ye le nc ha

e ? g u

ep

vity al ca

c li po

ol

Inferior vagal ganglion (X)

n

Meckel's cartilage

Or

o


m

T

Mandibular Mandibular arch

Medullary velum


Medullary velum

Inferior glossopharyngeal ganglion (IX) Arterial trunk Basal occipital bone Cell-sparse superarachnoid reticulum Spinal nerves

Dorsal root ganglia

See the following for higher magnification views of this and nearby sections. Plates 74A and B: mesencephalic tegmentum Plates 81A and B: pons and upper medulla

147

CTF2 (cells)

r Ce

e

ia

lc

l be

e

la

rN

l

(h

EP

t La

er

a

ch

em

i

e lc

h sp

b re

e) er

e

lla

Up r pe

M

e

u

l

l

a r

Glossopharyngeal receptor neurons (IX)?

y

we Lo r

C

Vagal sensory neurons (X)?

ch

N

Upper Dorsal rhombic lip

M

d

Vestibular nuclear neurons (VIII)?

ot

O

alami c Th

EN

DI

Central auditory neurons (VIII)?

rn

H

M

ed

el

ot

C E P H A L O B E N

P e

b re

n ar


Facial motor (VII) neurons? Sprouting facial (VII) nerve? Facial sensory neurons (VII)?

Neurons migrating from the remnants of rhombomeric NEPS

t

i

n

P o

le x P N e o iform H c o lay r t er ic a

T E L E N C E P ri m o r di a l p

P

Reticular formation formation?

E

N

n

Medial lemniscus?

Anterior hypothalamic NEP Migrating hypothalamic neurons

CTF1 (fibers)


Medial forebrain bundle? Lateral Reticular hypoformation? formation thalamus

Migrating preoptic neurons Preoptic NEP

lem

Parabrachial nucleus?

Bed nucleus of the stria terminalis

Brain surface (heavier line) Migrating olfactory neurons Migrating basal telencephalic neurons Migrating basal ganglionic neurons

l

ra

te

La

? us

sc

ni


NEP


Olfactory cortical Basal NEP? telencephalic NEP


Isthmal Inferior collicular NEP


Lateral ganglionic NEP

N

Red nucleus?

Strionuclear NEP


al) ct

Insular

NE

Subthalamic NEP

Limbic Hippocampal GEP? cortical (fornix) NEP

P

H nc A ep ha L li Superior c (t collicular e

Retrosplenial?

ic

E

e

P

S ub t h

EP

m a la

s

C

NE

N

Ventral complex

ncepha lic (teg se m (I M e lomotor II) nuclei e n ? ta u tegmen Oc rating t al ne ig ur M o

Epithalamic NEP

ns

l

N


EP N Posterior complex

Central complex?

e

N

l)

A

L

O

E

A

M

O


PH

N LO

PLATE 67B

M E S E

N E

R



P

Solitary tract? Ventralrhombic Lower rhombiclip lip Solitary nucleus Posterior intramural migratory stream (inferior olive neurons)? Solitary tract



148

PLATE 68A GW6.5 Sagittal, CR 15.0 mm, C9247 Level 5: Slide 19, Section 8 Future

Pia

dura?

ll Ce

-de

ns

ef

u

ll sku nd na i k es tur

RAAIINN FBBR OOF DEE SIID


EENNTTIR IREESS EECCT TIO IONN ISISF FRRO OMM RLI EGFH TT m S su es (f pe en ut r c ur v ep e en h aq t a ue r li du ic c ct le ) diencephalic superventricle (future third ventricle)

dorsal pool




ventral pool

Sphenoid bone

metencephalic pool

Carotid artery

Lateral nasal process

Facial (VII)? and vestibulocochlear (VIII) ganglia

Nerves VII and VIII boundary caps*

m ye le nc ep ha li c

Oral cavity

Otic vesicle

Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal) Nerve X (vagus)

Mandibular arch (I)


Trigeminal ganglion (V)?

Nerve II (optic)

Maxillary process


Frontonasal process

Meckel's cartilage

Medullary velum

po ol

Nerve IX


Arterial trunk

Vertebral artery?

Nerve X

Squamous occipital bone


Vagal (X) boundary cap* Superior vagal ganglion (X)

* Boundary caps are

Schwann cell GEPs?


Thal

CE DIEN

ating thala Migr m

Hippocampal? Posterior ganglionic/ amygdaloid NEP


CTF3 (cells and fibers) CTF2 (cells)


CTF1 (fibers) Lateral lemniscus?

Migrating amygdaloid neurons?



N

P o n



Facial sensory neurons (VII)?

n

b (h ell e

i

re

t

P

Ce

GEP? (optic nerve)

e

E

Upper rhombic lip

Undulations in the NEP surface are remnants of the rhombomeres more prominent in less mature specimens.

Up r pe


M e

d

ul la


ry

Low er

NE

P

The tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

y ar lit t ac tr

Vagal sensory neurons (X)?

Neurons migrating from the remnants of rhombomeric NEPs

Lower rhombic lip

So

Glossopharyngeal receptor neurons (IX)?

R H O M B E N C E P H A L O N

pl e

Lateral lemniscus and brachium of inferior colliculus?

ce M re e d b ia no e lla l tc r h m ar is p N he E P re ) ce La re te b e ra l no lla tc r h

A

H P

N e xifor o c m la o y r

o r dial

N

N

T E L E N C E P ri m

A

li c

Inferior collicular


Ventral complex



a

tal) ec

Insular Olfactory cortical NEP?

ep ha lic (te l ne gm ur on s

h

H

O

ating tegme Migr nt a

Retrosplenial?

Limbic cortical NEP

Superior collicular

P

(t

a

P N E

Me

E

L

c er t i

l

P?

C

es en ce p

EP )N tal en

N

P

O

E


Posterior Pre H complex P ns o r Epineu ic m i c N thalamic a NEP?

E al N tect

M

EP

A

N LO

M E S E N

nc se


L

PLATE 68B

Posterior commissure?

Solitary nucleus


Precerebellar NEP?

150

PLATE 69A GW6.5 Sagittal, CR 15.0 mm, C9247 Level 6: Slide 18, Section 8

ENTIRE SEC TIO NI SF RO M

Future dura?

ns

ef

u tu

S IN BRA OF

Ce

e ll- d

nd na ski e r

ll sku

LE FT

E ID


m su es (f pe en ut r c ur v ep e en h aq t a ue r li du ic c ct le )

dorsal pool



Pia

ventral pool

Sphenoid bone


Nerves VII and VIII boundary caps*

Maxillary Maxillary process

m ye le nc ep ha li c

Facial (VII)? and vestibulocochlear (VIII) ganglia Mandibular arch (I) Mandibular arch

Oral cavity Nerve VIII Otic vesicle

Meckel's cartilage


X eI rv e N

po ol

Medullary velum

Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal)

Superior vagal ganglion (X)

* Boundary caps are


Nerve II (optic)

metencephalic pool

Trigeminal ganglion (V) Nerve V


Carotid artery

Frontonasal process


Schwann cell GEPs? Vertebral artery?

Arterial trunk

Cell-sparse superarachnoid reticulum Squamous occipital bone

Vagal (X) boundary cap* Nerve X (vagal)

See the following for higher magnification views of this and nearby sections. Plates 76A and B: cerebellum Plates 77A and B: nerve V entry zone Plates 80A and B: nerves IX and X entry zones

151

PLATE 69B


D I ENCE

Lateral lemniscus and brachium of inferior colliculus?

Hippocampal? Posterior ganglionic/ amygdaloid NEP



CTF3 (cells and fibers) CTF2 (cells)


N

P

o

P

i n t



M

r

ll ed u

pe

Facial sensory neurons (VII)? Central auditory neurons (VIII)?

Up

Lateral lemniscus?


ry

a


E

e

Mesencephalic nucleus (V)

n

GEP? (optic nerve)

CTF1 (fibers)

Lateral lemniscus?

NE

P

Low er

Lower rhombic lip Precerebellar NEP?

Glossopharyngeal receptor neurons (IX)? Vagal sensory neurons (X)?

Neurons migrating from the remnants of rhombomeric NEPs

R H O M B E N C E P H A L O N

o



Retrosplenial?

M ed ia lc er C eb er no ell eb tc ar (h e h em lla is r N ph E La er P te e) ra lc er Up eb pe n el rr ho otc lar h m bi c lip

O

N e lay

L e xi

for

m

A a l pl

P ri m o r di

P


The tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

Inferior collicular

Migrating tegmental neurons? Mesencephalic (tegmental) NEP?

T

E N C E P E L H

Thalamic NEP (posterior complex)

Migrating amygdaloid neurons?


(

EP


li c

N

Insular

Superior collicular

H

N

Limbic cortical NEP

a

P

O

o

NE

h

E

al) ct

c

al

ng t

C

te

er

r

c ti

r at i

c ne ami hal

ns u ro

Mese nc ep

L

Mig

Labeled on this page: Central neural structures N

P

HA

Migrating superior collicular neurons?

A


N LO

M E S E N

Solitary tract Solitary nucleus?


152

PLATE 70A GW6.5 Sagittal, CR 15.0 mm, C9247 Level 7: Slide 16, Section 8 Labeled on this page: Peripheral neural and non-neural structures, brain ventricular divisions

Future dura?

LE FT

S

E ID

ll-

se den

f ut

u re

IN BRA OF

Ce

ENTIRE SEC TIO NI SF RO M

ll sku and n i sk


dorsal pool


ventral pool

Nerve V (opthalmic branch)

Pia Carotid artery

Trigeminal boundary cap* Trigeminal ganglion (V) Nerve V

EYE Pigment epithelium Intraretinal space Retinal NEP Vitreous body Sclera Pioneer retinal ganglion cells Eyelid

metencephalic pool


Medullary velum


Fused Fused maxillary maxillaryprocess process and mandibular and mandibular archarch (I)

myelencephalic pool

Nerve Nerve V (maxillary V (maxillaryand and mandibularbranches) branches) mandibular

Vestibulocochlear (VIII) ganglion

Otic vesicle

HyoidHyoid arch arch (II)

Nerve V (trigeminal) Nerve VIII (vestibulocochlear)

Nerve VIII

Nerve VIII boundary cap*

ArchesIII IIIand and IV? IV? Arches Vertebral artery?

Arterial trunk



* Boundary caps are

Schwann cell GEPs?

us ? m o ne ua bo S q o ra l p tem

See the following for higher magnification views of nearby sections. Plates 78A and B: cerebellum, pons, and medulla Plates 79A and B: nerves V and VIII entry zones

153

PLATE 70B


M E S E N


C

E

Migrating superior collicular neurons?

P

H

A

L O

N

Labeled on this page: Central neural structures Mesencephalic (superior collicular) NEP

Prim ord i

al pl e

x

rm ifo

l tica er lay c o r N

NEP Brain surface (heavier line)

eo



CTF1 (fibers) CTF2 (cells)

T

E L E N C

E

P

H

O N

L

A


Premigratory deep nuclear neurons and Purkinje cells sequestered in the cerebellar NEP NEP? ? Migrating Cajal-Retzius cells

The pontine NEP and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease. Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

EP

Peripheral VIII nerve (abundant glia)

A L O N

Medullary NEP

Vestibulo-auditory neurons Lateral lemniscus? (devoid of glia)

H


Vestibuloauditory neurons

Lower rhombic lip Cochlear NEP?


O

Glial channels in Retinal NEP?

M B E N C E P

Peripheral nerve V (abundant glia)

P ontine N

The telencephalic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase.

Central trigeminal tract (devoid of glia)

H

Retinal NEP

re) he i sp m he Lateral P( cerebellar NE llar notch e reb e Upper C rhombic lip Medial cerebellar notch

Trigeminal nuclear complex

R


GW6.5 Sagittal, CR 15.0 mm, C9247 Level 3: Slide 23, Section 8 See Level 3 in Plates 66A and B.

DORSAL NEOCORTEX

154

PLATE 71A

PLATE 71B Cell-dense future skin, skull, and dura Cell-sparse superarachnoid reticulum Brain surface (heavier line) Earliest settling Cajal-Retzius cells Migrating Cajal-Retzius cells

Blood islands outside pia Pia

Synthetic zone

Mitotic NEP cells predominate at ventricular border NEP - neuroepithelium The cortical NEP is in the "stockbuilding" phase when neural stem cells are increasing while few neurons (mainly Cajal-Retzius cells) are being generated.


Mitotic zone

NEP cell secretions empty into ventricle

Pseudostratified cortical NEP


Terminal bars of NEP cells




155

GW6.5 Sagittal CR 15.0 mm C9247 Between Levels 1 and 2: Slide 26 Section 9 See Level 1 in Plates 64A and B; Level 2 in Plates 65A and B.

HIPPOCAMPUS AND THALAMUS

156

PLATE 72A

PLATE 72B

Pseudostratified thalamic NEP

Synthetic zone


Terminal bars and secretory end feet of NEP cells


Cell-sparse future skull and skin

Mitotic zone


Future dura and dural blood vessels Blood islands near pia

Dorsal complex

Pioneer migrating thalamic neurons

Pia

at brain surface (heavier line)

Primordial plexiform layer Mitotic NEP cells

Synthetic zone Mitotic zone

Pseudostratified neocortical NEP

Thalamic NEP

Retrosplenial? Subicular?

Neocortical NEP Terminal bars and secretory end feet of NEP cells

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Primordial plexiform layer narrows over choroid plexus stem cells.

Ammon's horn Fornical GEP?

Limbic cortical NEP


"Budding" telencephalic choroic plexus Choroid plexus stem cells

Anterior complex



The vascular bed of choroid plexus is continuous with superarachnoid reticulum.

157

GW6.5 Sagittal, CR 15.0 mm, C9247 Level 1: Slide 27, Section 14

A. Anterior hypothalamus and infundibular recess

B. Posterior hypothalamus and mammillary recess

HYPOTHALAMUS See Level 1 in Plates 64A and B.

158

PLATE 73A

PLATE 73B

A.


Preoptic NEP

floor of diencephalic superventricle

Anterior optic recess hypothalamic Anterobasal NEP

Optic chiasmal GEP?

nuclear NEP?

Middle hypothalamic NEP

infundibular recess


d an , ) g l lo b e y s is h ry ta rio r p o p i ti u ste hy P o ro ( p eu n

Migrating anterior hypothalamic neurons Pituitary gland (anterior lobe, adenohypophysis)

Arcuate nuclear NEP?


ju n m ct es io en n M c be es en eph tw a ee ce ph lic n d su ie al pe nc ic rv ep te O g en ha c u m co l tr li o e m m nt ic c a pl o l n al ex to NE es d NE r (I P? II) P

Lamina terminalis

Pituicyte GEP

Rathke's pouch epithelium Rathke's pouch lumen

Interpeduncular nuclear NEP?

B. flo

or o f die nce

Posterior hypothalamic NEP (mammillary)

phal

ic su per

vent

ricle

mammillary recess

Middle hypothalamic NEP Brain surface (heavier line)

Migrating mammillary nuclear neurons?

Migrating tegmental neurons Migrating interpeduncular nuclear neurons?

159

160

PLATE 74A GW6.5 Sagittal CR 15.0 mm, C9247

MESENCEPHALIC TEGMENTUM

A.

Near Level 1: Slide 27 Section 7

B.

Level 4: Slide 21 Section 8

See level 1 in Plates 64A and B; level 4 in Plates 67A and B.

161

PLATE 74B

FONT KEY: Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

NEP - neuroepithelium

A.

c

p

t

M

e

P

Anterior

or

s Me

g tin tor u ro mo ? Sp ulo ons oc ax

2

Variously oriented layers of migrating tegmental neurons

en

presumed direction of axon growth in brain fiber tracts.

c tegmental NEP hali p ce

1

3

1

Oculomotor axons


Posterior

Oculomotor (III) neurons migrate only a short distance from NEP

ri

l

1. Predominantly radially radially-oriented oriented cells migrating parallel to the section plane. 2. Predominantly horizontally horizontallyoriented cells migrating perpendicular to the section Superficial layer plane. of predominantly horizontallylongitudinal fibers 3. Predominantly horizontally oriented cells migrating parallel to the section plane. Brain surface Arrows indicate the Arrows indicate the (heavier line)

B.

te

a

(future aqueduct)

Variously oriented layers of migrating tegmental neurons

presumed direction of neuron migration in the brain parenchyma.

An

lumen of the mesencephalic superventricle

E

2

s

e

1

t e g m e n

N

n

e

h

i c a l

Pioneer fibers of the medial longitudinal fasciculus?

Po

st

er

io

r

2

Oculomotor nerve roots

Superficial layer of predominantly longitudinal fibers Nerve III (oculomotor)

Deep layer of predominantly longitudinal fibers

162

PLATE 75A GW6.5 Sagittal CR 15.0 mm C9247 Near Level 3: Slide 24, Section 8 See Level 3 in Plates 66A and B.

MESENCEPHALIC TECTUM, ISTHMUS, AND CEREBELLUM

163

M

Migrating neurons surrounded by a fiber tract

PLATE 75B

Pi

on

e


ee

s n

io r

c

co

eurons ular n ollic or c eri ( up i c gs tin a l ra h ig p rm e

e

r pe Su

ABBREVIATIONS: NEP - Neuroepithelium CTF - Cerebellar transitional field Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

l li c

EP rN ula


Arrows indicate the presumed direction of axon growth in brain fiber tracts.

Cell-dense future skin, skull, and dura

t e c t a

l Pia with overlying blood islands

)

Inf e r i or c o lli c


N Pioneer migrating inferior collicular neurons (no surrounding fiber tract)

E

P NE ar ul

P Tr

oc

h

Is

le a rn

th m

Sprouting trochlear nerve fibers?

? u cl e a r N E P

al NEP

Trochlear (IV) nucleus?


CTF1 (fibers) CTF2 (cells) CTF3 (cells and fibers)


164

PLATE 76A

A.

B.

GW6.5 Sagittal, CR 15.0 mm, C9247 CEREBELLUM Level 1: Slide 27, Section 14 See Level 1 in Plate 64A and B.

Near Level 6: Slide 18, Section 5 See Level 6 in Plate 69A and B.

165

PLATE 76B

A.

Pioneer migrating inferior colliculus neurons

CTF1 (fibers) CTF2 (deep nuclear neurons)

Future fastigial nucleus?


Nuclear transitory zone

Sprouting trochlear nerve fibers? Migrating trochlear nuclear neurons?

r NEP (verm a l l e is eb ) r e C

Isthma

Nerve IV (decussation, trochlear)

Mitotic NEP cells rhombencephalic superventricle

l

N

EP Spinocerebellar and vestibulocerebellar fibers?

CT CTF

(fibers)

2 (deep nucle

C T F 3 ( d e ep n u c l


ere

bella

r

P E N

s

e ph

(h

e

re)

Lateral cerebellar notch Upper rhombic lip


ABBREVIATIONS: NEP - Neuroepithelium CTF - Cerebellar transitional field FONT KEY: Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Nuclear transitory zone

bers) and fi s n o eur ear n

Premigratory deep nuclear neurons and Purkinje cells sequestered in superficial cerebellar NEP?

C

ons) ar neur

i

Future dentate nucleus?

F1

Medullary velum

m

B.

Upper rhombic lip

Medullary velum Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.


166

PLATE 77A

TRIGEMINAL NERVE ENTRY ZONE

GW6.5 Sagittal CR 15.0 mm C9247 Near Level 6: Slide 19, Section 3

See Level 5 in Plate 68A and B; Level 6 in Plate 69A and B.

167

PLATE 77B Pontine NEP (trigeminal NEP)


Trigeminal nuclear complex (migrating and settling neurons)

Mesencephalic nucleus? Inward migrating neurons of the mesencephalic (V) nucleus

Penetrating trigeminal fibers

Ne

Penetrating trigeminal fibers

V

Nerve V

rve

Nerve V boundary cap

Nerve V

Trigeminal NEP

M igr

n P o

(source of all central trigeminal nuclei except mesencephalic)

a ti

t

ng

t ri

g

em

in

al

Me sen nuc ceph leu alic s?


nu

cle ar

plex n

euro n s

Central trigeminal fibers penetrate brain

Mesencephalic nuclear neurons migrate into the brain from the periphery FONT KEY: Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Auditory-vestibular NEP d u ll a r er me p Up

P N E

i n e com


y NE

P

Pioneer fibers of medial longitudinal fasciculus?

M igr

a ti n g c entral auditory neurons? t a L era Nucleus of the l l e m lateral lemniscus? Nucleus of the ni lateral lemniscus? sc us ?

Nerve V boundary cap (Schwann cell GEP?) Nerve V (trigeminal) Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.




168

PLATE 78A

GW6.5 Sagittal, CR 15.0 mm, C9247 Near Level 7: Slide 16, Section 3 LATERAL CEREBELLUM, PONS, AND MEDULLA


169

PLATE 78B


CT

Arrows indicate the presumed direction of axon growth in nerves and fiber tracts.

P

atin re m cl lls, fir ) g i s g r P atory dee p nu TF4 (ce r C fi be F2 in je fie d s) i c t e p l l s ( ?) n c e C e T d F 3 l i ( d lum e l e p s n n , u u c f l e i a r r ct c s n ) t e w u n r a ons intermi l ed with ve of n? i s ti n g m d li o i C g r T a F t nto 1 a ng i i n ( f g i b rg ng e i d t r ee p ula s ega s ti b n s e gr u s c n l e o r a u r d ve e n d an l cor y af f e r e n t s f r o m s pin a


ABBREVIATIONS: NEP - Neuroepithelium CTF - Cerebellar transitional field

llar

ai ,m

r

metencephalic pool

NEP

(he


q u es

mis


r ella re b

k ur

?) P? lls ce NE je kin ur (P F4 CT

Auditory-vestibular NEP Trigeminal NEP

Po at in

n ti

ig

gt rig

e mi

n al n

e Upp

ne NE P

r

Incoming trigeminal nerve fibers

Lower rhombic lip

Auditory (cochlear) NEP?

M

Pioneer fibers of the descending (spinal) trigeminal tract

myelencephalic pool


re)

ce the

phe

d) in t e re


euro ns?

Central trigeminal tract with no interstitial glia

rm

M i g r at

Nerve V filled with Schwann cells

Nerve VIII filled with Schwann cells


n ar bul esti ing v

ry neurons? ito ? ud cus a l nis tr a em n l e c al M igrating te r s? f la cu Nucle u s o nis m e l al L ater

Nerve VIII boundary cap (Schwann cell GEP?)

Nerve V boundary cap (Schwann cell GEP?)

yN llar edu o

EP

?

ebe

st

and Purkinje c (se e l l s so j o u r n i n g rons neu wave of mig

Medullary velum

ns

Cer

e ar

nl

Nuclear transitory zones

Lateral Upper cerebellar rhombic lip notch

eu

r

Migrating cochlear nuclear neurons

No interstitial glia in central fiber tracts

Otic vesicle

Lumen

Epithelium Vestibulocochlear ganglion (VIII)

VIII ganglion neurons migrating from germinal source in otic epithelium?


TRIGEMINAL AND VESTIBULOCOCHLEAR NERVE ENTRY ZONES

170

PLATE 79A GW6.5 Sagittal, CR 15.0 mm, C9247 Near Level 7: Slide 16, Section 13

PLATE 79B


M ig ra tin g

Pontin e an

ds

e tt l

in g

tri g e


neuroepithelium (NEP)

n euro ns?

s? euron Migrating and settling auditory n

Spinal tract (V)?

at gr

in

g

lea nuc r lea ch o c

rn

on eur

s?

M i

Central trigeminal tract with no interstitial glia

m in al

Mitotic cells

Incoming trigeminal nerve fibers

Lateral lemniscus? (no interstitial glia)

Nerve V boundary cap (Schwann cell glioepithelium, GEP)

Incoming VIII nerve fibers

Nerve VIII boundary cap (Schwann cell GEP)

Nerve V (trigeminal) filled with Schwann cells filled with Schwann cells Nerve VIII (vestibulocochlear) filled with Schwann cells Arrows indicate the presumed direction of axon growth in nerves and fiber tracts.


Otic vesicle epithelium

Vestibulocochlear ganglion (VIII)

Arrows indicate the presumed direction of neuron migration from germinal sources. FONT KEY: Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

171

172

PLATE 80A GW6.5 Sagittal, CR 15.0 mm, C9247 Near Level 6: Slide 18, Section 13

A.

Nerve IX

B.

Nerve X


ENTRY ZONES OF NERVES IX AND X

PLATE 80B

A.

Migrating vestibular nuclear neurons? Migrating central auditory neurons?

To solitary tract? To other tracts Bifurcating glossopharyngeal (IX) afferents Glossopharyngeal (IX) afferents penetrate brain.

Nucleus of the lateral lemniscus? Lateral lemniscus?

Interstitial glia absent in central fiber tracts

Nerve IX boundary cap (Schwann cell glioepithelium?) Nerve VIII Nerve VIII (vestibulocochlear)


Peripheral blood vessels Vestibulocochlear (VIII) ganglion

Copious Schwann cell cords

B.

Migrating sensory vagal neurons? Solitary nucleus?

Solitary tract?

(glossopharyngeal [IX] and vagal [X] afferents)

Solitary

nucleus

?

Vagal axons join solitary tract?

Peripheral blood vessels

Nerve X Vagal (X) afferents penetrate brain at multiple sites

FONT KEY: Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the presumed direction of axon growth in nerves and fiber tracts.

Vagal axons disperse among migrating vagal sensory neurons and solitary nuclear neurons?

Copious Schwann cell cords Nerve X (vagus)

Copious Schwann cell cords

173


MEDIAL PONS AND MEDULLA

174


PLATE ?B 81B


Mitotic cells near ventricular lumen

Pr

ed

Re

ti c

om

ul

ina

ar

ntly

P

reti cu la

rN EP ?

n


t i n

A bd

u ce

e

Migra ting a EP bduc (V ens I ne ur

)?

s? on

Migrating reticular neurons?

m U p p e ro f r


E

ns nu cle ar N

N

e?




o

neur on s ojo ur nz on

Arrows indicate the presumed direction of neuron migration from germinal sources.

Fa

m h o

Vestibular nuclear complex neurons (VIII)?

P cia

lm

otor nuclear N

( EP

S ?

R e m n a n t s

V II

)?

Ventrolaterally migrating facial motor neurons?

Sprouting nerve VI (abducens)? Reticular formation?

y NE P lar l N E P u r i c ed bome

Facial sensory neurons (VII)?


Sprouting VII nerve fibers segregate into bundles? Reticular formation?

Medial lemniscus intermingled with other unidentified fiber tracts?

Nerve VII facial genu

Lateral lemniscus?

neur ons in futur f uturee Settling neurons nuclei of the lateral lemnis cus?

175


A. ISTHMUS

B. UPPER PONS


NEUROEPITHELIUM AND MIDLINE RAPHE GLIAL STRUCTURE

176

PLATE 82A

PLATE 82B

A.

isthmal canal

I

s

t

h

m

a

l

N

E

P

sthmal NEP?) in ist (sequestered s n o r u al ne I sth m

EP? ine raphe G l d i m ith led w g n hmal neurons and morphocyte t i s i g m n i r s? journ i n te a n d so g P n E i t N a l Isthma Migr he glial struc Cell body laye

ture morphocyt es

rap r of midline

des (thin relative to pons) Midline raphe glial fiber palisa Scattered cell bodies of another type of glia migrating among the fibrous palisades (less dense than Pons) pons)

B. Arrows indicate the presumed direction of cell migration from germinal sources. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Pontine floor plate


Cell body layer of midline raphe g lial structu re

morphoc yt e s

Midline raphe glia l fiber pali sades (thic k rel

ative to is thmus)

Scattered cell bodies of another type of glia migrating among the fibrous palisades (more dense than isthmus)

Dense clumps of migrating glia line up beneath morphocyte layer.

177

GW6.5 Sagittal, CR 15.0 mm, C9247 Level 1: Slide 27, Section 14 NEAR PONTINE FLEXURE



178

PLATE 83A

PLATE 83B MORPHOCYTES are specialized glia that produce fibrous palisades in the region of the brain flexures and are the chief cell type in the midline raphe glial structure, first described in rats by Van Hartesveldt et al. (1986). See Volume 4, Plates 221A and B on pages 556-557 (Bayer and Altman, 2006). These fibers may provide structural stability as the brain curves during its rapid growth within the confines of the developing meninges, bone, and skin.

P Pontine NE


Mitotic cells

led with midline raphe GEP? g n i m r inte

w it ns or glia?) o r Tangentially mig u e n rating cells (

hin the midline raphe glial

s tru

cture

structure raphe glial e n i l d i m f ocytes) o C ell b o d y layer ( morph

lisades Thick midl ine raphe glial fiber pa Scattered cell bodies of another type of glia migrating among the fibrous palisades (note density increase posteriorly) FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


Arrows indicate the presumed direction of cell migration from germinal sources.

Dense clumps of radially migrating glia invade fibrous palisades.

179

GW6.5 Sagittal, CR 15.0 mm, C9247 Level 1: Slide 27, Section 14 MEDULLA (slightly anterior to medullary flexure)



180

PLATE 84A

PLATE 84B End feet of NEP cells protrude into the floor of the rhombencephalic superventricle

Less dense deep NEP (tangentially cut in two places)

Med

NEP intermingled ullary

M it

with

midl

More dense superficial NEP

The morphocytes in the lower medulla differ in two ways from others. First, they are less densely packed. Second, the large cells migrating downward may be morphocytes themselves rather than the smaller cells migrating in the pons and isthmus (see Plates 82 and 83).


ine

raph


zo n

e

e G EP ?

orphocytes) of midline raphe glia m ( r e y a l y l struc C e ll b o d ture d isper sed am on g

Thick midline raphe glial fiber pa lisade s

o tic

Syn

the

th e

tic

zo n

e

fib ers

Arrows indicate the presumed direction of cell migration from germinal sources.

181

182

PART PARTVII: VII: GW5.5 GW5.5 CORONAL CORONAL This specimen is embryo #1000 in the Minot Collection, designated here as M1000. The crown-rump length (CR) is 10 mm estimated to be at gestational week (GW) 5.5. Most of M1000’s forebrain and midbrain sections are cut (10 µm) in the coronal plane, but the plane shifts to predominantly horizontal in the posterior midbrain, pons, and medulla. We photographed 64 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and medulla. Fourteen of these sections are illustrated in Plates 85AB to 98AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of M1000’s brain and eye (Figure 6), and to show each illustrated section in situ (insets, Plates 85A to 98A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. Plates 99AB show high-magnification views of the telencephalic neuroepithelium. All parts of the telencephalic neuroepithelium are rapidly increasing their pool of neuronal and glial stem cells as they expand the shorelines of the enlarging telencephalic superventricle. However, the entire telencephalon is much smaller than in the GW6.5 specimens (Parts V and VI). The primordial plexiform layer adjacent to the cerebral cortical neuroepithelium is nearly devoid of cells except in far ventrolateral areas. The basal ganglionic and basal telencephalic neuroepithelia have only a thin layer of adjacent migrating neurons, many fewer than in the GW6.5 specimens. The posterior olfactory epithelium has only partially invaginated into a developing nasal cavity, while the anterior epithelium is a placode in the anterolateral head. Still, cellular densities outside the placode and invaginated epithelium may be supporting cells surrounding the first olfactory nerve fibers. The diencephalic neuroepithelium surrounds a superventricle that will narrow (shrinking shorelines) by GW6.5 in the preoptic, hypothalamic, and subthalamic areas. It is postulated that the superficial parts of the anterior hypothalamic and subthalamic neuroepithelia contain premigratory, postmitotic neurons that are sequestered there. More posteriorly, these neuroepithelia are surrounded by sequential waves of migrating neurons. In contrast, the thalamic neuroepithelium is in the “stockbuilding” stage, increas-

ing its population of neuronal and glial stem cells as the thalamic pool of the diencephalic superventricle expands. The eye is clearly connected to the ventral diencephalon by a thick, short stalk that is the glioepithelium of the future optic nerve. As in the more mature specimens, the retinal neuroepithelium is clearly differentiated from the pigment epithelium. The mesencephalon contains a stockbuilding neuroepithelium in the pretectum and tectum (virtually no adjacent migrating neurons). The tegmental and isthmal neuroepithelia are thick, but their population of stem cells begins to decrease as massive waves of migrating neurons leave. The subpial fiber band is considerably thinner than in the GW6.5 specimens. Both the pons and medulla have neuroepithelia that are thicker than at GW6.5, but are nevertheless shrinking as they unloaded their neuronal and glial progeny into an expanding parenchyma. In lateral areas, the neuroepithelium forms crescent-shaped evaginagions, the rhombomeres. The rhombomeres are associated with the entry zones of sensory cranial nerves V, VII, VIII, IX, and X; they will be the most distinctive features of the thombencephalon for the remainder of brain development. Cells are migrating and settling in longitudinal arrays at the pontine flexure. Most regions of the pons and medulla are characterized by large bands of migrating neurons with few nuclear divisions. A few cells are settling in the barely recognizable superior olivary complex and many are settling in the reticular formation throughout the pons and medulla. Some facial motor neurons are migrating from medial to lateral, leaving behind their axons in a small, but definite genu of the facial motor nerve. Migrating inferior olive neurons are in the posterior intramural migratory stream outside the precerebellar neuroepithelium in the posterior lower rhombic lip, but no neurons have settled in the inferior olivary complex. The solitary nucleus and tract cannot be identified, although solitary nuclear neurons are undoubtedly migrating outside the rhombomere 6 medullary neuroepithelium. The subpial fiber band is thick in the pons and medulla, especially at the entry points of the sensory nerves. The cerebellar neuroepithelium is much smaller than at GW6.5. Some early-generated deep nuclear neurons are migrating in the cellular layers of the cerebellar transitional field, but these and the fibrous layers are thinner and less definite than at GW6.5.

183

M1000 Computer-aided 3-D Brain Reconstructions B.

Angled front view

Pretectum

Side view am

u

T

s

Th

y p s o th al a m u

alamu

s o n

ce

Medullary velum

Rhombomere 2 Rhombomere 5

pe

m

r

BRAINSTEM FLEXURES

ulla

ed

er

w

we

rm

edu

1. Medullary lla

u ll

a

Lo

Lo

med

Upper rhombic lip

Up

p er

m ebellu Cer

P

a

ha i lona Su bth

Mammillary body

s

n

Inferior colliculus

2

Eye

s

Up

Eye

o

H

Infundibulum

n

P Preoptic area

3

Preoptic area

ph

g

Cereb

n gl S ia u bthal alo amu n s Hy po T thalam u s e

T

C

um

l co bra rt re

ll

u

n s a tele a l B B asa

4

th mu

be

Bas a l g a n g l Ba sal telen c e p

o

e

x

Is

re

m

s

P

corte

m tu

Ce

I s t h

Inferior colliculus

l ra

me e g

n

n t u m

s mu

l

al

la

x


g m e

u s

e

m

Occipital pole

us am

ith Ep a

h a l a

Superior colliculus

Epit ha s l

s

Superior colliculus

P o n

Pretectum

s

A.

1

2. Pontine

m

ed

u l la

Lower rhombic lip

3. Mesencephalic Spinal cord

4. Diencephalic

Spinal cord

C.

Top view

Rhombomere 2

Occipital pole Eye

u

C e

p

s

l B a s a

Scale bars = 1 mm

g

a

n

Medullary velum Lower rhombic lip

a C o

m e d u

r

l

l

o

w

o

C

s

m e d u l l a

p

o

us m a t l h a T m e g m e n t u

al

li a

ph

g

Basal telence

H

n

y

Preoptic area

S


Isthmus

Rhombomere 2 Rhombomere 5

Eye

ex

e p t u m

rt

C

Frontal pole

erebral c o

Bottom view

n

e r

D.

o

re

P

e

o r t e x

e r p p

C

U

l

P o n s

a

n s P o

r

be

e

P

E

r b


i n a l

m

Ve r m i s

a

llu m

l

u i t h a l a m

a

L

T h

d


r e t e c t u m

s

ere isph H em

Frontal pole


S

p

184

PLATE 85A GW5.5 Coronal CR 10 mm M1000 Level 1: Section 29

Non-neural structures labeled Interhemispheric fissure



Level 1: Computer-aided 3-D Brain Reconstruction Pia

Olfactory placode Hypothetical olfactory induction field

185

PLATE 85B



Dorsal limbic cortical NEP?


Neocortical NEP? anterior pool



Cell sparse cortical primordial plexiform layer (neurons have not yet migrated from NEP)

Ventral limbic cortical NEP? Telencephalic roof plate

(stem cells of telencephalic choroid plexus?)



Primordial plexiform layer absent in roof plate


186


Peripheral neural and non-neural structures labeled Interhemispheric fissure


See a high magnification view of the telencephalon in a nearby section in Plates 99A and B.



Pia

Nerve I (olfactory)? Olfactory placode


187

PLATE 86B

Central neural structures labeled Telencephalic/diencephalic roof plate (stem cells of choroid plexus?)

TELENCEPHALON

Primordial plexiform layer absent in roof plate

CEREBRAL CORTEX


Dorsomedial limbic cortical NEP?

dorsal pool

Neocortical NEP?


Cell sparse cortical primordial plexiform layer (neurons have not yet migrated from NEP)

future roof of third ventricle

foramen of monro

Ventrolateral limbic cortical NEP? Olfactory cortical NEP?

ventral pool



Septal NEP Pioneer migrating septal and basal telencephalic neurons* Telencephalic floor plate

*Note that the group of basal telencephalic migrating neurons may contain mitral cells heading for the future olfactory evagination.




188





Pia

See a high magnification view of the telencephalon in a nearby section in Plates 99A and B. Naso-optic furrow Nerve I (olfactory)?

Hypothetical olfactory induction field Nostril opening to olfactory invagination and nasal cavity Lateral nasal process Medial nasal process Nasal septum/roof of oral cavity

Placodal epithelium Olfactory epithelium


The GW5.5 Face and Neck

Figure 247C modified (Patten, 1953, p. 429.)

Nasal septum Medial nasal process

Frontal prominence

Naso-optic furrow Eye Lateral nasal process Nostril Mouth Mandible Hyo-mandibular cleft

Maxillary process Mandibular arch (I) Hyoid arch (II) Laryngeal cartilages?

189

PLATE 87B Central neural structures labeled DIENCEPHALON

Diencephalic roof plate

THALAMUS

(primordium of choroid plexus?) Dorsal complex?

Thalamic NEP

Primordial plexiform layer thinner in roof plate Thalamic primordial plexiform layer

Reticular nuclear


TELENCEPHALON


CEREBRAL CORTEX Dorsomedial limbic cortical NEP?

Neocortical NEP? dorsal pool

Ventrolateral limbic cortical NEP?

Primordial plexiform layer thinner in roof plate Telencephalic roof plate

(primordium of choroid plexus?)





foramen of monro

Corticoganglionic NEP Anterolateral ganglionic NEP

ventral pool

Pioneer migrating basal gangliionic neurons



Septal NEP? Pioneer migrating septal and basal telencephalic neurons* Telencephalic/diencephalic floor plate

(lamina terminalis in junction of preoptic area and septum?)

*Note that the group of basal telencephalic migrating neurons may contain mitral cells heading for the future olfactory evagination.




190





Dural blood vessels

See a high magnification view of the telencephalon and diencephalon in a nearby section in Plates 99A and B. Level 4: Computer-aided 3-D Brain Reconstruction

Pia

Naso-optic furrow Eye covering Olfactory epithelium Maxillary process

Nerve I (olfactory)? Nasal cavity

Placodal epithelium Medial nasal process

Nasal septum/roof of oral cavity Hypothetical olfactory induction field

191

PLATE 88B


DIENCEPHALON EPITHALAMUS


(primordium of pineal gland)

Epithalamic NEP

pineal recess

THALAMUS

Posterior complex?


Dorsal complex?

Thalamic NEP


Ventral complex?

thalamic pool

Premigratory thalamic neurons sequestered in thalamic NEP?

Reticular nuclear Anterior complex



subthalamic pool

Neocortical and limbic cortical NEP posterior pool


telencephalic superventricle (future lateral ventricle) foramen of monro

Sequestered and pioneer migrating reticular nuclear neurons? Sequstered and pioneer migrating anterior thalamic neurons? Cortical primordial plexiform layer Premigratory Cajal-Retzius cells and subplate neurons sequestered in cortical NEP?

Posterior ganglionic NEP

BASAL GANGLIA/ AMYGDALA

Amygdaloid NEP? Strionuclear NEP?

preoptic pool

Sequestered and migrating amygdaloid neurons?

Lateral area?

PREOPTIC AREA

Preoptic NEP

Sequestered and migrating posterior basal ganglia neurons?

Sequestered and migrating lateral preoptic area neurons?

Medial area?

Sequestered and migrating medial preoptic area neurons?

DIENCEPHALON

Diencephalic floor plate





192






Dural blood vessels Pia and pial blood vessels Naso-optic furrow

Orbitosphenoid process? Oral cavity

Maxillary process Palatal process Mandibular arch (I)

Nerve II (optic) Pigment epithelium Intraretinal space Retinal NEP Vitreous body Lens Sclera/cornea

Tongue

Eye


Hyo-mandibular cleft Meckel's cartilage in mandibular arch (I)

193

PLATE 89B

Central neural structures labeled MESENCEPHALON PRETECTUM?

Mesencephalic roof plate


Pretectal NEP? Pretectal primordial plexiform layer mesencephalic superventricle


(future aqueduct)

Posterior complex (dosal lateral geniculate)?

Brain surface (heavier line) Thalamic primordial plexiform layer

Ventral complex?

Thalamic NEP

Posterior complex (medial geniculate)?

thalamic pool

Reticular nuclear


SUBTHALAMUS

Subthalamic NEP

subthalamic pool

Retinal NEP

Sequstered and pioneer migrating subthalamic neurons?

Sequestered and pioneer migrating lateral preoptic area neurons?

Lateral area NEP? Medial area NEP?

Sequestered and pioneer migrating reticular nuclear neurons? Subthalamic primordial plexiform layer

PREOPTIC AREA

Preoptic and optic germinal zones


preoptic/hypothalamic pool optic recess

Optic nerve (II) and tract GEP?

HYPOTHALAMUS Migrating anterobasal nuclear neurons?

Anterior hypothalamic NEP?

Glial channels for optic chiasm and tract?




194




Future dura (internal border of brain case) and blood vessels


Pia and pial blood vessels Internal carotid artery

Orbito-sphenoid process?

Naso-optic furrow

Maxillary process Oral cavity Mandibular arch (I)

Hyo-mandibular cleft Pigment epithelium Intraretinal space

Tongue

Retinal NEP Sclera

Eye

Hyoid arch (II) Meckel's cartilage? in mandibular arch Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule) Rathke's pouch (primordium of adenohypophysis)


195

PLATE 90B

Central neural structures labeled MESENCEPHALON PRETECTUM?

Mesencephalic roof plate (posterior commissural GEP?)

Posterior commissure (pioneer fibers)

Pretectal NEP? mesencephalic superventricle



Posterior complex (dosal lateral geniculate)? Ventral complex?

Thalamic NEP

Pretectal primordial plexiform layer

(future aqueduct)

Thalamic primordial plexiform layer thalamic pool



Sequestered and pioneer migrating reticular nuclear neurons


SUBTHALAMUS

Subthalamic primordial plexiform layer

Subthalamic NEP?

subthalamic pool

HYPOTHALAMUS

Hypothalamic NEP


Sequential waves of migrating subthalamic neurons

Sequential waves of migrating lateral hypothalamic area neurons?

Lateral area NEP? hypothalamic pool


Anterior NEP?

Migrating anterior hypothalamic neurons

infundibular recess

Diencephalic floor plate (median eminence?)



196





Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Future dura (internal border of brain case) and blood vessels

Middle cerebral artery?

Ali-sphenoid process? Anterior cardinal vein

Meckel's cartilage?

P

Fused maxillary process and mandibular arch arch(I)

e

al b por tem s ou tr

o ne

Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule) Primordia of sphenoid and basal occipital bones


Trigeminal ganglion (V) Rathke's pouch (primordium of adenohypophysis)

Otic placode


197

PLATE 91B

Central neural structures labeled MESENCEPHALON PRETECTUM/SUPERIOR COLLICULUS?

Mesencephalic roof plate (posterior commissural GEP?)

Posterior commissure (pioneer fibers) Pretectal primordial plexiform layer

Pretectal?


Tectal NEP Superior collicular?



TEGMENTUM

Tegmental NEP

Migrating mesencephalic tegmental neurons

(reticular formation, red nucleus, oculomotor (III) complex?)

SUBTHALAMUS


(future thirdventricle)

subthalamic pool

Subthalamic NEP?

HYPOTHALAMUS Lateral area?

Hypothalamic NEP

Middle?

hypothalamic pool

Arcuate?

Subthalamic primordial plexiform layer Sequential waves of migrating subthalamic neurons

Sequential waves of migrating lateral hypothalamic area neurons? Migrating middle hypothalamic neurons Medial forebrain bundle?

DIENCEPHALON infundibular recess


(median eminence and pituicyte GEP of the neurohypophysis?)



198



Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Pia and pial blood vessels Future dura (internal border of brain case) and blood vessels

Circle of Willis artery? Middle cerebral artery?


Ali-sphenoid process?

Vestibular ganglion (VIII)?

ior c ardinal vein




An

ter


B a s i l a r

Budding spiral ganglion (VIII)?

a r t e r y

Sella turcica?


Inferior vagal ganglion (X)? Inferior glossopharyngeal ganglion (IX)? Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule)


199

PLATE 92B Central neural structures labeled MESENCEPHALON

Mesencephalic roof plate (commissural GEP?)

TECTUM

Commissure of the superior colliculus? (pioneer fibers)


Brain surface (heavier line) mesencephalic superventricle (future aqueduct)


TEGMENTUM

Tegmental NEP

Successive waves of migrating mesencephalic tegmental neurons



Pioneer migrating mammillary neurons

Hypothalamic NEP

(posterior, mammillary)

Hypothalamic NEP

(middle)

HYPOTHALAMUS

DIENCEPHALON

Medial forebrain bundle? Pioneer migrating middle hypothalamic neurons

infundibular recess


(pituicyte GEP in neurohypophysis?)



200

PLATE 93A


GW5.5 Coronal CR 10 mm M1000 Level 9: Section 255



Circle of Willis artery (posterior cerebral)?



Basilar artery

Nerve V boundary cap Nerve V (trigeminal) Trigeminal ganglion (V)

Nerve VIII boundary cap Nerve VIII (vestibulocochlear) Facial ganglion (VII)

Schwann cell GEP in boundary caps? Vestibular ganglion (VIII)


Inferior glossopharyngeal ganglion (IX)?


Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule) Anterior cardinal vein



PLATE 93B


(commissural GEP?)

MESENCEPHALON


TECTUM





TEGMENTUM

Tegmental NEP


DIENCEPHALON

(posterior tip of mammillary body)


PONS

Pontine floor plate

(midline raphe glial structure GEP)

Longitudinal domains of migrating and settling pontine neurons Trigeminal nuclear complex



Medial pontine NEP Pontine floor plate


Central trigeminal tract Migrating raphe nuclear complex neurons?


Lateral lemniscus?

RHOMBENCEPHALON

Medial lemniscus? Medial longitudinal fasciculus? Midline raphe glial structure





202

PLATE 94A


GW5.5 Coronal CR 10 mm M1000 Level 10: Section 269 Pia and pial blood vessels


Cell-dense primordial mesenchymal brain case (skin/bone) Basilar artery Future dura (internal border of brain case) and blood vessels

Nerve V boundary cap Schwann cell GEP in boundary caps? Nerve VIII boundary cap Nerve VIII (vestibulocochlear) Vestibular ganglion (VIII)


Superior glossopharyngeal ganglion (IX)? Superior vagal ganglion (X)?

Vertebral artery Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule)


203 Central neural structures labeled MESENCEPHALON

PLATE 94B

Mesencephalic roof plate (commissural GEP?)

TECTUM Superior collicular NEP

Brain surface (heavier line) Premigratory superior collicular neurons sequestered in the superior collicular NEP?



(future aqueduct)

TEGMENTUM Medial forebrain bundle?

Tegmental NEP



PONS

Medial lemniscus?

Pontine floor plate


Trigeminal motor nucleus (V)? Trigeminal sensory nuclear complex (V)

Medial pontine NEP

Central trigeminal tract


Sequential waves of migrating pontine neurons Premigratory facial motor nuclear (VII) neurons intermingled with abducens (VI) nuclear neurons?




Posterior extension of trigeminal nuclear complex?


Nerve VII genu (facial) interspersed with migrating facial motor neurons? Migrating raphe nuclear complex neurons?



Lateral lemniscus?

MEDULLA

RHOMBENCEPHALON

Superior olivary complex neurons? Medial lemniscus?


Midline raphe glial structure Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.




204


PLATE 95A GW5.5 Coronal CR 10 mm M1000 Level 11: Section 285 Pia and pial blood vessels

Cell-sparse superarachnoid reticulum (brain parenchymal expansion zone) Nerve III sheath (oculomotor)



Nerve VIII boundary cap (Schwann cell GEP?) Spiral ganglion (VIII) budding from otic epithelium?


Superior glossopharyngeal ganglion (IX)?




Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule)


205



(commissural GEP?) Brain surface (heavier line)



PLATE 95B



TEGMENTUM

Tegmental NEP PROPOSED RHOMBOMERE IDENTITIES

Medial forebrain bundle? R2 Successive waves of migrating mesencephalic tegmental neurons

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

R3 R4

PONS


R5

Pontine floor plate


Medial pontine NEP Pontine reticular formation

R2

(trigeminal NEP)

Trigeminal sensory nuclear complex (V) Central trigeminal tract Migrating trigeminal neurons?


Trigeminal sensory nuclear complex (V)


R3

Migrating sensory neurons that will receive input from the facial ganglion (VII)?

(facial sensory NEP?)

Lateral pontine NEP

R4

Migrating auditory and vestibular neurons?

R5

Lateral lemniscus?


Nucleus of the lateral lemniscus?



Migrating raphe nuclear complex neurons? Posterior intramural migratory stream (inferior olive neurons)?



Medial lemniscus?

MEDULLA

RHOMBENCEPHALON


SPINAL CORD ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere


Spinal floor plate

Spinal germinal zones

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.


Ventral funiculus Ventral gray

Ventral NEP

Intermediate NEP

Intermediate gray central canal

Dorsal NEP Spinal roof plate

Lateral funiculus Dorsal funiculus Dorsal gray


206


Peripheral neural and non-neural structures labeled Pia and pial blood vessels




Nerve VIII boundary cap (Schwann cell GEP?) Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule)

Medullary velum



NerveX boundary cap (Schwann cell GEP?) Superior vagal ganglion (X)?

Nerve X (vagis) Nerve XI (spinal accessory)

Dorsal root boundary cap (Schwann cell GEP?)

Cell-sparse superarachnoid reticulum (parenchymal expansion zone)

207



MESENCEPHALON

(commissural GEP?)

Brain surface (heavier line) Superior collicular primordial plexiform layer

TECTUM


PROPOSED RHOMBOMERE IDENTITIES


R4

(future aqueduct)

TEGMENTUM

Tegmental NEP

Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

R5

Migrating mesencephalic tegmental neurons Mesencephalic reticular formation

Isthmal NEP

R6

isthmal canal

ISTHMUS

PLATE 96B


CTF1 (fibers) CTF2 (deep neurons) CTF3 (fibers)

CEREBELLUM

Cerebellar NEP PONS

CTF4 (cells)

metencephalic pool


Lateral pontine NEP

Premigratory deep neurons and Purkinje cells sequestered in the superficial cerebellar NEP?

R2

Metencephalic roof plate


(upper rhombic lip)





R4



R5

Migrating auditory and vestibular neurons?

myelencephalic pool

R6 Migrating solitary nuclear neurons? (glossopharyngeal receptors)


(reticular formation, raphe complex, prepositus, vagal motor [X], and hypoglossal [XII])


Posterior intramural migratory stream (inferior olive neurons)? Spinocerebellar tracts? Migrating raphe nuclear complex neurons?

MEDULLA Medullary/ spinal reticular formation

RHOMBENCEPHALON SPINAL CORD Ventral? (merging with

medial medullary NEP)

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.

Ventral gray?

Spinal NEP

Intermediate gray Intermediate

central canal

Dorsal Spinal roof plate



208







Otic vesicle and epithelium (future cochlea, semicircular canals, utricle, and saccule)

Medullary velum




209


PLATE 97B

MESENCEPHALON

Mesencephalic roof plate (commissural GEP?) Brain surface (heavier line)

TECTUM




R5 mesencephalic superventricle


(future aqueduct)

R6

Inferior collicular primordial plexiform layer R7

ISTHMUS

Isthmal NEP

Successive waves of migrating isthmal neurons?

isthmal canal

CEREBELLUM

Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

Layers of the cerebellar transitional field (CTF) CTF1 (fibers) CTF2 (deep neurons)

Cerebellar NEP


metencephalic pool


MEDULLA

myelencephalic pool

R5?


CTF3 (fibers) CTF4-5? (deep neurons) Premigratory deep neurons and Purkinje cells sequestered in the superficial cerebellar NEP?



R6

R7

Migrating glossopharyngeal receptor neurons? Migrating vagal sensory (X) neurons? Spinocerebellar tracts?

Lower medullary NEP

(gracile and cuneate NEPS merge with dorsal spinal NEP)

Migrating gracile and cuneate nuclear neurons?

RHOMBENCEPHALON


ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.

Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. Lower medullary roof plate


210


Peripheral neural and non-neural structures labeled Pia and pial blood vessels




Medullary velum


211

PLATE 98B



TECTUM

(commissural GEP?)

Inferior collicular primordial plexiform layer


Trochlear NEP in the isthmus

mesencephalic superventricle (future aqueduct) See enlargement on left isthmal canal

ISTHMUS

Trochlear NEP

CTF1 (fibers) CTF2 (deep neurons) CTF3 (fibers) CTF4-5? (deep neurons)

ph

er e

metencephalic pool

He

m

Uncrossed trochlear nerve (IV) fibers?


is

Ce re

b el lar NE P

Trochlear nucleus (IV)?

Premigratory inferior collicular neurons sequestered in superficial NEP? Migrating and settling isthmal neurons

Ve rm is

Migrating trochlear neurons?


CEREBELLUM Metencephalic roof plate (upper rhombic lip)


RHOMBENCEPHALON


MEDULLA

(lower rhombic lip)

Precerebellar NEP?

Migrating precerebellar neurons? myelencephalic pool


ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Diamonds indicate symmetric areas of low cell density that are postulated to contain sprouting axons from local neurons.

Lower medullary NEP (gracile and cuneate)

Lower medullary roof plate


PLATE 99A GW5.5 Coronal, CR 10 mm M1000 Between Levels 2 to 4 A. Section 85

C

E B E R R A

(CEREBRAL CORTEX at higher magnification)

TELENCEPHALON AND DIENCEPHALON

L

B. Section 83

T E X O R

SEPTUM

C

(ENTIRE TELENCEPHALON)

G BA AN S G AL LI A

212

BASAL TELENCEPHALON

ER

R

S

D AN IA A L AL NG D A YG L G M A ASA B

RTEX

AM

CO

AL U

IC T P O EA E PR AR

EB

L

TH

(TELENCEPHALON AND PART OF THE DIENCEPHALON)

C

A

C. Section 123


See Levels 2 to 4 in Plates 86A and B to 88A and B.

213

PLATE 99B

Dorsomedial limbic cortical NEP? Primordial plexiform layer absent in roof plate

Since the cortical primordial plexiform layer has few to no cells, the migration of cortical neurons has not begun.

Telencephalic/diencephalic roof plate (primordium of choroid plexus?)

A. Section 85 B. Section 83



Neocortical NEP




Neocortical NEP?

Dorsomedial limbic cortical NEP?

Less dense superficial NEP may contain sequestered premigratory Cajal-Retzius cells and subplate neurons.


(future lateral ventricle) Ventrolateral limbic cortical NEP? Corticoganglionic NEP Telencephalic floor plate (lamina terminalis?)

The neuroepithelium (NEP) in the telencephalon is "stockbuilding" neuronal precursors and is expanding the shoreline of the telencephalic superventricle.


Septal NEP? Basal telencephalic NEP

Pioneer migrating basal ganglia neurons Less dense superficial NEP may contain sequestered premigratory neurons. Pioneer migrating septal and basal telencephalic neurons*

C. Section 123

The NEP in the diencephalon is "unloading" postmitotic neurons and the shoreline of the diencephalic superventricle is shrinking. *The basal telencephalic migrating neurons may contain mitral cells heading for the future olfactory evagination.

Sequstered and pioneer migrating anterior thalamic neurons? Cortical primordial plexiform layer

Anterior thalamic NEP

Premigratory Cajal-Retzius cells and subplate neurons sequestered in superficial cortical NEP?



Preoptic NEP Lateral area? Medial area?


foramen of monro



Neocortical and limbic cortical NEP

Strionuclear NEP? Amygdaloid NEP? Corticoganglionic NEP? Sequestered and migrating posterior basal ganglia neurons? Posterior ganglionic NEP? Sequestered and migrating amygdaloid neurons? Sequestered and migrating strial bed nuclear neurons? Sequestered and migrating lateral preoptic area neurons? Sequestered and migrating medial preoptic area neurons?

214

PART PARTVIII: VIII: GW5.5 GW5.5 SAGITTAL SAGITTAL

Carnegie Collection specimen #6516 (designated here as C6516) with a 10.5 mm crown-rump length (CR) is estimated to be at gestational week (GW) 5.5. C6516 was fixed in corrosive acetic acid, embedded in a celloidin/ paraffin mix, and was cut in 8-µm sagittal sections that were stained with aluminum cochineal. Various orientations of the computer-aided 3-D reconstruction of M1000’s brain are used to show the gross external features of a GW5.5 brain (Figure 7). Like most sagittally cut specimens, C6516’s sections are not parallel to the midline; Figure 7 shows the approximate rotations in front (B) and back views (C). We photographed 65 sections at low magnification from the left to right sides of the brain. Five of the sections, mainly from the left side of the brain, are illustrated in Plates 100AB to 104AB. Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify the approximate midline, non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (lowcontrast images) identify central neural structures. Plates 105AB to 114AB show high-magnification views of many parts of the developing brain. The telencephalon is the smallest major brain structure, composed mainly of a “stockbuilding” neuroepithelium surrounding an expanding telencephalic superventricle. The primordial plexiform layer consists of discontinuous cell-sparse areas. The cortical neuroepithelium nearly reaches the pial surface with some of its outermost feathered edges. It is postulated that the superficial cortical neuroepithelium has postmitotic Cajal-Retzius cells sequestered there prior to migration. Few migrating neurons are adjacent to the very thick basal ganglionic and basal telencephalic neuroepithelia; nevertheless, these neuroepithelia are beginning to form mounds in the floor of the telencephalon. The olfactory epithelium is well established in the nasal cavity and olfactory nerve fibers are growing toward the brain. The diencephalon is the larger forebrain structure. The “stockbuilding” neuroepithelium surrounds a dorsally expanding superventricle in the future thalamic area. The thick neuroepithelium in the hypothalamic and subthalamic areas is depleting its population of stem cells and post-

mitotic, premigratory neurons are postulated to be sequestered in its superficial parts. Some migrating and settling young neurons are outside the neuroepithelium in the ventral diencephalic parenchyma adjacent to a thin subpial fibrous band. The mesencephalon, a prominent arch between the mesencephalic and diencephalic flexures, is relatively smaller than at GW6.5. The roof (tectum and pretectum) of the mesencephalon contains a stockbuilding neuroepithelium adjacent to a thin cell-sparse layer. In contract to the GW6.5 specimens, fibers in the posterior commissure are absent. The tegmental and isthmal neuroepithelia are rapidly unloading their neuronal progeny in dense bands in the adjacent parenchyma. The outermost clumps of young neurons appear to interact with axons in the subpial fiber band. The rhombencephalon is the largest brain structure. Both the pons and medulla have neuroepithelia that form crescent-shaped rhombomeres in lateral areas. In the sagittal plane, it is easy to see that rhombomeres are unloading their neuronal and glial progeny into parenchymal expansions at the entry zones of sensory cranial nerves V, VII, VIII, IX, and X. Neurons migrating in these areas are tentatively identified as receptors of the incoming sensory axons. For example, trigeminal nuclear neurons (mainly those in the principal sensory nucleus) are generated in rhombomere 2 and migrate outward to mingle with incoming afferents from the trigeminal ganglion. Medially, the pons and medulla contain longitudinal bands of migrating cells, but nuclear subdivisions are generally absent in the parenchyma. The genu of the facial motor nerve forms fascicles adjacent to a neuroepithelium medial to rhombomere 3, the presumptive source of neurons that will be receptive to axons of the facial ganglion. The subpial fiber band is definitely thicker in lateral areas where the axons from sensory ganglia enter the brain. As in the GW6.5 specimens, peripheral nerves have dense glia (Schwann cells), while central fiber tracts are clear. The cerebellum stands out as the most immature rhombencephalic structure. All parts of the cerebellar neuroepithelium are stockbuilding neuronal and glial stem cells. Relatively indistinct layers are in the cerebellar transitional field.

215

EXTERNAL FEATURES OF THE GW5.5 BRAIN lam

M a m m ill a r bo d y

lamus

lu m

Hy

potha

Sub

Ce

reb

ellum Upper rhombic lip

bu

A perfect sagittal cut through the brain is parallel to the midline from anterior to posterior. Based on Level 1, sections of C6516's brain rotate an estimated 2.5º counterclockwise from the anterior (B, front view) and posterior midlines (C, back view). In the sections illustrated on the following pages, anterior parts (top and left) are tilted toward the observer, while posterior parts (bottom and right) are tilted away from the observer.

di

Eye

3 s

Tha

t h alamus

y

I nf

un

2

BRAINSTEM FLEXURES

Medullary velum

o

Basal telencephalon Preoptic area

us

ganglia al s a

Inferior colliculus

hm

B

4

Ist

Cerebral cortex

gmentu

m

Te

n

Side view

Superior colliculus

Pretectum

Epithalamus

us

A.

P

1. Medullary 2. Pontine

M

3. Mesencephalic

e

4. Diencephalic

d

u

1

Superior colliculus

B.

Front view

Epithalamus

Posterior midline

C.

Superior colliculus

Back view

Spinal cord

Pretectum

Thalamus

a l l

Anterior midline

Lower rhombic lip

Inferior colliculus Isthmus

-2.5º Vermis

Ce

Cerebral cortex (occipital pole)

um ere ell isph reb Hem

Rhombic lip border

Right side

Cerebral cortex

Left side Eye

Medulla

Spinal cord Scale bars = 1 mm

Medullary velum

Left side

Figure 7. A, The lateral view of the left side of a computer-aided 3-D reconstruction of the brain and upper cervical spinal cord in M1000, the preceding GW5.5 specimen, which has a similar crown-rump length to C6516 (10 mm and 10.5 mm, respectively). External features are identified as in Figure 6B. The heavy numbered lines refer to brainstem flexures (boxed key). B, Front view of the brain in A. The angled line shows how C6516's sections rotate left (arrow) from the anterior midline. C, Back view of the brain in A. The angled line shows how C6516's sections rotate left (arrow) from the posterior midline.

Pons

Right side

-2.5º Medulla

Spinal cord

216

PLATE 100A GW5.5 Sagittal, CR 10.5 mm C6516, Level 1: Slide 11 Section 6




ricle )

of s e

l latera

Medullary velum

cav i t y

Hyoid arch (II) Basal occipital bone?

x yn ar Ph


n um col

Body

central canal

E LIN ID M ID HT S RIG

E

Intervertebral disks

Medullary velum

LE F T SID EO F B RA IN

ral Ve r t e b

myelencephalic pool

Future larynx?

Anterior Anteriorcardinal cardinal vein vein

Body

Or al

Plates 105A and B: cerebral cortex Plates 106A and B: basal telencephalon Plates 107A and B: septum and hypothalamus Plates 108A and B: midbrain tegmentum and isthmus Plates 109A and B: isthmus and cerebellum

Mandibular arch (I)

tongue Primordia of

See the following for higher magnification views of this section.

sively more

Future sphenoid bone

Medial nasal process/ nasal septum

c t io n sh i f t s pr og re s

ventral pool

Rathke's pouch epithelium (primordium of anterior pituitary gland)




foramen of monro


metencephalic pool

Plan e

dorsal pool


subtha hypothal lamic/ amic pool

Pia

Frontal prominence


dienc epha su lic (futu perven re th t ird v ricle ent

tha lam ic p ool

Future dura (internal border of Cell-sparse superarachnoid reticulum

217 S E N C E P H M E A

te

Me se n

LO HA Th alamic NEP

EP

DIENC


Migrating reticular formation neurons

oth

H

yp

r rio te An

Cerebellar

NEP (vermis) Cerebellar NEP

Upper rhombic lip


R


E P

Basal Preoptic telencephalic NEP NEP Septal NEP

CTF2 (deep nuclear neurons)

e

Cer

al a m ic N EP

Middle/lateral

ebral

co

rt

i

Anterior complex

CTF1 (fibers)


NEP Subthalamic

m r

Reticular nuclear

Sprouting nerve IV among migrating trochlear neurons

Isthmal NEP

P

fo

Migrating tegmental neurons

o n t i n

yer la P NE al c

Epithalamic NEP

Inferior collicular

EP

Posterior alic (tegme nt commissure commissural eph a c GEP?

tal) N

N

e

EP

P

ec

Pr

N

E N C E P H A L E L ordial pl ex O N T m ri i

i

N

al

(t

EP lN ta

N l)


c

ph

Superior colliculus collicular

PLATE 100B

L

c


Mesence

O


H

M

per

Lamina terminalis

M

e u

Migrating reticular formation neurons

d

The preoptic NEP, hypothalamic NEP, subthalamic NEP, tegmental NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

l l

Lower rhombic lip

a r y

Migrating hypoglossal (XII) and vagal motor (X) neurons?

P

Fibrous processes

Lower N E


Midline raphe glial structure Cell body layer

Midline raphe GEP?

Ventral

Ventral funiculus

Ventral gray Intermediate gray niculus Dorsal fu

Spinal NEP

Intermediate Dorsal

ay Dorsal gr

L N A S P I

Lower rhombic lip

R D C O

L O N H A E P N C B E

Up



O

Sprouting nerve VII adjacent to facial motor NEP?

Migrating basal telencephalic and septal neurons

Precerebellar NEP?

218




thalamic pool



Pia


sub thalamic pool



ventral pool

optic recess


Sphenoid bone

Mesenchyme associated with nerve I (olfactory)?

ye

le nc

cav

more late

Medullary velum

ep

i

ha li

ty

ng u e

c

Hyoid arch (II)

m

Mandibular arch (I)

Oral

P rimordia of to

Maxillary process

ssively

hypothalamic pool

f section shifts p Plane o rogre


Frontal prominence

Medullary velum

foramen of monro



ral

metencephalic pool

dorsal pool

po ol

Pharynx

Arch III?



Anterior cardinal vein

n

D

um ol c al e br Ve r t

central canal central canal

M IDL

T LEF

INE

SIDE

OF

IN BRA

AN

DS

AL PI N

CO

R

219

alamic NEP?

P E

Subth P ala m i c NE

N EP

Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

O N A L P H C E

R6

e

Migrating reticular neurons? Ascending fiber tracts from spinal cord

Precerebellar NEP?

Migrating raphe neurons? Raphe nuclei NEP?

P

Ventral funiculus

Lowe r

N E

Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei

Reticular NEP?

a r y l l

R5

Sprouting axons of local neurons

Ventral Lower rhombic lip

u

Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

R6

Migrating glossopharyngeal receptor (IX) neurons

d

R4

R5

M

Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion.

N

R4


r

R3

E

R3

B

Migrating sensory neurons that will receive input from facial (VII) ganglion

Cerebellar notches

N

Th

h a l a mic

Hy p o t h

R2

Upper rhombic lip

Lateral

Medial

pe

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus.

Cerebellar NEP

Migrating pontine reticular formation neurons? Migrating trigeminal (V) nuclear complex neurons

Pontine

c

E

mi

Thala

DIENC

bt

N ic

a

O rt

L

lay er

co

T


Migrating auditory and vestibular (VIII) neurons

R2


Migrating reticular nucleus neurons

Up


CTF1 (fibers)

M

Migrating ganglionic neurons

cular

O


Optic nerve GEP

o lli

H

Medial ganglionic NEP

Migrating basal telencephalicneurons

rc


Su

Cerebral

orm

P

rio


Reticular nuclear


R

E N C E P E L Primordial ple H A xif

ic

fe

P

am

Pretectal NEP

NEP

NE

al


O

N

N

N

L

Superior a l co i l

c t al)

NE

P

LO A H EP

enceph

(te

l

M

es

PLATE 101B

ar ul lic

Labeled on this page: Central neural Brain surface structures (heavier line)

M

In

ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere

E N C E P H A E S c


Ventral gray

The telencephalic NEP, thalamic NEP, tectal NEP, and cerebellar NEP form expanding shorelines of the superventricle as stockbuilding NEP cells increase. The hypothalamic NEP, subthalamic NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

Ventral gray

Ventral funiculus

Intermediate gray?

Spinal NEP

Ventral gray Ventral

Dorsal

Dorsal gray

Gracile and cuneate nuclear NEP?

Intermediate

A L I N S P

R D C O




Cell-dense primordial mesenchymal brain case (skin/bone) Future dura (internal border of brain case) An card terior inal vein

Cell-sparse superarachnoid reticulum Pia


dorsal pool


metencephalic pool


(optic recess)



Frontal prominence


ventral pool

Lateral nasal process Nerve I (olfactory)?

Sphenoid bone

Nasal/olfactory epithelium Maxillary process


O l

ngu e


Hyoid arch (II)

See higher magnification views of the rhombencephalon from this section in Plates 110A and B and Plates 111A and B.

myelencephalic pool

Otic vesicle

y cav i t

P rimordia of to

ra

Mandibular arch (I)

Arch III? Inferior vagal ganglion X?

A

Sympa

i n te r

ard or c

in a l

Cell-sparse superarachnoid reticulum vein

a k g run t c i thet

ENT I RE S E C

Medullary velum

Spinal nerves

ng

lia

FRO TI O N IS


?

Dorsal root ganglia

M LE

FT S

I

F DE O

BRA

IN

D AN

SP

A IN

O LC

RD

shifts progressively f section more l at era l Plane o

220

221


CTF3 (deep nuclear neurons and fibers)

DI


CTF2 (deep nuclear neurons) CTF1 (fibers)

m ic N E P

NEP

Cerebellar NEP Medial Lateral


P

Strionuclear NEP?

R3

N

Optic nerve GEP

R2

P

L O N H A


R7

E

R6


P


Migrating vagal sensory (X) neurons

r we Lo

R5

Precerebellar NEP?

R7

NE


Lower rhombic lip

ry

R4


lla

Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion.

R6

du

R3

R5

Me


er Upp


R2

C

R4



E


ic

tine

gli on

Cerebellar notches

Upper rhombic lip

B

Gan

Migrating trigeminal (V) nuclear complex neurons Migrating sensory neurons that will receive input from facial (VII) ganglion

Po n

N

E

Anterolateral

NE

Amygdaloid/ posterolateral P

Cereb


Subthala

L A

l c or t

Thalamic NEP

ra

al

E P HAL

R H O M

T E L E N C E P H

ic

NC

(Posterior complex)


O

E


ON


Labeled on this page: Central neural structures N

PLATE 102B

MESENCEPHALON (lateral edge)

Cuneate nuclear NEP? Migrating cuneate nuclear neurons?

Ascending fiber tracts from spinal cord

Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

s iculu a l fu n Ventr

Lateral funiculus

Ventral gray

Spinal NEP Dorsal

Intermediate gray?

A L I N S P

R D C O

The telencephalic NEP, thalamic NEP, and cerebellar NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. The subthalamic NEP, pontine NEP, and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

222



Cell-dense primordial mesenchymal brain case (skin/bone) Cell-sparse superarachnoid reticulum Pia

Nasal cavity


myelencephalic pool

Nerve VIII

A n t e r boundary cap*

Nerve VIII

Maxillary process

Nasal epithelium


P r i m o r di

Placodal epithelium Oral cavity


a of

Mandibular arch (I)

eral


io

Choroid fissure

Medullary velum

to n

*Boundary caps are Schwann cell GEPs?


gu

l at more y l e v i s s P la n e of section shifts progre

Nerve I (olfactory) Olfactory epithelium

Nerve II

metencephalic pool

Nerve V boundary cap*

Otic vesicle

Ne rv eI X

(future lateral ventricle, lateral pool)

r cardinal vein


e

Nerve II (optic)

Hyoid arch (II)

Nerve V (trigeminal) Nerve VIII (vestibulocochlear)

Nerve X boundary cap*

Arch III?


Nerve X al Superior din r a ganglion (X) rcn Basal o i ter vei occipital An bone?

Arch IV?

Nerve X (vagus)

See a higher magnification view of the rhombencephalon from a nearby section in Plates 112A and B.

Sympathe

Inferior ganglion (IX) Inferior ganglion (X)

tic

n k ga trun

Do

EN T

gli

a?

l rsa

IRE

roo

SEC

t g

TI

g an

lia

SF ON I

L ROM

EF

ID TS

EO

F

A BR

IN

AN

P DS

AL IN

CO

RD

223

PLATE 103B

Labeled on this page: Central neural structures FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


N

tical N or

al

G a n gli o

Sprouting olfactory nerve (I) axons

n ic

N

Migrating basal ganglionic neurons Optic nerve GEP

Cerebellar NEP Cerebellar notches

R2

Pontine NEP Central trigeminal tract Migrating auditory and vestibular (VIII) neurons

R4

Lateral lemniscus

R5

Medullary NEP

Lower rhombic lip

L

R6

N

R7

O

Migrating vestibulocochlear (VIII) ganglionic neurons from germinal source in otic vesicle epithelium

A

Peripheral neural structures

Migrating glossopharyngeal (IX) ganglionic neurons from germinal source in glossopharyngeal placode

Upper rhombic lip

Lateral

Medial

Migrating trigeminal (V) nuclear complex neurons?

EP

Ce


Primary olfactory cortical NEP? Posterolateral/ amygdaloid Anterolateral


B E H N C E P

TELE


CTF1 (fibers)

R H O M

NCEP

re

br

HA


EP

c

L

O


Migrating glossopharyngeal sensory (IX) neurons Migrating vagal sensory (X) neurons

Migrating vagal (X) ganglionic neurons from germinal source in vagal placode?


The telencephalic NEP and cerebellar NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. The pontine NEP and medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

R2


R4


R5


R6


R7

Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

224



Neural retina Retinal NEP


EYE

Intraretinal space Pigment layer Pigment epithelium of retina

Pia


Nerve V


Nerve V boundary cap*


(future lateral ventricle, lateral pool)

metencephalic pool



Nerve II

Nasal cavity

Maxillary process

Nasal epithelium

myelencephalic pool



Maxillary placodal epithelium

Oral cavity Mandibular arch (I)

Nerve II (optic) Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal) Nerve X (vagus)

Nerve X

Lingual epithelium


Hyoid arch (II) Inferior ganglion (IX)


Arch IV?

sup Celler spa ret arach rse icu no lum id


Otic vesicle


E

O

F

BR

AI

N

See higher magnification views of the rhombencephalon from nearby sections in Plates 112A and B to Plates 114A and B.

SI

D

ENT

S IRE

EC

TIO

S NI

OM FR

F LE

T

P l ane o f s e c ti o n s h ift s


Medullary velum Nerves VII+VIII boundary caps* Nerves VII+VIII

eral re lat

Choroid fissure

ly mo essive progr

Nerve I (olfactory) Olfactory epithelium

225

PLATE 104B

Labeled on this page: Central neural structures FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

b re

r

y

a ry olf a c

to

Migrating trigeminal (V) nuclear complex neurons? Central trigeminal tract (devoid of glia)

cerebellar notch

Upper rhombic lip


R2 Migrating primary olfactory cortex neurons Migrating Cajal-Retzius neurons Migrating auditory and Optic vestibular (VIII) neurons nerve Lateral lemniscus GEP (devoid of glia) Peripheral nerves have dense glia.

EP rN lla here) e eb isp Lateral

r m Ce (he


P ri m

TEL


Pontine NEP R4

Medullary NEP R5

Lower rhombic lip

RH O M BEN CE P H A L O N

Ce

AL

l

CTF1 (fibers)

co r t ical

EP

ENCEPH

ra


N

ON



Migrating vestibulocochlear (VIII) ganglionic neurons from germinal source in otic vesicle epithelium

PROPOSED RHOMBOMERE IDENTITIES R2


R4


R5


The telencephalic NEP and cerebellar NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. The pontine NEP and medullary NEP form shrinking shorelines of the superven-tricle as stockbuilding NEP cells decrease.




GW5.5 Sagittal, CR 10.5 mm, C6516 Level 1: Slide 11, Section 6 DORSAL CEREBRAL CORTEX

226

PLATE 105A

Primordial mesenchymal brain case (skin/bone/dura/arachnoid)


Future pia

The subarachnoid reticulum is indistinguishable over the cortex.

Earliest migrating Cajal-Retzius cells The superficial cerebral cortical NEP is postulated to contain postmitotic, premigratory CajalRetzius cells.

The primordial plexiform layer forms discontinuous cell-sparse areas outside the cerebral cortical NEP.

Synthetic zone


Mitotic NEP cells NEP - neuroepithelium The cerebral cortical NEP is in the "stockbuilding" phase when neural stem cells are increasing and few neurons (CajalRetzius cells) are being generated.



Mitotic zone

Pseudostratified NEP

P pre ostmit m o neu igratotic ron ry, s?

Cerebral cortical NEP

PLATE 105B

227


GW5.5 Sagittal, CR 10.5 mm, C6516 Level 1: Slide 11, Section 6 BASAL TELENCEPHALON

228

PLATE 106A

PLATE 106B

Postmitotic, premigratory neurons?

Frontal prominence

Synthetic zone Mitotic zone

Mitotic NEP cells




ventral pool



Septal NEP

Mitotic zone Synthetic zone

Pioneer migrating basal telencephalic neurons

Medial nasal process/ nasal septum

NEP - neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Differentiating field

Pioneer migrating septal neurons


229


GW5.5 Sagittal, CR 10.5 mm, C6516 Level 1: Slide 11, Section 6 SEPTUM/DIENCEPHALON

230

PLATE 107A

PLATE 107B

Successive waves of migrating hypothalamic neurons

NEP - neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


foramen of monro



Middle/lateral hypothalamic NEP


(future third ventricle) hypothalamic pool

ventral pool

Septal NEP

Preoptic NEP

Anterior hypothalamic NEP


Pioneer migrating septal neurons

Pioneer migrating preoptic neurons

Lamina terminalis (closure site of anterior neuropore)

Rathke's pouch epithelium (primordium of anterior pituitary gland) Sphenoid bone

Sphenoid bone Oral epithelium Oral cavity

231

GW5.5 Sagittal, CR 10.5 mm, C6516 Level 1: Slide 11, Section 6 MIDBRAIN TEGMENTUM See Level 1 in Plates 100A and B.

232

PLATE 108A

PLATE 108B



e

s

c n e

e

ta

l)

N


E

P

M

(future aqueduct)

lic (tegme n pha


Mitotic NEP cells


Blood islands and blood vessel primordia invade NEP

Successive waves of migrating tegmental neurons

Subthalamic NEP

Predominantly longitudinal fiber tracts

Cell-sparse superarachnoid reticulum Successive waves of migrating subthalamic neurons Brain surface (heavier line)

Migrating isthmal neurons Brain surface (heavier line)

Isthmal NEP

233

234

PLATE ?A 109A GW5.5 Sagittal, CR 10.5 mm, C6516 Level 1: Slide 11, Section 6 ISTHMUS and CEREBELLUM See Level 1 in Plates 100A and B.

235

PLATE 109B


Upper rhombic lip

ABBREVIATIONS: CTF - Cerebellar transitional field NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

mis) CTF2 (deep nuclear neurons)

Cere

CTF1 (fibers)

(ver P r NE bella




(future fourth ventricle) metencephalic pool

Mitotic NEP cells




Migrating trochlear (IV) neurons? Sprouting nerve IV axons?

Mesencephalic (tectal, inferior collicular) NEP

Isthmal NEP Mitotic NEP cells



Blood islands and blood vessel primordia invade NEP rac ib e r t f rons? l u a e n n tu d i mal lo n g i g isth y l n t i t n a a Migr o m in P red

ts

A higher magnification view of the R2 to R7 neuroepithelium is in Plates 111A and B. See Level 3 in Plates 102A and B.

GW5.5 Sagittal, CR 10.5 mm, C6516 Near Level 3: Slide 8, Section 10 PONS/MEDULLA

236

PLATE 110A

FONT KEY: R2 ventricular divisions - capitals Germinal zone - Helvetica bold R3 Transient structure - Times bold italic Permanent structure - Times Roman or Bold R4 ABBREVIATIONS: R5 CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium R6 R - Rhombomere R7 CTF3

(deep nuclear neurons and fibers)

PROPOSED RHOMBOMERE IDENTITIES Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

Migrating cuneate nuclear neurons? Lower rhombic lip

Precerebellar NEP?

CTF2 (deep nuclear neurons) Upper rhombic lip

Cuneate nuclear NEP?

E

P

CTF1 (fibers)

Lo we r

PLATE 110B

Medullary velum

NEP



myelencephalic pool

M


e

r lla

metencephalic pool


b re

U

e


C

P o n t i n e

N

R2

E

P R3

r ppe

e

d

u

a l l

r

y

N


R7


R6


R5


R4


Ganglion VIII boundary cap (Schwann cell GEP?) Vestibulocochlear ganglion (VIII)

Otic vesicle

Petrous temporal bone Inferior ganglion (X)

Brain surface (heavier line) Hyoid arch (II) Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.


Migrating trigeminal (V) nuclear complex neurons neurons?

Migrating sensory Migrating auditory and vestibular (VIII) neurons neurons that will receive input from facial(VII) (VII)ganglion? ganglion facial

Arch III

Oral cavity/pharynx

237

R2 R3 R4 R5 R6 R7

PROPOSED RHOMBOMERE IDENTITIES Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


GW5.5 Sagittal, CR 10.5 mm, C6516 Near Level 3: Slide 8, Section 10 PONS/MEDULLA

238

PLATE 111A

PLATE 111B



U p p e r

m e d u l l a r y


Blood islands and blood vessel primordia invade NEP


P o n t i n e

N E P

R7

R6

R5

R2

R3

(sensory IX neuronal NEP)

(sensory X neuronal NEP)

(vestibuloauditoryNEP)

R4

(trigeminal NEP)

N E P

(vestibuloauditoryNEP)

(facial sensory NEP) Sprouting axons of local neurons

Migrating trigeminal (V) nuclear complex neurons Brain surface (heavier line)

Migrating sensory neurons that will receive input from facial (VII) ganglion


Migrating glossopharyngeal receptor (IX) neurons Migrating vestibular and auditory (VIII) neurons Ganglion VIII boundary cap (Schwann cell GEP?)

Migrating vestibular and auditory (VIII) neurons

Migrating vagal sensory (X) neurons?



239

240

PLATE 112A

GW5.5 Sagittal, CR 10.5 mm, C6516 Between Levels 4 and 5: Slide 7, Section 6 RHOMBENCEPHALON See Level 4 in Plates 103A and B; Level 5 in Plates 104A and B. PROPOSED RHOMBOMERE IDENTITIES R2 R4 R5 R6 R7

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

241

PLATE 112B

CTF3 (deep nuclear neurons and fibers) CTF2 (deep nuclear neurons) CTF1 (fibers)


P

Upper rhombic lip

metencephalic pool

Peripheral nerves and boundary caps are filled with string-like arrays of Schwann cells, but all internal fiber tracts are free of interstitial glia. Apparently peripheral nerve gliogenesis precedes central fiber tract gliogenesis.

Medullary velum

Boundary caps may be the germinal sources (glioepithelia) of Schwann cells.

Medial cerebellar notch rhombencephalic superventricle

Wherever peripheral afferents enter the central nervous system, there is a swelling of the superficial fiber tracts to accommodate the larger number of axons at these sites.


R2

Pontine NEP

Mi


Central trigeminal tract Lateral lemniscus


R4 vesti bular

Mandibular arch (I) Mandibular placodal epithelium

R6 Migrating glossopharyngeal sensory (IX) neurons

Nerve VIII boundary cap

R7

Migrating vagal sensory (X) neurons?

Glossopharyngeal afferent fibers

Vagal Nerve X afferent boundary cap fibers

Nerve IX boundary cap

Lumen

Otic vesicle


neu ron s

Epithelium

Oral cavity


and auditory (V I I I)


Maxillary process


R5


e I X

Nerve V

ti ng

Medullary NEP

rv

Tr i ga gem n g in (V lio n a l )

g ra

Lower rhombic lip

myelencephalic pool

Ne

Migrating trigeminal (V) nuclear complex neurons


Pharynx

Inferior glossopharyngeal ganglion (IX)

Arch III? Hyoid arch (II)

X


Ne rv e

N

er

r


sph

lla

(hemi

Cerebe

e)

E

ABBREVIATIONS: CTF - Cerebellar transitional field NEP - Neuroepithelium R - Rhombomere

Schwann cell arrays in the vagus nerve (These arrays are present in all peripheral nerves and boundary caps.)


Placodal germinal source of ganglion IX?

242

PLATE ?A 113A

GW5.5 Sagittal, CR 10.5 mm, C6516 Lateral to Level 5: Slide 6, Section 11 RHOMBENCEPHALON See Level 5 in Plates 104A and B. See level 5 in Plates ?? A and B. PROPOSED RHOMBOMERE IDENTITIES R2 R4

See Plates ?? A and 114A and B. B.

R5

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

243

PLATE PLATE 113B ?B

Upper rhombic lip


r N EP he r

CTF1 (fibers)

e)


metencephalic pool


As seen in Plates 112A and B, peripheral nerves are filled with stringlike arrays of Schwann cells, while internal fiber tracts are free of interstitial glia. The boundary caps of these nerves may be the germinal sources (glioepithelia) of Schwann cells.

isp

(hem

Cereb

ella



Medullary velum

The swellings at the entry zones of the trigeminal and vestibulocochlear nerves are especially prominent in this section. Evidently, massive numbers of fibers are entering the central nervous system myelensimultaneously. cephalic pool



Cerebellar notch

Migrating trigeminal (V) nuclear complex neurons Nerve V boundary cap


R2


mi na

ga

l

nglion (V)

n d a ud

Lower rhombic lip

Cochlear nuclear NEP?

itory neuro s n



Vestibular and auditory afferent fibers

Neural retina (retinal NEP)

Facial ganglion (VII)? Epithelium

Oral cavity

Pe

tr

ne

Lumen

Otic vesicle

bo

Choroid fissure

Superior glossopharyngeal ganglion (IX)

l

Optic nerve GEP


ou

or

a

Anterior cardinal Nerve VIII vein with many intersitial Schwann cells

Glial channels in Retinal NEP?

M ax ill ar yp ro ce ss

EYE

lar a

R5

Trigeminal afferent fibers

ge Intraretinal space

Medullary NEP

R4

Migr a tin g ve sti b u

Tr i

Pigment epithelium

Pontine NEP

s te mp

Mandibular arch (I) Arrows indicate the presumed direction of axon growth in brain fiber tracts. Arrows indicate the presumed direction of neuron migration from germinal sources.



Hyoid arch (II) Lingual epithelium

244

PLATE 114A

GW5.5 Sagittal, CR 10.5 mm, C6516 Lateral to Level 5: Slide 6, Section 11 ENTRY ZONES OF NERVES V AND VIII

See Level 5 in Plates 104A and B. PROPOSED RHOMBOMERE IDENTITIES

R2 R4

R5

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

245

PLATE 114B

CTF1 (fibers)

Cerebellar NEP


(hemisphere)




Cerebellar notch

Pontine NEP

Migrating trigeminal (V) nuclear complex neurons


Medullary velum


R2

Medullary NEP

R4 Mig

ve V Ne r


bo

un

da

ry

rat

ing

ves

R5 tib

cap

glio i

neurons

g nal

Vestibular and auditory afferent fibers


an

Tr i g e m

r and auditory

n (V)

Nerve V with many intersitial Schwann cells

ula

These neurons carry primary sensory information from touch and pressure receptors in the face.

Vestibular ganglion? (primary sensory information from maculae of the semicirular canals, utricle, and saccule)

Nerve VIII with many intersitial Schwann cells


Boundary caps may be the germinal sources (glioepithelia) of Schwann cells.

Arrows indicate the presumed direction of axon growth in brain fiber tracts. Arrows indicate the presumed direction of neuron migration from germinal sources.

Vestibulo-cochlear ganglion (VIII) Facial ganglion (VII)? (primary sensory information from the taste buds of the anterior tongue)

Maxillary process

Oral cavity

Lumen

Spiral ganglion? (primary sensory information from the cochlea)

Otic vesicle Epithelium

(germinal source of vestibulocochlear ganglionic neurons)

Petrous temporal bone surrounds otic vesicle.

246

PART PARTIX: IX: GW5 GW5 CORONAL CORONAL

Carnegie Collection specimen #8314 (designated here as C8314) with a 7.1 mm crown-rump length (CR) is estimated to be at gestational week (GW) 5. C8314 was fixed in formalin, embedded in a celloidin/paraffin mix, and was cut in 8 µm transverse sections that were stained with azan. Sections of the prosencephalon and anterior mesencephalon are cut in the coronal plane, but the plane shifts to predominantly horizontal in the posterior mesencephalon, pons and medulla. We photographed 39 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and rhombencephalon. Eleven of these sections are illustrated in Plates 115AB to 124AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of C8314’s brain and eye (Figure 8), and to show each illustrated section in situ (insets, Plates 115A to 124A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural and peripheral neural structures; labels in B Plates (lowcontrast images) identify central neural structures. At this stage of development, the forebrain is a unitary prosencephalon because a telencephalon cannot be clearly distinguished from the diencephalon. There are no paired telencephalic vesicles. The most anterior brain sections are tentatively identified as the future telencephalon, while sections of the optic vesicle and posterior to it are more clearly identified as diencephalic. All parts of the prosencephalic neuroepithelium are rapidly increasing their pool of neuronal and glial stem cells as they expand the shorelines of the enlarging prosencephalic superventricle. Cell migration is virtually absent. The olfactory placode has not yet invaginated, but forms a thick epithelium in the anterolateral surface of the head. The evaginated optic vesicle forms a C-shaped curve around the developing lens,

defining an inner retinal neuroepithelium and an outer pigment epithelium. The eye is much closer to the diencephalon than at GW5.5 (Part VII) and does not yet form a stalk-like extension that will be the optic nerve. The mesencephalon contains a stockbuilding neuroepithelium in the pretectum and tectum; cell migration has not yet begun. The tegmental and isthmal neuroepithelia are also stockbuilding their stem cell populations. Only a few pioneer neurons have migrated out. The subpial fiber band is very thin in the tegmentum, but thickens slightly in the isthmus. The most prominent neuroepithelial structures in the rhombencephalon are the laterally-placed rhombomeres. These are crescent-shaped evaginations of stockbuilding neuroepithelium separated by narrow mounds jutting into the ventricular lumen. Blood islands and sprouting pioneer axons form clefts in between the evaginations. The rhombomeres are associated with the entry zones of sensory cranial nerves V, VII, VIII, IX, and X. In the coronal plane, the trigeminal ganglion (sensory axons of V) can be easily associated with rhombomere 2, and the vestibulocochlear ganglion (source of VIII axons) with rhombomeres 4 and 5. The subpial fiber band is thicker where sensory afferent axons enter the brain. A thin, definite layer of migrating neurons lines the superficial border of each rhombomere as neuronal and glial progeny move into a small parenchyma. Medial pontine and medullary neuroepithelia are thinner, and may actually be shrinking as more neurons and glia migrate outward. The small stockbuilding cerebellar neuroepithelium is only identifiable in the most posterior sections of the rhombencephalon; the few neurons accumulating outside it are probably the earliest-generated deep nuclear neurons.

247

R5 R6

rm

edu

N C E P H S E A Fut ur e co

Su o t h b thalamus T ala m u s

o

s

Upper rhombic lip

P

R3 R4

2 Medullary velum

R5 pe rm ed u lla R6

BRAINSTEM FLEXURES

R7

R7?

1. Medullary

lla

Lo

we

n

R2

P o n s

E

al o Pre n opt area ic

O

T N

P

O

R

ulla

3

p

m ellu reb Ce

R4 med

Hy

Inferior colliculus

I

m u t h

R

Lo

R3

t

4

Eye

Up

p er

gm en

Future basal ganglia and basal telencephalon Choroid fissure Preoptic area Infundibulum Mammillary body R2

Rhombomeres

Up

s

n

e

s

o

ral reb ce ex tr

m

Fu

P

m ellu

ye

Cereb

ia

s ntum I s t h m u

tex

gl an t u r e b a s al g ph tu ce re b a s a l telen

Fu

Inferior colliculus

Superior colliculus

u

Su b t ha lam us Hypoth alam u

us

u s

s

O

Th a

P

m

re cerebra Futu lc or

Te g m e

L

Ep ith

N

mus ala us lam

Superior colliculus

a

a l a m

C E P H A L E N O

Side view

pithal

E

h

S

Pretectum

Pretectum

Angled front view

s

A.

C8314 Computer-aided 3-D Brain Reconstructions B.

we

2. Pontine

1

3. Mesencephalic

rm

ed u

lla

Lower rhombic lip

4. Diencephalic Spinal cord

Spinal cord

C.

Future hemisphere

R2 Eye

N

Top view

C e r e b e l l u m

r e t e c t u m

t h a l a m p i u


P

R

P

O

E

a l a m u T h s

re cerebral corte x Futu

s

N C E P H A S E L

O


P

D.

o

I s t h m n

u

s

Future vermis

s

R2 R3

R4

P

l

a

R5

R6 l

l

R7

a

w

e r

i n a l

C o

m e d u

r

l

m e d u r p e

S

o

p

P o n s

I s t h m u s

a h a l

t o

m e g m e n t u

s

u

m

Choroid fissure

U

Futu r e b a s a

p

ce

T

en

H y

b asal tel

n s P o

R

F

ture

P r e o p t i c

Fu

O

Scale bars = 0.5 mm

u re sept u m p ga n g h alo n li a

ut

N C E P H A S E L

O

a r e a

N

l

L

Eye

d

Bottom view

p

248

PLATE 115A


GW5 Coronal CR 8 mm C8314 Level 1: Section 12

Primordial mesenchymal brain case (skin, bone, and meninges)

Pia and pial blood islands

Frontonasal process Olfactory placode Maxillary process


Level 2: Section 42 Primordial mesenchymal brain case (skin, bone, and meninges)

Pia and pial blood islands

Frontonasal process

Pioneer olfactory nerve (I) fibers Olfactory placode Maxillary process


249

PLATE 115B


Level 1: Section 12 ANTERIOR PROSENCEPHALON Brain surface (heavier line)

Prosencephalic roof plate

Prosencephalic primordial plexiform layer

prosencephalic superventricle

Prosencephalic NEP

(future lateral and third ventricles)

(future telencephalic)

Prosencephalic floor plate

ANTERIOR PROSENCEPHALON

Level 2: Section 42



Prosencephalic NEP (future thalamic)


Prosencephalic NEP (future telencephalic)



Prosencephalic NEP

(future preoptic/hypothalamic)

Prosencephalic floor plate NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


250

PLATE 116A



Blood islands in developing pia


Blood islands in developing dura

Formative superarachnoid reticulum Invaginating lens placode

EYE Intraretinal space Retinal NEP Lens vesicle Choroid fissure Pigment epithelium

Rathke's pouch?

Cephalic (maxillary) placode

Oral cavity

Maxillary process

Primordium of mandible Mandibular arch (I)

Primordium of tongue Lateral swellings Medial swellings

Hyoid arch (II)


Arch III Arytenoid swellings

Arch IV

Larynx

Pharynx Vagal ganglion (X) Nerve X (vagus)

Glottis

Sympathetic trunk


Figure 247B modified (Patten, 1953, p. 429.)

Frontonasal process

Oral cavity

Eye Olfactory placode Maxillary process

Mandible Hyo-mandibular cleft

Mandibular arch Hyoid arch Arches III and IV

251

PLATE 116B



THALAMUS Diencephalic roof plate


Thalamic NEP SUBTHALAMUS diencephalic superventricle

Subthalamic NEP PREOPTIC AREA/ HYPOTHALAMUS Preoptic NEP


Subthalamic primordial plexiform layer Preoptic primordial plexiform layer

Retinal NEP optic recess

Preoptic/hypothalamic NEP Diencephalic floor plate

infundibular recess

Preoptic/hypothalamic primordial plexiform layer

DIENCEPHALON

Non-neural structures Lingual epithelium

Multiple loci in the placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands.




252

PLATE 117A GW5 Coronal CR 8 mm C8314 Level 4: Section 97

Peripheral neural and non-neural structures labeled Blood islands in developing pia Blood islands in developing dura


Formative superarachnoid reticulum

EYE Intraretinal space

Rathke's pouch epithelium (primordium of adenohypophysis)

Retinal NEP Lens vesicle Choroid fissure Pigment epithelium

Oral cavity

Maxillary process

Cephalic (maxillary) placode

Mandibular arch (I)

Primordium of mandible Primordium of tongue Lateral swellings Medial swellings

Hyoid arch (II) Arch III Arytenoid swellings

Arch IV

Larynx Glottis

Pharynx

Vagal ganglion (X) Nerve X (vagus) Sympathetic trunk


253

PLATE 117B

Central neural structures labeled DIENCEPHALON Diencephalic roof plate

EPITHALAMUS (primordium of pineal gland) Epithalamic NEP THALAMUS

Epithalamic/thalamic primordial plexiform layer pineal recess


Thalamic NEP SUBTHALAMUS Subthalamic NEP




PREOPTIC AREA/ HYPOTHALAMUS Preoptic NEP?

Preoptic(?) primordial plexiform layer

Retinal NEP Anterior hypothalamic NEP

infundibular recess Hypothalamic primordial plexiform layer

Diencephalic floor plate (primordium of median eminence)

Non-neural structures Lingual epithelium





PLATE 118A


GW5 Coronal CR 8 mm C8314 Level 5: Section 117 Primordial mesenchymal brain case (skin, bone, and meninges)

Blood islands in developing pia Blood islands in developing dura

Formative superarachnoid reticulum

Internal carotid artery?

Sp he no id pr im or diu m?

254


ss ) ce ch ro ar y p ular r a ill dib ax n M s ma n i (jo

Rathke's pouch epithelium (primordium of adenohypophysis) Maxillary/mandibular placode

Oral cavity Lateral primordium Medial primordium of tongue of tongue

Ganglion IX placode Ganglion IX

Mandibular arch (I) Hyoid arch (II)

Arch III

Epiglottis Pharynx

Vagal ganglion (X) Anterior cardinal vein?

Nerve X

Notochord


255

PLATE 118B


Mesencephalic roof plate (posterior commissural GEP)


Pretectal/thalamic primordial plexiform layer

Pretectal NEP


THALAMUS Thalamic NEP

SUBTHALAMUS


Subthalamic NEP HYPOTHALAMUS Anterior/middle hypothalamic NEP Diencephalic floor plate (primordium of median eminence and neurohypophysis)

Subthalamic primordial plexiform layer infundibular recess

Hypothalamic primordial plexiform layer

DIENCEPHALON

Non-neural structure Multiple loci in the placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands.



256


Peripheral neural and non-neural structures labeled Blood islands in developing dura


Formative superarachnoid reticulum Anterior cardinal vein?

Sp he no id pr im or diu m?


Circle of Willis arteries? Anterior cardinal vein?

Notochord

Fused maxillary process and mandibular arch

Nerve V?

Meckel's cartilage

Branchial placodes Glossopharyngeal ganglion (IX)

Hyoid arch (II)

l bone tempora Petrous

Notochord

Pharynx

Nerve IX (glossopharyngeal) Nerve X (vagus) Anterior cardinal vein?


257

PLATE 119B


Mesencephalic roof plate (posterior commissural GEP)

PRETECTUM

Posterior commissure pioneer fibers

Pretectal NEP

Pretectal/tectal primordial plexiform layer

TECTUM

Tectal (superior collicular) NEP TEGMENTUM

Tegmental NEP

HYPOTHALAMUS

Hypothalamic (mammillary) NEP Diencephalic floor plate

DIENCEPHALON




Tegmental primordial plexiform layer Pioneer migrating tegmental neurons

Mammillary primordial plexiform layer





Peripheral neural and non-neural structures labeled Blood islands in developing dura




Basilar artery



Nerve V (trigeminal) Anterior cardinal vein Vestibulo-cochlear ganglion (VIII)

Otic vesicle Lumen

b one

Epithelium

Pe

r al

258

trous temp o

Nerve and ganglion IX (glossopharyngeal)

Nerve and ganglion X (vagal)


259

PLATE 120B



TECTUM


Mesencephalic roof plate mesencephalic superventricle


(future aqueduct)

TEGMENTUM

Tegmental NEP

Tegmental primordial plexiform layer

Mesencephalic floor plate

Pioneer migrating tegmental neurons


PONS

Medial lemniscus?

Pontine floor plate


Sequential waves of migrating pontine neurons

Medial pontine NEP

Peripheral neural structure



Migrating vestibulocochlear ganglionic neurons originating in otic vesicle epithelium

Medial medullary NEP Medullary floor plate

Sequential waves of migrating medullary neurons


Medial lemniscus

MEDULLA



Ventral funiculus

Spinal floor plate



Ventral gray

Ventral NEP Intermediate gray Lateral funiculus

Intermediate NEP ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

central canal


Dorsal funiculus Dorsal gray

Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.







Cell-sparse Cell sparse superarachnoid reticulum Basilar artery Anterior cardinal vein Nerve V boundary cap (Schwann cell GEP?) Nerve V (trigeminal) Trigeminal ganglion (V) Nerve VII and VIII boundary caps (Schwann cell GEPs?) Anterior cardinal vein Vestibulocochlear ganglion (VIII)

Lumen

tem

Epithelium

por a l b o n e

Otic vesicle

us

260


r Pe t

o


Nerve and ganglion X (vagal)


261



MESENCEPHALON


TECTUM





R2

(future aqueduct)

TEGMENTUM

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

R3

Tegmental NEP Tegmental primordial plexiform layer

R4


(midline raphe glial structure GEP) Midline raphe glial structure Pioneer migrating tegmental neurons

PONS

PLATE 121B

R5


R6

Pontine floor plate

(midline raphe glial structure GEP) Medial lemniscus? Migrating trigeminal neurons from R2 NEP Central trigeminal fibers

R7

R2 R3

Lateral pontine NEP

Migrating facial sensory neurons from R3 NEP

R4



Lateral lemniscus?

Migrating vestibular and auditory neurons from R4+R5 NEPs

R5


R6 R7?


Migrating solitary neurons (glossopharyngeal receptors) from R6 NEP

(hypoglossal [XII] and vagal [X] motor?)

MEDULLA

Migrating vagal sensory neurons from R7 NEP?


Sprouting hypoglossal (XII) and vagal (X) nerve axons? Migrating medial medullary neurons

Ventral NEP

(merges with medial medullary NEP)



Intermediate gray

Intermediate NEP


Migrating ventral spinal neurons

central canal




262

PLATE 122A








Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear) Nerve V boundary cap

Nerve VII boundary cap? Nerve VIII boundary cap

Otic vesicle

l bo ne

Epithelium

mp

ora

Lumen

te

Schwann cell GEPs?


(Schwann cell GEP?)

Pe t r o

us

Nerve IX (glossopharyngeal) Nerve X (vagal)

Nerve XI (spinal accessory)


263

PLATE 122B



TECTUM




R2


TEGMENTUM/ISTHMUS

Tegmental NEP

R3 R4

Tegmental primordial plexiform layer Pioneer migrating tegmental neurons

isthmal canal

Isthmal primordial plexiform layer

R5

Pioneer migrating isthmal neurons Isthmal floor plate

R6

Isthmal NEP


PONS

Midline raphe glial structure Medial lemniscus? Pontine floor plate

R7


Medial pontine NEP


Migrating trigeminal neurons from R2 NEP Central trigeminal fibers

R2 (trigeminal NEP) Clefts in NEP define rhombomere boundaries

R3 (facial sensory NEP)



R4 (vestibulo-auditory NEP)

Lateral lemniscus?




Clefts in NEP define rhombomere boundaries

R6 (glossopharyngeal NEP) R7? (vagal sensory NEP)

Solitary tract?

Migrating vagal sensory neurons from R7 NEP

Midlateral medullary NEP MEDULLA

(reticular formation?)




Migrating midlateral medullary neurons

Intermediate NEP

(merges with midlateral medullary NEP)

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Migrating intermediate spinal neurons Intermediate gray

Dorsal NEP

central canal

Spinal roof plate




264

PLATE 123A



Blood islands in developing dura Blood islands in developing pia





Nerve VII boundary cap? Nerve VIII boundary cap

Otic vesicle Epithelium

us

Lumen

t e mp o r a l b o n

e

t Pe Nerve IX boundary cap

ro

Schwann cell GEPs?

Nerve X boundary cap Schwann cell GEPs?


265

PLATE 123B






R2



R3

(future aqueduct)


R4

Inferior collicular primordial plexiform layer

R5

ISTHMUS


Isthmal NEP

isthmal canal

Pioneer migrating isthmal neurons

R6

R7

RHOMBENCEPHALON PONS/CEREBELLUM


Cerebellar NEP Pioneer cerebellar deep nuclear neurons Central trigeminal fibers

R2 (trigeminal NEP)

metencephalic pool Migrating trigeminal neurons from R2 NEP






Lateral lemniscus?




R6 (glossopharyngeal NEP)



R7 (vagal sensory NEP)

Migrating vagal sensory neurons from R7 NEP

Midlateral medullary NEP (reticular formation?)

Posteromedial medullary NEP (merges with dorsal spinal NEP, gracile and cuneate?)

MEDULLA

Migrating cuneate nuclear neurons?

ABBREVIATIONS: NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


meyelencephalic pool

Migrating gracile nuclear neurons?

Medullary roof plate


266

PLATE 124A


GW5 Coronal CR 8 mm C8314 Level 11: Section 222 Blood islands in developing pia




267

PLATE 124B





Tectal (inferior collicular) NEP Inferior collicular primordial plexiform layer

ISTHMUS

Isthmal NEP (trochlear?)

isthmal canal

Pioneer migrating isthmal (trochlear nucleus?) neurons


Medial cerebellar NEP (vermis) Pioneer migrating cerebellar deep nuclear neurons

Lateral cerebellar NEP (hemisphere) Metencephalic roof plate (upper rhombic lip)

metencephalic pool


Lateral medullary velum

Myelenencephalic roof plate (lateral lower rhombic lip) Precerebellar NEP?

meyelencephalic pool

Pioneer migrating precerebellar neurons

Migrating cuneate nuclear neurons?

Posteromedial medullary NEP

(gracile and cuneate?)

Migrating gracile nuclear neurons?

MEDULLA ABBREVIATIONS: NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Medial medullary velum

Myelenencephalic roof plate (medial lower rhombic lip)



268

PART PARTX: X: GW5 GW5 SAGITTAL SAGITTAL

Carnegie Collection specimen #8966 (designated here as C8966) with a 7.1 mm crown-rump length (CR) is estimated to be at gestational week (GW) 5. C8966 was preserved in Zenker’s fixative, embedded in a celloidin/ paraffin mix, and was cut in 10-µm sagittal sections that were stained with hematoxylin and eosin. Various orientations of the computer-aided 3-D reconstruction of C8314’s brain are used to show the gross external features of a GW5 brain (Figure 9). Like most sagittally cut specimens, C8966’s sections are not parallel to the midline; Figure 9 shows the approximate rotations in front (B) and back views (C). We photographed 29 sections at low magnification from the left to right sides of the brain. Seven of the sections, mainly from the left side of the brain, are illustrated in Plates 125AB to 131AB. Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify the approximate midline, non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (lowcontrast images) identify central neural structures. Plates 132AB to 133AB show high-magnification views of the rhombencephalon. The anterior part of the prosencephalic superventricle is tentatively identified as the future telencephalic superventricle, an enlargement of the diencephalic superventricle. The prosencephalic neuroepithelium is stockbuilding its various populations of neuronal and glial stem cells. The presumptive basal ganglionic and basal telencephalic neuroepithelia do not form mounds in the floor of the telencephalon. There is a definite lamina terminalis in the ventral prosencephalon that marks the site of closure of the anterior neuropore. The olfactory epithelium, in an anterolateral placode, is already producing nerve fibers.

The mesencephalon, arching between the mesencephalic and diencephalic flexures, is characterized by stockbuilding neuroepithelia surrounding an expanding mesencephalic superventricle. Cells have not yet migrated from the tectal and pretectal neuroepithelia. A few cells are migrating outside the tegmental and isthmal neuroepithelia. There is a very thin subpial fiber band. The rhombencephalon is the largest brain structure. Rhombomeres 2 through 7 form well-defined swellings in the lateral neuroepithelium. The rotation of C8966’s sections does not clearly show all the entry points of the cranial sensory nerves. However, rhombomere 2 is clearly associated with incoming axons from the trigeminal ganglion (V afferents), and rhombomeres 4 and 5 with the vestibulocochlear ganglion (VIII afferents). The association of rhombomere 3 with a tentatively identifiable facial ganglion (VII afferents) is less clear. Each rhombomere has a thin layer of pioneer migrating neurons, most are receptors for incoming sensory axons. However, there are many fewer fibers entering the brain from these ganglia than at GW5.5, and no fibrous swellings are in the very thin subpial fiber band. Sections near the midline show that rhombomeres do not extend into the medial pontine and medullary neuroepithelia. There is a thicker layer of migrating cells outside the medullary neuroepithelium, and the subpial fiber band is thicker as the brain blends with the spinal cord. The cerebellum stands out as the most immature and smallest rhombencephalic structure. In spite of that, a cerebellar notch can be identified laterally where the cerebellar and pontine neuroepithelia join. The most lateral sections cut the cerebellar neuroepithelium tangentially, allowing a few indistinct layers in the cerebellar transitional field to be identified.

269

EXTERNAL FEATURES OF THE GW5 BRAIN

th al

us

po

3

m

m

Hy

Upper rhombic lip

s

Future basal ganglia and basal telencephalon Preoptic area

Inferior colliculus

th

Choroid fissure

4

A perfect sagittal cut through the brain is parallel to the midline from anterior to posterior. Sections of C8966's brain rotate an estimated 13º clockwise from the midline, 6.5º to the right side of the anterior midline (B, front view), and 6.5º to the left side of the posterior midline (C, back view). In the sections illustrated on the following pages, anterior parts (top and left) are tilted away from the observer, while posterior parts (bottom and right) are tilted toward the observer.

Is

Eye

e

e re

PROSENCEPHALON

en gm tu

m

Future cerebral cortex

Superior colliculus

Pretectum

Thalamus

C

Side view

b e ll u

Epithalamus

Subthalamus a m us T

A.

n

Infundibulum Mammillary body

o

Rhombomere (R) 2 R3

P

2

R4

BRAINSTEM FLEXURES

Medullary velum

M

R5

1. Medullary

e

2. Pontine

R6

d

3. Mesencephalic

R7?

u

B.

Front view

1

a l l

Anterior midline

4. Diencephalic Lower rhombic lip

C.

Pretectum

Spinal cord

Epithalamus Thalamus

Pretectum

Back view

Superior colliculus

PROSENCEPHALON

Eye Choroid fissure

Eye

6.5º

e e b l l

e

r

Right side

C

m

R2 R3

Left side

R4

R5 R6 R7?

Medulla

Spinal cord Scale bars = 0.5 mm

Medullary velum

Left side

Figure 9. A, The lateral view of the left side of a computer-aided 3-D reconstruction of the brain and upper cervical spinal cord in C8314, the preceding GW5 specimen, which has a similar crown-rump length to C8966 (8 mm and 7.1 mm, respectively). External features are identified as in Figure 8B. The heavy numbered lines refer to brainstem flexures (boxed key). B, Front view of the brain in A. The angled line shows how C8966's sections rotate right (arrow) from the anterior midline. C, Back view of the brain in A. The angled line shows how C8966's sections rotate left (arrow) from the posterior midline.

Inferior colliculus

u

Isthmus Pons

Right side Rhombic lip border

Medulla

6.5º

Spinal cord

Posterior midline

270

PLATE 125A

GW5 Sagittal, CR 7.1 mm, C8966 Level 1: Slide 6 Section 2



R


IG

(future aqueduct)

H

thalamic pool

T



subthalamic/ hypothalamic pool

isthmal canal


Rathke's pouch epithedlium (primordium of anterior pituitary gland)

metencephalic pool

Arch III?

Arch IV?

rh om be n (f ce ut p ur h e f al ou ic rt s h up ve e nt r ri ve cl n e) tr ic le

Hyoid arch (II)

Bas ilar arte ry

Prim

o rd i

a of

Mandibular arch (I)

tong ue Or al ca vi ty

Cephalic placodes

myelencephalic pool

Medullary velum

S I D E


L E F T


O F

B R A I N

Pharynx

Medullary velum

M

I D

N E L I

DE


SI

(f

halice ncep l meserventriucct) e e aqued sup utur

(future lateral ventricles)

271

co

i

E DIENC

Pioneer migrating thalamic neurons

M

Epithalamic NEP


e

A tal) NEP

Inferior collicular


N L O

Superior collicular

(tec

E

P

Isthmal NEP

N

o

n

t

in

e

Migrating pontine (reticular formation?) neurons

Migrating trochlear (IV) neurons?

P

ic

P

lic

am H y p o t h al

E

NEP

Anterior

nc

e

l)

Middle

C

H

Mese

Pioneer migrating tegmental neurons

Posterior Preoptic NEP Lamina terminalis (site of anterior neuropore closure)

N

ha

cepha lic (tegm

E

p

en

Subthalamic NEP?

N


S

ta

Basal telencephalic and septal NEP

E

P retectal N EP

ala mic NEP

es

Th

P

M

al

br Cere

rt

PLATE 125B

N LO HA

n

LO ON N((FFU PHAL UTT NCE UURR SE O EE TTE PPRR EL

N)) ON LLO HAA H P P E CE P NNC l NE ca LEE


E

P

Pioneer migrating deep nuclear neurons Upper rhombic lip

M

id

li n e e

G EP ?

H

ph

R

ra


O M

B

E N C

Upper

E P

H A

E P Migrating gracile and cuneate nuclear neurons?

Reticular, vagal motor, and hypoglossal NEP?

Gracile and cuneate nuclear NEP? Lower rhombic Lower lip

N

N


O


L

M e d u l l a r y

Migrating medullary (reticular formation?) neurons

Nearly all parts of the NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. Medial parts of the medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of axon growth in brain fiber tracts.


GW5 Sagittal, CR 7.1 mm, C8966 Level 2: Slide 5, Section 2

PLATE 126A Primordial mesenchymal brain case (skin, bone, and meninges)

M

thalamic pool

me s


en

ce ut

L

I

N

ph e

ic

ur

aq

) ct

cle tri en rv

du

pe

ue

su


D

al

(f


I

E



Branchial placodes


Cell-sparse Cell sparse superarachnoid reticulum myelencephalic pool

Sympathetic trunk ganglia? Nerve X boundary cap (Schwann cell GEP)? Nerve X (vagus)?

Medullary velum

O F

I D E S

T

Arch IV?

Medullary velum

L

E

F

ral late e r mo ely

of s ect ion shi fts pro gr ess iv

nx

ne

ary

P la

Ph

isthmal canal

rh om be n (f ce ut p ur h e f al ou ic rt s h up ve e nt r ri ve cl n e) tr ic le

ca vi ty ra l

Hyoid arch (II)

O

Arch III?


metencephalic pool

of ton gu e

Mandibular arch (I)

Cephalic placodes


B R A I N


Pr im or dia

272

See a higher magnification view of the entire rhombencephalon, parts of the mesencephalon, and diencephalon in Plates 132A and B.

273

r

co al

rti

c

D I E NC

P T h a l a mic NE

Pre

Epithalamic NEP

Ce

re

b

HALON) CEP N E NEP EL al

tec M ta E l N Posterior S EP commissural E GEP? N

C E

P P

M Brain surface (heavier line)

NE P

Migrating trigeminal nuclear complex (V) neurons from R2 NEP Migrating facial (VII) neurons from R3 NEP

Isthmal (trochlear nuclear) NEP?


R2

R3

O N

Inferior collicular

P

N E

Isth ma l

Middle H y p o t i c h a l a m

alic (tectal) N E

Anterior

ph

Posterior

L

ce Superior collicular

al) NEP nt

Lamina terminalis (site of anterior neuropore closure)

en

ephalic (tegm enc e es

Preoptic NEP

A

es

Basal telencephalic and septal NEP


H

M

NCEPHALO OSE N( PR FU

TU RE

T

PLATE 126B

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals R - Rhombomere Germinal zone - Helvetica bold Transient structure - Times bold italic ON L A Permanent structure - Times Roman or Bold EPH

Pontine NEP Upper rhombic lip

R4 Migrating vestibular and auditory neurons from R4+R5 NEPs

R

H

O

U pper

R5

M

B

E

Medullary NEP


N

R6

C

Migrating solitary nuclear neurons (IX glossopharyngeal receptors) from R6 NEP

E

R2

P

L

er

R3 R4

O

R7

Lower rhombic lip

A

Lo w

H

Migrating vagal sensory (X) neurons from R7 NEP

N R5

Nearly all parts of the NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. Medial parts of the medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

R6 Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

R7

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

274

PLATE 127A

GW5 Sagittal, CR 7.1 mm, C8966 Level 3: Slide 4, Section 7 ENTIRE SECTION I S FR O



M L E

FT

DE

me s

thalamic pool


SI

OF

en

ce ut

e

ic

aq

) ct

cle tri en rv

du

pe

ue

su


N

ph

ur


AI

al

(f

BR


Maxillary process?



Cephalic placodes Cell-dense Cell dense mesenchyme

Nerve IX boundary cap (Schwann cell GEP)?

Nerve X (vagus)


eo

fs



h if

Medullary velum

ns

Lumen

Epithelium

ts p



tio

Otic vesicle

rog ress ivel y


r

Arch III Inferior glossopharyngeal ganglion (IX)

m or e lat

c

a

metencephalic pool

ec

O

eral

v

of ton gu e

a l

Branchial placodes

Pr im or dia

Hyoid arch (II)

i t y

Mandibular arch (I)

myelencephalic pool

Pl

an

275

D IEN C E

o

Thala

bra

lc

ON) HAL EP C P EN l N E EL r t i c a

PLATE 127B



ON AL H P

NE mic

Pre

P

Epithalamic NEP

M tec


E

tal

S

NE

E

N

P

C

E

P

H A

L

Basal telencephalic and medial basal ganglionic NEP

M

N lic (tec

NE


tal )

Inferior collicular

P



Sprouting nerve IV (trochlear)?

Isthmal NEP

R2

Migrating facial sensory (VII) neurons from R3 NEP

R3

Migrating vestibulocochlear ganglionic (VIII) neurons from germinal source in otic epithelium? epithelium?

EP ar N bell ere) Cer(heemisph

Migrating trigeminal nuclear complex (V) neurons from fromR2 R2NEP? NEP


pha


Migrating subthalamic Migrating tegmental neurons? neurons

Middle/ EP lateral i c N am al th

Superior collicular

ce

Posterior

Anterior H ypo

en

anterior neuropore closure) Pioneer migrating basal telencephalic and basal ganglionic neurons

O

es

Mesencephali c( t

Subthalamic NEP? Lamina terminalis (site of

Preoptic NEP

EP ntal) N me eg

ALON (FU CEPH TU SEN RE O Cere PR T


Medial cerebellar notch?

Pontine NEP

R H

R4

Pioneer migrating deep nuclear neurons?

O

Upper rhombic lip

M

B

A

L

O

N

R6

H

R5

P

R4

E

R3

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

Lower rhombic lip

C

R2

R6

Medullary NEP

N


R5

E



Nearly all parts of the NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase. Medial parts of the medullary NEP form shrinking shorelines of the superventricle as stockbuilding NEP cells decrease.

276

PLATE 128A

GW5 Sagittal, CR 7.1 mm, C8966 Level 4: Slide 3, Section 24 ENTIRE


SECTI

ON I S FR O

M L EFT SID E

Labeled on this page: Peripheral neural and non-neural structures, brain ventricular OF divisions BR


me s


thalamic pool

en

ce ut

ph e

ic

ur

aq

) ct

cle tri en rv

Otic vesicle

eral gre

ssiv ely

Branchial placode

fts

shi Medullary velum

Epithelium Lumen

Otic canal (marks the initial invagination of the otic placode)

Nerve VII (facial) Nerve VIII (vestibulocochlear)

See a higher magnification view of the rhombencephalon from this section in Plates 133A and B.

e ct

io n



of s

ganglion (VIII)

metencephalic pool

ne

O Vestibulocochlear

Pla


pro

Nerve VII and VIII boundary caps (Schwann cell GEPs)? Nerves VII and VIII

r

a l

c

a

Pr im or dia

of ton gu e

v i t y

Cell dense Cell-dense mesenchyme


m or e lat

Placodal epithelium (maxillary)

Arch III?

du

optic recess

Maxillary process

Hyoid arch (II)

pe

ue

l

/ ic oo m p la m ic a h a bt al su oth p hy

Mandibular arch (I)

su


IN

al

(f


A


PLATE 128B



Labeled on this page: Central neural ALON) structures EPH NC

DI ha

T

E NC

H EP

l a mic N

AL

EP

ON Pretecta l

Epithalamic NEP

I st

h

m

NE P ic

Migrating isthmal neurons?

R3

CTF2 (deep nuclear neurons) CTF3 (deep nuclear neurons and fibers)

EP r Nre) lla he be isp re em Ce (h

R2

RH OM

Pontine NEP

Upper rhombic lip Lower rhombic lip

B

R4

al N P? E

CTF1 (fibers)


Migrating trigeminal nuclear complex (V) neurons from R2 NEP

N L O

Inferior collicular

A

Superior collicular




Migrating vestibulocochlear ganglionic (VIII) neurons from germinal source in otic epithelium

H

ic

m la ha Hypot


E ) NEP P ectal c (t E ali ph C ce en es

am

Preoptic NEP


S

M

t h al

Mesencepha lic (teg m Migrating tegmental neurons

P NE

NEP tal) en

S ub

Optic nerve (II) GEP?

E


Basal telencephalic and lateral basal ganglionic NEP

Pioneer migrating basal telencephalic and basal ganglionic neurons

M

NEP ?

N

ALON (FU NCEPH TU OSE RE PR e b ral Cer co r

LE l NEP ca TE ti

E N C

P

H

A

L

ON

from R4 NEP?

E

Migrating auditory Migrating vestibular and auditory (VIII)R4 neurons (VIII) neurons from NEP

PROPOSED RHOMBOMERE IDENTITIES R2 R3 R4

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

All parts of the NEP in this section form expanding shorelines of the superventricles as stockbuilding NEP cells increase.

278

PLATE 129A


ENTIRE SECTIO N

IS FR O

M L E

FT



SID

E

OF


BR

AI

N

mes supeencepha lic (futu rvent re aq ricl e ue


duct )

Nerve I (olfactory) Olfactory placode Maxillary process



eral

Sphenoid bone?

optic recess

Facialganglion ganglion Facial (VII)placode? placode? (VII)

m or e lat rog res sive ly

ts p h if

ns tio sec

Facial ganglion (VII)? Nerve VII? Nerve VIII

of

al Orvity a c

Hyoid arch (II)

ne

Branchial placodes

Nerve V boundary cap (Schwann cell GEP)?

P la

Mandibular arch (I)

Ant erio r ca rdin al v ein?

Cell dense mesenchyme

Vestibulocochlear ganglion (VIII) Petrous temporal bone

Otic vesicle Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear)

Epithelium


Lumen

(future fourth ventricle, metencephalic pool)

Medullary velum


PLATE 129B




am

M N

E


P

C

ic NEP

E

N

al

e ctal

Me sen ceph alic (t egme ntal )N E Migrating tegmental neurons Migrating subthalamic Brain surface neurons? (heavier line)

P

h

Pret

Epithalamic NEP

E

T


S

Cerebra

l

c

DIENCEPHALON

al N E P

PROSENCEPHALON (FUTURE TELENCEPHALON)

tic or

E P

H

en

P

NE l)

a l ic

N L O

ph (tecta


A

ce

Pioneer migrating basal telencephalic and basal ganglionic neurons

Hypothalamic and preoptic NEPs


Cer e (he be mi lla

sp he re

Cochlear nucleus nuclear NEP?

RH OM

Pontine NEP

EP rN ) Upper rhombic lip

B

E

Lower rhombic lip

C E

A

L


N


Migrating facial sensory (VII) neurons from R3 NEP?

R2

R 3?

Migrating facial ganglionic (VII) neurons from germinal source in pacode?

Central trigeminal fibers

CTF2+3 (deep nuclear neurons and fibers)

H


Medial cerebellar notch CTF1 (fibers)

P


es

P

Basal telencephalic and lateral basal ganglionic NEP

NE

M

Su

ami c bthal

ON PROPOSED RHOMBOMERE IDENTITIES R2 R3

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion.


280

PLATE 130A



ENTIRE

SECTI


ON IS FROM

Labeled on this page: Peripheral neural and non-neural structures, brain ventricular divisions LEF

T S IDE


OF

BR

A

IN

Nerve I (olfactory)

Olfactory placode

EYE

optic recess


Placodal epithelium (maxillary) Sphenoid bone? ior ter An

ter a


re l a

Mandibular arch (I)

l

? ein lv ina rd ca

Cell-dense Cell dense mesenchyme

mo

Maxillary process

pro

gre

ssiv ely

Nerve V boundary cap (Schwann cell GEP)?

ion

ect fs

eo


an

Facial ganglion (VII) placode?

Pl

Ant erio r ca rdin al v ein?

shi

fts

Facial ganglion (VII)

Hyoid arch (II)



Otic vesicle

Epithelium

Lumen

281

PLATE 130B Labeled on this page: Central neural structures PROSENCEPHALON (FUTURE TELENCEPHALON)

Cerebral cortical NEP MESENCEPHALON Basal telencephalic and lateral basal ganglionic NEP


Pretectal NEP Posterior commissure GEP?

Opthalmic germinal zones

Migrating Pioneer migrating basal Subthalamic subthalamic neurons? telencephalic and basal NEP ganglionic neurons

Pigment epithelium germinal zone

Migrating pretectal and tectal neurons?


DIENCEPHALON

Retinal NEP Optic nerve (II) GEP?



Migrating facial ganglionic (VII) neurons from germinal source in pacode

Central trigeminal fibers

Pontine NEP R2 RHOMBENCEPHALON


PROPOSED RHOMBOMERE IDENTITIES IDENTITY PROPOSED RHOMBOMERE R2


ABBREVIATIONS: CTF - Cerebellar transitional field GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from germinal sources.


282

PLATE 131A



ENTIRE SECT IO


Labeled on this page: Peripheral neural and non-neural structures, brain ventricular divisions N IS FR OM LEFT

SIDE

BR

A

IN


OF

Cell dense mesenchyme


EYE

gress ively m or

Olfactory placode

e later al

Sprouting Nerve I (olfactory) axons

Pigment epithelium Intraretinal space Retinal NEP

Mandibular arch (I)

An ter ior


car din al

vei n?


tio n

f sec ne o

Trigeminsl ganglion (V) placode?

Pla

An ter ior ca rd in al ve in ?

Maxillary process

s h ift

Sphenoid bone?


Hyoid arch (II)

s pro

Choroid fissure

283

PLATE 131B Labeled on this page: Central neural structures

PROSENCEPHALON (FUTURE TELENCEPHALON)

MESENCEPHALON



Basal telencephalic and lateral basal ganglionic NEP?


Migrating tectal neurons

Peripheral neural structures structures originoriginating from the ating from central nervous central nervous system: system: Opthalmic germinal zones Pigment epithelium germinal zone Retinal NEP Optic nerve (II) GEP?

All parts of the NEP in this section form expanding shorelines of the superventricles as stockbuilding NEP cells increase. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from germinal sources.



HYPOTHALAMUS, MESENCEPHALON, AND RHOMBENCEPHALON PROPOSED RHOMBOMERE IDENTITIES R2 R3 R4 R5 R6 R7


284

PLATE 132A GW5 Sagittal, CR 7.1 mm, C8966 Level 2: Slide 5, Section 2

PLATE 132B

Cerebellar NEP

Upper rhombic lip

(vermis)

Mesencephalic (tectal, inferior collicular) NEP

ABBREVIATIONS: FONT KEY: NEP - Neuroepithelium ventricular divisions - capitals R - Rhombomere Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

Medullary velum

tectal pool

Trochlear nuclear NEP?

metencephalic pool

isthmal canal


Lower rhombic lip

Reticular NEP?


Migrating pontine reticular formation neurons?

EP

la


Trigeminal NEP

Facial R3? sensory NEP

Medullary NEP Upper

R4

Vestibuloauditory NEP

R5

Vestibuloauditory NEP

Migrating facial sensory (VII) neurons

a t h o p H y


R2

Migrating trigeminal nuclear complex (V) neurons

ic

Me se


Pontine NEP

Middle

hypothalamic pool

Rathke's pouch epithelium (primordium of anterior pituitary gland) Oral cavity

myelencephalic pool

Migrating vestibular and auditory (VIII) neurons

R6

Glossopharyngeal NEP

Lo we r


Isthmal NE P

m

ic (tegme hal nt p e c n

EP )N al


N

tegmental pool

(future aqueduct)

R7 Vagal sensory NEP

Migrating vagal sensory (X) neurons Migrating solitary nuclear neurons (IX glossopharyngeal receptors)

Nerve X Nerve X boundary (vagus)? cap (Schwann cell GEP?)

Sympathetic trunk ganglia?

Oral cavity

Primordia of tongue Mandibular arch (I)

Hyoid Pharynx arch (II)

285

CEREBELLUM AND PONS



Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.


286

PLATE 133A

PLATE 133B

Otic vesicle


*Note that incoming afferent axons do not form a characteristic "bulge" at the brain surface as in the other GW5 specimen (C8314). That indicates C8966 is less mature than C8314, and massive fiber ingrowth occurs during GW5.


Migrating vestibular and auditory (VIII) neurons from R4 NEP

R4

vestibuloauditory NEP


Pioneer VII and VIII afferent axons enter brain*

Nerve VIII (vestibulocochlear) Schwann cells plentiful Schwann cells scarce

Pontine NEP

R3

Cerebellar notch


facial sensory NEP

Schwann cells migrate into peripheral nerve from GEP in boundary cap?

R2




Nerve VII boundary cap

Ep ith eli um

Lumen


trigeminal NEP

CTF2 (deep nuclear neurons) CTF1 (fibers)

tion l ina ana ag ) c c inv de Oti itial laco e in ic p s th ot ark the (m of

Lower rhombic lip


Migrating facial sensory (VII) neurons from R3 NEP Migrating trigeminal nuclear complex (V) neurons from R2 NEP


Nerve VII (facial, full of Schwann cells) Facial ganglion (VII)?


Arrows indicate the presumed direction of neuron and glia migration from germinal sources.

287

288

PART PARTXI: XI: GW4.5 GW4.5 CORONAL CORONAL

This specimen is embryo #2300 in the Minot Collection, designated here as M2300. The crown-rump length (CR) is 6.3 mm estimated to be at gestational week (GW) 4.5. M2300’s prosencephalic and anterior mesencephalic sections are cut (8 µm) in the coronal plane, but the plane shifts to predominantly horizontal in the posterior mesencephalon, pons, and medulla. We photographed 48 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and medulla. Fourteen of these sections are illustrated in Plates 134AB to 146AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of M2300’s brain and optic vesicle (Figure 10), and to show each illustrated section in situ (insets, Plates 134A to 146A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. The prosencephalon is considerably smaller than at GW5 (Part IX) with a stockbuilding neuroepithelium surrounding a small prosencephalic superventricle. Anterior sections are tentatively identified as the future telencephalon, while sections of the optic vesicle and posterior to it are more clearly identified as diencephalic. Cell migration is absent in both future telencephalic and diencephalic parts of the prosencephalon. The olfactory placode forms a thick epithelium in the anterolateral surface of the head that is much closer to the brain than at GW5. There is a thinner epithelium connecting the olfactory placode and the lens placode. The evaginated optic vesicle is just beginning to curve around the lens placode. However, a definite inner retinal neuroepithelium (thick with presumptive glial chan-

nels adjacent to the lens) and an outer pigment epithelium (thin) can be differentiated. The mesencephalon contains a stockbuilding neuroepithelium in the pretectum and tectum; cell migration has not yet begun. The tegmental and isthmal neuroepithelia, thicker than at GW5, are stockbuilding their stem cell populations. Only a few pioneer neurons have migrated out, perhaps these are sequestered in the outer parts of the neuroepithelium itself rather than outside it. The subpial fiber band is very thin in the tegmentum, but thickens slightly in the isthmus. The most prominent neuroepithelial structures in the rhombencephalon are the rhombomeric evaginations. In this specimen, several sections show how closely rhombomeres are associated with sensory cranial ganglia. The trigeminal ganglion (source of V sensory axons) is nearly attached to the brain surface at rhombomere 2. The vestibulocochlear ganglion (source of VIII axons) is attached to the rhombomere 4 brain surface. The otic vesicle touches the rhombomere 5 brain surface. The short nerve extending from the large vagal ganglion (source of X sensory axons) touches the rhombomere 7 brain surface. The subpial fiber band is thin throughout the rhombencephalon; even though sensory axons are touching the brain, they have yet to enter it. A very thin layer of migrating neurons lines the superficial border of some rhombomeres; for the most part, cell migration has not yet started. The small stockbuilding cerebellar neuroepithelium is only identifiable in the most posterior sections of the rhombencephalon. There are no migrating neurons outside the cerebellar neuroepithelium, only a thin cell-free fibrous layer.

289

M2300 Computer-aided 3-D Brain Reconstructions A. B. s

H

me du l

th ala m

po

P

ll R7

1. Medullary

we

P R O

S

la

a ull ed rm

3. Mesencephalic

1

4. Diencephalic

Ventral rhombic lip

Spinal cord

Spinal cord

Optic vesicle

R2

Figure 10. A, The left side of the 3–D model viewed from the front at a 45º heading; this view is used to "peel away" sections of each level in the following Plates. B, A straight view of the left side. C, A straight down view of the top. D, An upward view of the bottom, angled (120º) to look into the mesencephalic and diencephalic flexures.

P

n

Optic vesicle

P

s

Futu r e b a s

R6

R7

la

in a l

C o

med u

l

R5

er

r medu ll pe

U

p

a

ow

s o

P

n s o

t h a l a

m

n

u s

a r e a

m e g m e n t u m u s h I s t

P

al

T

ce

o

en

H y p

F

b asal tel

P r e o p t i c

ut u re sept u m ph ng li a a l o n

ture

R4

r

R3

ga

N C E P H A S E L O O R

Scale bars = 0.5 mm

Upper rhombic lip

R2

N

Fu

m u s C e r e b e l l u m

Su

h

In

fer

perior colliculu

ior colliculu

s

s

r e t e c t u m

s

t h a l a m p i u E

a l a m u T h s

re telencephalo n Futu

R P

Bottom view

o

t

s

L

O

I P

D.

Medullary velum

N C E P H A L S E O

N

Top view

d

C.

Medullary velum

Lo

Low er

R6

a

BRAINSTEM FLEXURES

R7

R5

ed u

R6

e d u ll a

R3 R4

Infundibulum Mammillary body

Dorsal rhombic lip

R2

per m Up

Up

rm

3

o n s

Sub th al Te

O

R

s n

N O

S u bthal am Hy pothalam us us T

o

R4 R5

pe

Preoptic area

Rhombomeres

R3

P

Hy

P

E P H A N C L

Optic vesicle

m llu

s

Pr eoptic area

gme

4

Future telencephalon

R2

Inferior colliculus

s hmu Ist

h mu

Optic vesicle

E

Epithalamu s lamus u Tha am

e reb Ce

m en

L

Superior colliculus

N

O

um nt

us t e l F uture e n c ep h alo n

eg

llum e be Ce r

Thal am

Pretectum

A

Inferior colliculus

tu m I s t

s mu

Epithala

Side view

S E N C E P

Pretectum

us

Superior colliculus

Angled front view

S

p

290

PLATE 134A


GW4.5 Coronal CR 6.3 mm M2300


Blood islands in developing meninges

Olfactory placode


Level 2: Section 35


Blood islands in developing meninges Frontonasal process?

Olfactory placode

Mesenchymal densities between placode and future central olfactory structures


Preplacodal epithelium

291

PLATE 134B


Level 1: Section 5 ANTERIOR PROSENCEPHALON








Level 2: Section 35 PROSENCEPHALON Prosencephalic roof plate


Prosencephalic NEP (future thalamic)





Prosencephalic NEP

(future preoptic/hypothalamic)


NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


292

PLATE 135A



Primordial mesenchymal brain case (skin, bone, and meninges) Dense mesenchyme between optic vesicle and brain

OPTIC VESICLE Pigment epithelium Intraretinal space Retinal NEP Lens placode

Future oral cavity

Preplacodal epithelium

Mandibular arch (I)

Lateral tongue primordia Part of the mandibular arch placodal epithelium gives rise to the thyroid gland.


Figure 247A modified (Patten, 1953, p. 429.)

Frontal prominence Optic vesicle Olfactory placode Future oral cavity Tongue and mandible primordia

Maxillary process Mandibular arch (I)

Hyo-mandibular cleft Hyoid arch (II) Arches III and IV


293

PLATE 135B




(future choroid plexus in roof of third ventricle)


Thalamic NEP


SUBTHALAMUS

Subthalamic NEP Preoptic area NEP

Subthalamic primordial plexiform layer Preoptic area primordial plexiform layer


optic recess

Anterior hypothalamic NEP Diencephalic floor plate (future chiasmal GEP)

Glial channels in retinal NEP?

Hypothalamic primordial plexiform layer Chiasmal glial channels?

PREOPTIC AREA/ HYPOTHALAMUS



294

PLATE 136A




Dense mesenchyme between optic vesicle and brain

OPTIC VESICLE Pigment epithelium Intraretinal space Retinal NEP Lens placode

Maxillary process Future oral cavity

Preplacodal epithelium Lingual epithelium Lateral tongue primordia

Mandibular arch (I)

Arterial trunk

Hyoid arch (II)



295

PLATE 136B

Central neural structures labeled DIENCEPHALON THALAMUS/EPITHALAMUS


(future pineal gland?) Brain surface (heavier line)

Thalamic/epithalamic NEP

SUBTHALAMUS

Subthalamic NEP


pineal recess?

Thalamic/epithalamic primordial plexiform layer



optic recess

Anterior hypothalamic NEP Diencephalic floor plate (future chiasmal GEP)

Hypothalamic primordial plexiform layer Chiasmal glial channels?

PREOPTIC AREA/ HYPOTHALAMUS



296

PLATE 137A


GW4.5 Coronal CR 6.3 mm M2300 Level 5: Section 85 Formative superarachnoid reticulum (cell sparse)


Sphenoid primordium?

OPTIC VESICLE

Pigment epithelium Intraretinal space Retinal NEP

Maxillary process Rathke's pouch epithelium (primordium of adenohypophysis) Future oral cavity

Preplacodal epithelium Lingual epithelium Lateral tongue primordia

Mandibular arch (I)

Medial tongue primordia

Hyoid arch (II)

Arch III

Arterial trunk

Arch IV?

Laryngo-tracheal groove



297

PLATE 137B




(future pineal gland?)

Thalamic/epithalamic NEP SUBTHALAMUS

Subthalamic NEP

optic recess

pineal recess?





Hypothalamic primordial plexiform layer infundibular recess

Middle hypothalamic NEP Diencephalic floor plate

(future median eminence and neurohypophyseal GEP)

HYPOTHALAMUS



298 Peripheral neural and non-neural structures labeled


Formative superarachnoid reticulum (cell sparse)



d oi ? en dium h Sp mor i pr

Rathke's pouch epithelium (primordium of adenohypophysis)

Maxillary process

Future oral cavity Preplacodal epithelium Lingual epithelium

Mandibular arch (I)

Lateral tongue primordia Medial tongue primordia

Hyoid arch (II)

Laryngo-tracheal groove

Arch III Arch IV?

Ph

ary


nx Vagal ganglion (X) placode? Vagal ganglion (X)

Dorsal aorta

Notochord


299 Central neural structures labeled MESENCEPHALON

Pretectal/tectal primordial plexiform layer

PRETECTUM/TECTUM



Pretectal/tectal NEP

PLATE 138B

Pioneer posterior commissure fibers



(future aqueduct)

TEGMENTUM

Tegmental NEP

Tegmental primordial plexiform layer First tegmental neuronal migration Hypothalamic primordial plexiform layer

Posterior hypothalamic (mammillary) NEP

mammillary recess

Diencephalic floor plate HYPOTHALAMUS

DIENCEPHALON

Peripheral neural structure Migrating vagal ganglionic neurons from germinal source in vagal placode?



300

PLATE 139A


GW4.5 Coronal CR 6.3 mm M2300 Level 7: Section 115 Primordial mesenchymal brain case (skin, bone, and meninges)


Cell-sparse formative superarachnoid reticulum

id ? no m he iu Sp ord im pr

Fused maxillary process and mandibular arch (I)

Notochord

Trigeminal ganglion (V) placode?

Future oral cavity Lingual epithelium Medial tongue primordia

Hyoid arch (II)


Arch III

Vagal ganglion (X) placode?

Arch IV


Arytenoid swelling

Glottis Larynx

Vagal ganglion (X)

Pharynx

Dorsal aorta Sympathetic trunk?

Notochord

Dorsal root ganglion Dermatome


Dorsal root of spinal nerve

301



TECTUM



Tectal NEP

PLATE 139B


(future aqueduct)

TEGMENTUM Tegmental primordial plexiform layer

Tegmental NEP Mesencephalic floor plate

Medial lemniscus?

(midline raphe glial structure GEP?)


MESENCEPHALON

Peripheral neural structures Migrating trigeminal ganglionic neurons from germinal source in trigeminal placode?

Migrating vagal ganglionic neurons from germinal source in vagal placode?

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

SPINAL CORD Ventral funiculus Ventral gray Lateral funiculus

FONT KEY: ventricular divisions - capitals central canal Germinal zone - Helvetica bold Transient structure - Times bold italic Dorsal Permanent structure - Times Roman or Bold funiculus

Spinal floor plate

(ventral commissural GEP)

Ventral NEP

Intermediate NEP Dorsal NEP Spinal roof plate


302

PLATE 140A



Fused maxillary process and mandibular arch (I) Anterior cardinal vein




Hyoid arch (II)


Cell-sparse formative superarachnoid reticulum Arch III


Facial ganglion (VII) placode? Glossopharyngeal ganglion (IX)?

Glossopharyngeal ganglion (IX) placode?

Notochord

Vagal ganglion (X) Dorsal aorta Sympathetic trunk?

Notochord




Dermatome



MESENCEPHALON


TECTUM

Mesencephalic roof plate Tectal NEP


isthmal canal

TEGMENTUM/ISTHMUS

Tegmental/isthmal NEP

Tegmental/isthmal primordial plexiform layer Migrating isthmal neurons Migrating central trigeminal neurons


PONS

PLATE 140B

R2 (trigeminal NEP)

Presumptive migrating ganglionic neurons from germinal sources in branchial placodes?


Trigeminal ganglionic neurons

Medial pontine + R3 NEP

(abducens [VI], facial motor [VII]?)

Migrating abducens (VI) and facial motor (VII) neurons?

MEDULLA

Facial ganglionic neurons

Medial medullary NEP Migrating medial medullary neurons



Glossopharyngeal ganglionic neurons Vagal ganglionic neurons

Lower medullary NEP? RHOMBENCEPHALON



R3

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion.

SPINAL CORD Spinal floor plate Ventral funiculus Ventral gray

ABBREVIATIONS: Lateral GEP - Glioepithelium funiculus NEP - Neuroepithelium R - Rhombomere central FONT KEY: canal ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Dorsal Permanent structure - Times Roman or Bold funiculus


Ventral NEP



304

PLATE 141A



Trigeminal boundary cap (Schwann cell GEP?)



Vestibulocochlear ganglion (VIII)?


Facial ganglion (VII)? Hyoid arch (II)


Glossopharyngeal ganglion (IX) Glossopharyngeal ganglion (IX) placode?

Vagal ganglion (X)

Dermatome




305



MESENCEPHALON


TECTUM



Tectal NEP

Tegmental/isthmal primordial plexiform layer

(future aqueduct)

Migrating tegmental/isthmal neurons Migrating trigeminal (V) neurons Central trigeminal tract

isthmal canal

TEGMENTUM/ISTHMUS

Tegmental/isthmal NEP

PONS


R2 (trigeminal NEP) rhombencephalic superventricle (future fourth ventricle)


Migrating facial sensory (VII) neurons? R4 (vestibulo-auditory NEP)

Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R3 Facial sensory NEP germinal source of sensory neurons that receive input from the facial (VII) ganglion. R4+5 Vestibulo-auditory NEP - germinal sources of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.


R5 (vestibulo-auditory NEP) Migrating auditory and vestibular neurons

Lower medullary NEP

Migrating vestibulocochlear ganglionic neurons (VIII) from germinal source in otic vesicle epithelium Arrows indicate the presumed direction of neuron migration from germinal sources.


(vagal motor [X], hypoglossal [XII], blends with ventral spinal NEP)

MEDULLA

PLATE 141B




RHOMBENCEPHALON

SPINAL CORD Spinal floor plate


Ventral funiculus Ventral gray

Ventral NEP


Intermediate gray ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere

Lateral funiculus Dorsal gray

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Dorsal Transient structure - Times bold italic Permanent structure - Times Roman or Bold funiculus

Intermediate NEP central canal


306







Trigeminal ganglion (V) VIII nerve boundary caps* Vestibulocochlear ganglion (VIII) Migrating vestibulocochlear ganglionic neurons from germinal source in the otic epithelium Hyoid arch (II)


Otic vesicle Epithelium Lumen

Glossopharyngeal ganglion (IX)

Vagal ganglion (X)

Dermatome



307



MESENCEPHALON

PLATE 142B


TECTUM



isthmal canal


ISTHMUS

Isthmal NEP


CEREBELLUM

R2

Cerebellar NEP

Migrating cerebellar deep nuclear neurons R3

PONS Central trigeminal tract


R2 (trigeminal NEP) Migrating trigeminal (V) neurons

R4


R3 (facial sensory NEP) Migrating facial sensory (VII) neurons?

R5

R4 (vestibulo-auditory NEP) R6 R5 (vestibulo-auditory NEP)

R6 (glossopharyngeal) NEP

Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

Migrating auditory and vestibular neurons Migrating glossopharyngeal receptor neurons (solitary nucleus?)

Lower medullary NEP



MEDULLA

RHOMBENCEPHALON


SPINAL CORD Ventral gray Ventral funiculus?

Intermediate gray Lateral funiculus

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: Dorsal ventricular divisions - capitals funiculus Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Ventral NEP

Spinal germinal Intermediate zones NEP

central canal


308

PLATE 143A




VII nerve boundary cap?* Vestibulocochlear ganglion (VIII) VIII nerve Migrating vestibulocochlear ganglionic neurons from germinal source in the otic epithelium

VIII nerve boundary cap*



Vagal ganglion (X)

Dermatome Dorsal root of spinal nerve


309

PLATE 143B

Central neural structures labeled MESENCEPHALON TECTUM?


Mesencephalic roof plate Posterior tip of tectal NEP?

Tectal primordial plexiform layer? mesencephalic superventricle (future aqueduct)

ISTHMUS isthmal canal


Isthmal NEP CEREBELLUM


Fibrous layer in superficial cerebellum

R2

Cerebellar NEP

PONS

R3


R2 (trigeminal NEP) Central trigeminal tract


R3 (facial sensory NEP) Migrating facial sensory (VII) neurons?

R4

R4 (vestibulo-auditory NEP) Migrating vestibular and auditory neurons

R5


R6

R6 (glossopharyngeal) NEP Migrating glossopharyngeal receptor neurons (solitary nucleus?) R7 R7 (vagal sensory) NEP Migrating sensory vagal neurons?

Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


Lower medullary NEP

MEDULLA


RHOMBENCEPHALON

Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere

Ventral funiculus? Ventral gray

SPINAL CORD Ventral NEP

Lateral funiculus Intermediate gray

Dorsal gray


Dorsal funiculus

Intermediate NEP Spinal central canal


germinal zones

310

PLATE 144A



Remnant of otic placode Primordial mesenchymal brain case (skin, bone, and meninges)


Nerve X boundary cap (Schwann cell GEP?) Nerve X (vagus)

Nerve XI (spinal accessory)


311

PLATE 144B

Central neural structures labeled RHOMBENCEPHALON CEREBELLUM

Medial metencephalic roof plate


Vermis

Cerebellar NEP


Hemisphere

metencephalic pool

Lateral metencephalic roof plate in upper rhombic lip


PONS


Auditory (cochlear) NEP? Migrating cochlear nuclear neurons?

Medullary velum

Lateral myelencephalic roof plate in lower rhombic lip PROPOSED RHOMBOMERE IDENTITIES


R4

Migrating vestibular and auditory neurons R5 (vestibulo-auditory NEP)

R5

myelencephalic pool

R6 (glossopharyngeal) NEP Migrating glossopharyngeal receptor neurons (solitary nucleus?)

R6

R7

R7 (vagal sensory) NEP Migrating sensory vagal neurons?

Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

Midlateral medullary NEP

(blends with intermediate spinal NEP)

MEDULLA Lateral funiculus? Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere

Intermediate gray

SPINAL CORD Intermediate NEP

central canal Dorsal gray Dorsal funiculus




312

PLATE 145A





Nerve IX boundary cap (Schwann cell GEP?)


313

PLATE 145B



Medial metencephalic roof plate


Vermis

Cerebellar NEP


metencephalic pool

Hemisphere

Lateral metencephalic roof plate in upper rhombic lip

Medullary velum

MEDULLA


Precerebellar NEP


Migrating precerebellar neurons

Lateral myelencephalic roof plate in lower rhombic lip

Migrating vestibular and auditory neurons R5 (vestibulo-auditory NEP)

R6 (glossopharyngeal) NEP Migrating glossopharyngeal receptor neurons (solitary nucleus?)

myelencephalic pool

R7 (vagal sensory) NEP? Migrating sensory vagal neurons? PROPOSED RHOMBOMERE IDENTITIES


(blends with dorsal spinal NEP)

R5

Cuneate nuclear NEP? Migrating gracile and cuneate nuclear neurons?

R6

Gracile nuclear NEP?


R7

Medial myelencephalic roof plate

Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei. Arrows indicate the presumed direction of neuron migration from germinal sources.


314

PLATE 146A





315

PLATE 146B


RHOMBENCEPHALON Medial metencephalic roof plate metencephalic pool

Medullary velum


MEDULLA

Precerebellar NEP Migrating precerebellar neurons myelencephalic pool Cuneate nuclear NEP?

Lateral myelencephalic roof plate in lower rhombic lip



NEP - neuroepithelium Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Migrating gracile and cuneate nuclear neurons? Gracile nuclear NEP?



316

PART PARTXII: XII: GW4 GW4 SAGITTAL SAGITTAL

Carnegie Collection specimen #9297 (designated here as C9297) with a 4.5-mm crown-rump length (CR) is estimated to be at gestational week (GW) 4. C9297 was embedded in a celloidin/paraffin mix and was cut in 8-µm sagittal sections that were stained with azan. Various orientations of the computer-aided 3-D reconstruction of C836’s brain are used to show the gross external features of a GW4 brain (Figure 11). Like most sagittally cut specimens, C9297’s sections are not parallel to the midline; Figure 11 shows the approximate rotations in front (B) and back views (C). We photographed 18 sections at low magnification from the left to right sides of the brain. Nine of the sections, mainly from the left side of the brain, are illustrated in Plates 147AB to 155AB. Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify the approximate midline, non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (lowcontrast images) identify central neural structures. Plates 132AB to 133AB show high-magnification views of the rhombencephalon. The prosencephalon is the smallest major brain structure with little distinction between a future telencephalon and diencephalon. The entire prosencephalic neuroepithelium is rapidly stockbuilding its various populations of neuronal and glial stem cells. The lamina terminalis in the ventral prosencephalon marks the site of closure of the anterior neuropore. A cell-dense area adjacent to the olfactory placode may be supporting cells associated with growth of the olfactory nerve toward the brain. The mesencephalon is much smaller than at GW5. The stockbuilding pretectal and tectal neuroepithelia have a relatively short anteroposterior length and blend with the

presumptive cerebellar neuroepithelium in the dorsomecial rhombencephalon. The stockbuilding tegmental and isthmal neuroepithelia form a distinctive arch between the mesencephalic and diencephalic flexures. There is a very thin subpial fiber band in the tegmentum and isthmus. The rhombencephalon is the largest brain structure. Rhombomeres 2 through 7 form well-defined swellings in the lateral neuroepithelium. Most sensory cranial ganglia and the otic vesicle are located directly lateral to the rhombomeres with which they interact. The trigeminal ganglion (V afferents) appears in sections lateral to the last section that contains rhombomere 2. The vestibulocochlear ganglion (VIII afferents) is lateral to the last section that contains rhombomere 4; the otic vesicle is lateral to the last section that contains rhombomere 5. The presumptive superior glossopharyngeal ganglion (IX afferents) is lateral to the last section with rhombomere 6, and the large superior vagal ganglion (X afferents) is lateral to the last section with rhombomere 7. The association of rhombomere 3 with the sensory part of nerve VII is less clear, but the facial ganglion (VII afferents) is near its presumptive placodal source in lateral sections. Each rhombomere has a thin layer of pioneer migrating neurons that are only visible in most lateral sections, where the outer edges of the rhombomeric neuroepithelium are cut tangentially. Virtually no fibers have yet entered the brain from these ganglia. Sections near the midline show a smooth neuroepithelium. Some migrating cells are outside the lower medullary neuroepithelium, and the subpial fiber band is thicker as the brain blends with the spinal cord. The cerebellum stands out as the most immature and smallest rhombencephalic structure. The most lateral sections show a very thin layer of migrating neurons outside the cerebellar neuroepithelium.

317

EXTERNAL FEATURES OF THE GW4 BRAIN

Ve n tr

al die n c

ep

entu

Is

3

thm

s

h

u C

us

Rhombomere (R) 2

n

T

on

4

ct

m ellu eb er

Optic vesicle

m eg

Te

m

C E P H N E e n c e p hal o l Te

n

Pretectum

l n rsa Do phalo n ce d ie

m

A

L

O

N

al

Side view

P R

A.

O

S

P

o

R3 R4

R5

M

A perfect sagittal cut through the brain is parallel to the midline from anterior to posterior. Sections of C9297's brain rotate an estimated 9.6º counterclockwise from the midline, 4.8º to the left Upper side of the anterior midline (B, rhombic front view), and 4.8º to the right lip side of the posterior midline (C, back view). In the sections illustrated on the following pages, anterior parts (top and left) are tilted toward the observer, while posterior parts (bottom and right) are tilted away from the observer. Medullary velum

e d

R6

BRAINSTEM FLEXURES R7?

l a u l

1

MESENCEPHALON

Front view

3. Mesencephalic 4. Diencephalic

Lower rhombic lip

Anterior midline

B.

1. Medullary

Spinal cord

C.

Pretectum

Back view

Tectum

Left side -4.8º

e e b l l

Right side

Posterior midline

e

u

r

PROSENOptic CEPHALON vesicle

Optic vesicle

C

m

Right side

R4 R5

Left side

Pons

R6 Rhombic lip border

R7?

RHOMBENCEPHALON

Spinal cord

Scale bars = 0.25 mm

Figure 11. A, The lateral view of the left side of a computer-aided 3-D reconstruction of the brain and upper cervical spinal cord in C836, the following GW4 specimen, which has a similar crown-rump length to C9297 (4.5 mm and 4.0 mm, respectively). External features are identified as in Figure 12B. The heavy numbered lines refer to brainstem flexures (boxed key). B, Front view of the brain in A. The angled line shows how C9297's sections rotate left (arrow) from the anterior midline. C, Back view of the brain in A. The angled line shows how C9297's sections rotate right (arrow) from the posterior midline.

Medullary velum

Medulla

-4.8º

Spinal cord

Medulla

318

PLATE 147A

GW4 Sagittal, CR 4.5 mm, C9297 Level 1: Slide 4, Section 32 LE



FT

(future aqueduct)

SI

DE

O

F

A

I

ncepha d

ie

metencephalic pool

A RE E A


IN


DL

li

c

p su


MI

elenceph re t a tu ventricl lic fu super e

er

ve

ntri

cle

BR

N


fu

tu

r

e

Medullary velum

Cephalic preplacode

al Or ty vi ca

uee nggu toton oof f diaia oordr imim PPr r

Mandibular arch (I)

Hyoid arch (II)



Branchial placodes

Medullary velum

Pharynx

RIG HT

SI D

Larynx?

myelencephalic pool E

OF BR A

Gut

IN ND

SP

IN

A

L

C

O

RD


A

Vertebral/ basilar artery?

PLATE 147B

SENCEPHAL ME ON

ct ete Pr

P al NE




Epithalamic

PR OS E

Dorsal diencephalic NEP

Isthmal NEP

NC E


P Anterior

) ALON

GE

P


Fibrous processes

R H O M B E N C E P H A L O

A N D DIENCEPH

Posterior

a

ine ra p he gl i al

CE

Preoptic

h

l

dl

EN

ON

t

a

Mi

EL

o H y p

AL

i c

Ventral diencephalic NEP

Pontine NEP


ET

ro


Upper rhombic lip

m

Basal telencephalic and septal

se

(F U T U R

Cerebral cortical

M id dl e

ALON

h ncep alic NE

P

PH

PH


Thalamic


Upp


er

Migrating medullary (reticular formation?) neurons Ascending fiber tracts from spinal cord


N


L

fu n

ic u l

Lower rhombic lip Precerebellar NEP?

E P Low er

lu s n ic u a l fu r t n Ve y gra tr a l Ven

al a te r

Reticular, raphe, vagal motor (X), and hypoglossal (XII) NEPs?

N

M e d u l l a r y


y gra ate i d e erm Int

us

ray sal g Dor us nicul al fu Dors

Gracile and cuneate nuclear NEPs?

Spinal NEP

AL S PIN

D COR


All parts of the NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase.

319

320

PLATE 148A

GW4 Sagittal, CR 4.5 mm, C9297 Level 2: Slide 4, Section 24 P lan



eo

fs

ec

ti o

(future aqueduct)

ns

hi

fts

pr og re s

y

le

vl

r

ic

ate el or m


n

t

r

v

e

pe alic su

l

metencephalic pool

p

h


ra

elenceph re t a tu ventricl lic fu super e

si

en future di

c

e


Cephalic preplacode

l

of


ton

O F

e

v i t y c a

gu

B R A I N

Hyoid arch (II) Notochord

myelencephalic pool

Pharynx

MID LIN

E RI G HT

S

Medullary velum

ID E

Basilar artery?

OF BR AI N

AN

D

SP

AL IN

CO

RD

al tr al n ce can

Labeled on this page: Peripheral neural and nonneural structures, brain ventricular divisions

S I D E

ia

a

ord

r

im

Plates 156A and B: prosencephalon Plates 157A and B: isthmus, cerebellum, and pons Plates 160A and B: midline raphe glial structure

Branchial placodes


O

Pr

Mandibular arch (I)

See the following for higher magnification views of this section.

F T L E

Medullary velum

321

PLATE 148B

CEPHA MESEN LO N ctal NEP ete Pr


m

SE N CE P

T

h

a

l

a

HA L PH

LO

NA

i c


(reticular, raphe, abducens [VI], and facial motor [VII]?)

l o H y p Preoptic

ND

DIE N

Anterior

C E P H A LO N )

t

h

Upper rhombic lip

Pontine NEPs

a

a

r

P

CE A


m


Cerebellar NEP


M id dl e

nc e os

EN

Basal telencephalic and basal ganglionic

Isthmal NEP

Migrating pontine (reticular, raphe, abducens [VI], and facial motor [VII]?) neurons

r U ppe


Medullary NEPs

(reticular, raphe, vagal motor [X], and hypoglossal [XII]?)


B E N C E P H A L O N O M R H

(F U T URE TEL

ephalic NEP

ON


Cerebral cortical

Posterior

c

Dorsal diencephalic NEP i

PR O

ic al EP ph ) N e l c a en nt es e M egm (t Migrating tegmental neurons

Epithalamic

Pioneer migrating cerebellar deep nuclear neurons?



Lower


Fibrous processes

Midline raphe glial GEP

Lower rhombic lip

tral Ven e diat rme Inte


Spinal NEP

sal Dor

I N S P

A

L

C

O

R

D


322

PLATE 149A



(future aqueduct)

eo fs

enc di

l r a

S I D E

f


L E F T

c a v i t y

Hyoid arch (II)

ral



O

Branchial placodes

e late mor

Medullary velum

Cephalic preplacode

Mandibular arch (I)

ivly ress rog ts p

metencephalic pool


e


h if

futur

ns

ephal


tio


ec

telencep re rventr ha l ic e tu le i u sup

an

c

Pl

ic superventricle


Medullary velum

Lung bud?

Pharynx

O F B

myelencephalic pool

R

A

I

N

A N

Notochord

D S P

I A

L C

O

R

D

E L I N M I D

a

c

n

N

r n t e c

l

a

l

a

323

P

Te gm en ta l

ic m

Migrating tegmental neurons?

halamic



Cerebellar NEP

Subt ior

i c

er

m

po

st

l a

P

e/

a

dl

h

M

id

Anterior


R2

Migrating facial (VII) neurons from R3 NEP


R5

Migrating medullary neurons

Medullary NEP

r

l ra ? nt te e ia V ed rm te In

r

PR O S EN CE P

te pla r ) o Flo GEP? ( All parts of the NEP form expanding shorelines of the superventricles as stockbuilding NEP cells increase.

R6

Lo we

R6


l medullary NEP (reticula r , r a Media phe r [X], and hypoglossal , [XII l moto ] ?) vaga

R5

Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

Upper

R4

Pontine NEP

R3


R3

Medial cerebellar notch? Upper rhombic lip

R4


R2


C E P H A L O M B E N O N R H

TELENC TURE EPH (FU AL N O ON L a A l h i p c e AN NE H enc s P D o

Ventral Basal telencephalic diencephalic and basal NEP ganglionic

t H y p o

E P


Cerebral cortical

Preoptic

L

la

P NE

Pioneer migrating cerebellar deep nuclear neurons

N

a

Epithalamic

Mesencephalic NEPS

l

Th

T e c t

a

N) LO A PH CE N E DI

tectal NEP re

PLATE 149B

A

ON


MESE NCE PH

R

oof

pla

Sp

in

N al Do

te

C I N S P

A

L

EP

l rsa

O

R

D

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

PLATE ?A 150A

GW4 Sagittal, CR 4.5 mm, C9297 Level 4: Slide 4, Section 8 P lan

rv pe

en

cephal

ic

su

hi

fts

pr og re s

s

ral

di

pr su ose pe nc rv ep en ha t r l ic ic l e

ns

metencephalic pool Medullary velum

O F

B R A I N

c a v i t y

Branchial placodes

S I D E

l r a

Hyoid arch (II)


O

Mandibular arch (I)


Cephalic recess preplacode

F T L E

optic

Lung bud?

ti o

Carotid artery?

futu r e


ec

ate

Cell-sparse superarachnoid reticulum Rathke's pouch epithelium (primordium of anterior pituitary gland)

fs

el or m

ic al ph cle

le

eo

ly iv

e n t ri c


future t e super len ve ce nt ri

324

Arch III

Pharynx

A N D

myelencephalic pool

S P I N

A


L C

O R

Vertebral/basilar artery?

D

325


H

ic


ar e ll b re P Ce NE

ic


EP


EP

P

r

SENCEPHALON ( PRO F UT UR R7

Lower rhombic lip


Ascending fiber tracts from spinal cord?

nuc lear NEP

R6

Migrating spinal motor neurons? Ventral funiculus

a l t r n V e


Medullary NEP

R7?


Lower

R5

R6

ac ile an dc un eat e

R4

R5

s p

i n

a

l

N

i n

E

a

Gr

P

l

N

E

P p E s N e t l a i a i n e d p m D r s t e R I n a l O s r C D o

I N S P

A

L

N C E P H M B E A L O N H O

R4



R


Upper

R3

Pontine NEP

R3


R2

Upper rhombic lip

R2


N le Optic vesic







Migrating pretectal and tectal neurons

lam

Diencephalic NEPs

Subtha

Cerebral cortical

m

an dG

LON AND EPHA DIE NC E NC L encephalic EP s TE o N H E A

P

la

O AL N

E

a Th

Tectal NEP

Pretectal NEP

Epithalamic

N) LO

PLATE 150B

MESENC EP

P

Arrows indicate the presumed direction of axon growth in brain fiber tracts. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

PLATE 151A



Labeled on this page: Peripheral neural and non-neural structures, Pl an brain ventricular eo fs divisions e ct


n io sh

ift

alic enceph rhomb ventricleicle) ventr supefr ourth

(f u t u r

Maxillary process?

e

O r

Mandibular arch (I)

a

l Arch III

L E F T

Hyoid arch (II)

v i t y c a

S I D E


l

Medullary velum

Cephalic preplacode

Branchial placodes

tera

Cell-dense mesenchyme

a re l mo

optic recess

metencephalic pool

ly siv res rog sp


myelencephalic pool

O F

Ph

ar

yn

x

Arch IV

B R A I N


Lung bud?

A N

D

Primitive gut?

S P

I

Vertebral artery?

N A L

Vertebral column

O

R

D

See Plates 158A and B for a high magnification view of the rhombencephalon.

C

326

327

PLATE 151B

Labeled on this page: Central neural structures Epithalamic

CE


an d

R2

os

e

P

eN Optic vesicl


)

Pontine NEP

R3 R4

R2 R3

R6

R7?

Lower rhombic lip


Ascending fiber tracts from spinal cord?

Lateral funiculus? Intermediate gray

In

Dorsal gray

Dorsal funiculus

S P

I N

A

r te

L

m

ed

D

o

ia

te

a rs

C

l

O

s

N

s

E

in

p

Ventral gray Ventral funiculus


P

al

N

EP

R7


Medullary NEP

Prece r e b e llar NE P?

R5


R6

Lower

R4

R5



Upper


C E P H A L O N B E N O M

D I EN C E P H A L O N

E

H

CE

AND


R

EN

ON

P laEr) rlN llaehere eb b e rer spP e) C Ce(ehemNi Espher mi Upper (he rhombic lip



nc

T U R E TEL

Cerebral cortical



P

ephalic N E

Diencephalic NEPs

Pr

(F U

AL

ic

G EP

EN

LO N

H

a

m la

t h alamic Sub

OS PR

PH A

Th

P

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

p

in

R

a

l

D



PLATE 152A


Plane of section shif ts p rog res s


ivl y

mo

re l

ate


ra l



L T EF

DE SI

a l c

y/Ph

Hyoid arch (II)

Nerve IX boundary cap?*

arynx

Arch III


Arch IV

Nerve X boundary cap?*


r io

rc

ar

d in

al v ein ?


Lung bud?

myelencephalic pool

vit



ra

Branchial placodes

Mandibular arch (I)

Nerve VIII boundary cap*

O

Nerve VII (facial) Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal) Nerve X (vagus)

a l ic

Maxillary process

eph

Nerve VII boundary cap?*

Preplacodal epithelium (maxillary)

An

)?

Primitive gut?

cc

es

so

ry

Vertebral column

Dorsal root ganglia Ne

AIN BR OF


nc icle e) mbe ntr tricl rho perveth ven su e four ur (fut

metencephalic pool

optic recess

te

328

r

XI ve

(s

n pi

al

a

Medullary velum

329

PLATE 152B

Labeled on this page: Central neural structures PROSENCEPHALON

C


Upper rhombic lip

R2

ne R3 N E

i

le

NE

P and

a lam

o

b th

Su

EP

e


nt

O p tic v e s i c

Cerebellar NEP


Po

ce sen pha ro rebral c

RHOMBENCEPHALON


G

P NE lic rtical

ic ?

(FUTURE TELENCEPHALON AND DIENCEPHALON)


P

P

R4

R6


r nuclear NEP?

R7

Cochlea



Lower rhombic lip

P N E


R5

y a r M e d u l l


PROPOSED RHOMBOMERE IDENTITIES R2 R3 R4 R5 R6 R7



330

PLATE 153A


Plane o f

se c

ti o n

shi

fts


pro

gre

ssi vly

mo re l

at

er a


l



optic recess


T LEF

Nasal process

Nerve V boundary cap?*

Nerve VIII boundary cap?*

EO

O

r

F BR

Maxillary process

al


SID


vi

ty

Germinal epithelium

ryn

Otic vesicle

x


Lumen

ha

Branchial placodes

/P

Hyoid arch (II)

AIN

ca

Mandibular arch (I)

Nerve IX boundary cap?*

Arch III

Nerve X boundary cap?* Cell-sparse superarachnoid reticulum

Arch IV



Vagal ganglion (X)



Sy


See a higher magnification view of the rhombencephalon in Plates 159A and B.

mp

at

tr tic he

un

k

g

g an

lia

?

331

PLATE 153B


PROSENCEPHALON (FUTURE TELENCEPHALON AND DIENCEPHALON) RHOMBENCEPHALON

Prosencephalic (cerebral cortical)

NEP


icle NE es

Optic v

Migrating trigeminal nuclear complex (V) neurons from R2 NEP Migrating facial sensory (VII) neurons from R3 NEP?

R2 R3?

P

GEP and






Pontine NEP

R6

Medullary NEP



PROPOSED RHOMBOMERE IDENTITIES R2 R3 R5 R6 R7

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


PLATE 154A

GW4 Sagittal, CR 4.5 mm, C9297 Level 8: Slide 3, Section 16 P l a n e o f s e c ti o n s


h ifts p

rogre

ssiv

Ante rio r

ly m ore lat era l

in rd ca


Labeled on this page: Non-neural structures, brain ventricular divisions

al

n? vei

optic recess

FT

SID

Nasal process

LE


EO

FB

Maxillary process

al

ca

vi

ty

Mandibular arch (I)

Y OD

Or

/P ha

ry

Branchial placodes

nx


Hyoid arch (II) Arch III Arch IV

ca

vei

n?

n

i t e

te

s

ri

or

nal rdi

s

o

m

A

332

R

e

m

n

a

n

t

s

o

f

333 Labeled on this page: Peripheral neural structures

PLATE 154B Optic vesicle

(opthalmic germinal epithelia)

Pigment epithelium?



Vestibulocochlear ganglion (VIII) Migrating vestibulocochlear ganglionic (VIII) neurons

e pi t

Olfactory placodal epithelium (germinal source of primary olfactory neurons)

Surface of optic evagination from the brain (heavier line)

h

e

Otic vesicle

u m l i

Lumen Luman

l


Retinal NEP?

m i n a e r

Sprouting nerve I (olfactory)

Oral and pharyngeal placodes

G

Intraretinal space

Retinal NEP?

Glossopharyngeal ganglion (IX) Placodal germinal source of ganglion IX? Placodal germinal source of ganglion X? Superior vagal ganglion (X)

Migrating vagal ganglionic (X) neurons? Inferior vagal ganglion (X)

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of neuron migration from germinal sources.

(vestibulocochlear ganglionic germinal epithelia)

334

PLATE 155A



Plane of section

s hifts p

rogre

ssiv ly m ore l

optic recess


Labeled on this page: Non-neural structures, brain ventricular divisions ate

ra

l


Nasal process

Maxillary process Mandibular arch (I) Branchial placodes

Hyoid arch (II)

Ph a rynx


Oral cavity

Arch III Arch IV?


it

e

s

R

em

na

n

ts

of

s

om

DY F BO EO SID FT LE


335

PLATE 155B

Labeled on this page: Peripheral neural structures Optic vesicle

Sprouting opthalmic branch of Nerve V (trigeminal) with plentiful glia

(opthalmic germinal epithelia)

Pigment epithelium

a l s p a ce

t


Retinal NEP Placodal epithelium (maxillary) Optic nerve (II) GEP?

Olfactory placodal epithelium (germinal source of primary olfactory neurons)

Migrating facial ganglionic VII neurons?

Placodal germinal source of ganglion VII?

Oral and pharyngeal placodes

Vestibulocochlear ganglion (VIII) Facial ganglion (VII)

inal epi the rm

In trare


in

m liu

Lumen

Ge

Surface of optic evagination from the brain (heavier line)

Inferior Glossopharyngeal ganglion (IX)

Placodal germinal source of ganglion X?

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of neuron migration from germinal sources.

(vestibulocochlear ganglionic germinal epithelia)

Superior Glossopharyngeal ganglion (IX)


Migrating vagal ganglionic (X) neurons?

Otic vesicle

Superior vagal ganglion (X) Nerve X (with plentiful glia) Inferior vagal ganglion (X)

336

PLATE 156A

SUBDIVISIONS OF THE PROSENCEPHALIC NEP

GW4 Sagittal CR 4.5 mm C9297 Level 1: Slide 4, Section 24






ic am

al

Th

Superficial fibrous layer may contain glial channels.


Pioneer migrating tegmental neurons?

Superficial fibrous layer may contain glial channels intermingled with pioneer axons from spinal cord, medulla, and pons.

P

m


le

a

Rathke's pouch (evagination of the oral cavity)

a

l


dd

Future telencephalic NEP

i c

N


E P


Mi

al rebr Ce


Posterior (mammillary recess)

E

corti ca l

N

Pioneer migrating lateral mammillary nuclear neurons?

sa

lt

el

Preoptic NEP


en ce p

ha

li c

and

s ept


h

Ba

PROSENCEPHALON (FUTURE TELENCEPHALON AND DIENCEPHALON)



MESENCEPHALON

NE

P

PLATE ?B 156B

Anterior

H

y

p

o

t

Rathke's pouch epithelium produces the diverse cell types in the anterior pituitary gland (adenohypophysis).

al N E P

Oral epithelium End of Rathke's pouch epithelium? Preplacodal epithelium

337



ISTHMUS, CEREBELLUM, AND PONS

338

PLATE 157A

PLATE 157B NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.



Medullary velum


Upper rhombic lip



Cerebellar NEP (vermis/hemisphere)


Sprouting nerve IV (trochlear)?

Migrating tectal neurons

Medial pontine NEP

Tr oc hl ea rn uc le a rN EP ?


Me (teg senc me eph nta alic l) N EP

(future aqueduct)


(reticular, raphe, abducens [VI], and facial motor [VII]?)

Isthmal NEP




Migrating isthmal neurons Migrating tegmental neurons

MESENCEPHALON (TECTUM, TEGMENTUM, AND ISTHMUS)

Migrating pontine (reticular formation, abducens [VI], and facial motor [VII]?) neurons

Superficial fibrous layer may contain glial channels intermingled with pioneer axons from spinal cord and medulla.

Superarachnoid reticulum RHOMBENCEPHALON (CEREBELLUM AND PONS)

339

RHOMBOMERES IN PONS AND MEDULLA

GW4 Sagittal CR 4.5 mm C9297 Level 5: Slide 3, Section 40 Slide 3, Section 40 PROPOSED RHOMBOMERE IDENTITIES R2

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R3 Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. R4+5 Vestibulo-auditory NEP - germinal sources of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R6 Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. R7 Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


340

PLATE ?A 158A

PLATE ?B 158B





Primordial mesenchymal brain case (skin, bone, and meninges) Brain surface (heavier line)

Medullary velum

Upper rhombic lip metencephalic pool

myelencephalic pool


Mitotic cells

Medullary NEP Cerebellar NEP

Pontine NEP

(hemisphere)

R4

Cerebellar notch?

R3

R2

(trigeminal NEP)

(facial NEP)

(vestibuloauditory NEP)

R5

(vestibuloauditory NEP)

R6

(glossopharyngeal NEP)

Pioneer migrating cerebellar deep nuclear neurons Superficial fibrous layer



Migrating vestibular and auditory (VIII) neurons from R4 and R5 NEPs

Migrating glossopharyngeal sensory neurons (IX) from R6 NEP

R7

(vagal sensory NEP)


Ascending axons from spinal cord?

341

GW4 Sagittal, CR 4.5 mm, C9297 Level 7: Slide 3, Section 24 See Level 7 in Plates 153A and B.


Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R3 Facial sensory NEP - germinal source of sensory neurons that receive input from the facial (VII) ganglion. R4+5 Vestibulo-auditory NEP - germinal sources of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R6 Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. R7 Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

RHOMBENCEPHALON AND SENSORY CRANIAL NERVE ENTRY ZONES

342

PLATE 159A

PLATE 159B

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold ABBREVIATIONS: NEP - Neuroepithelium R - Rhombomere


Note that R4, R5, and R7 NEPs are not in the plane of this section.

Pontine NEP Migrating facial sensory (VII) neurons from R3 NEP?


1

Medullary NEP



R6 glossopharyngeal NEP

Nerve VIII boundary cap?*

1 Nerve V boundary cap?*



IX ?* rve cap Ne ary d un bo

R2 trigeminal NEP

R3? facial sensory NEP


Nerve X boundary cap?*



Lumen

Germinal epithelium


Migrating glossopharyngeal sensory neurons (IX) from R6 NEP


* Boundary caps are

Schwann cell GEPs?

Otic vesicle

343



LOWER MEDULLA AND SPINAL CORD (MIDLINE RAPHE GLIAL STRUCTURE)

344

PLATE 160A

PLATE 160B ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Medullary velum



(future fourth ventricle) myelencephalic pool

LEFT SIDE OF BRAIN

MIDLINE

End feet of NEP cells protrude into ventricule

Medullary NEP

(reticular, raphe, vagal motor [X], and hypoglossal [XII]?)

Midline raphe glial GEP (in cell body layer, no end feet)

RIGHT SIDE OF SPINAL CORD

central canal

Spinal NEP (ventral) Cell body layer

Fibrous processes

Midline raphe glial structure Notochord Oral cavity RHOMBENCEPHALON (LOWER MEDULLA)

Ventral commissural GEP? Ventral commissure in spinal cord SPINAL CORD

345

346

PART PARTXIII: XIII: GW4 GW4 CORONAL CORONAL

Carnegie Collection specimen #836 (designated here as C836) with a 4-mm crown-rump length (CR) is estimated to be at gestational week (GW) 4. C836 was fixed in corrosive acetic acid, embedded in paraffin, and was cut in 15-µm transverse sections that were stained with aluminum cochineal. Sections of the prosencephalon and anterior mesencephalon are cut in the coronal plane, but the plane shifts to predominantly horizontal in the posterior mesencephalon, pons, and medulla. We photographed 36 sections at low magnification from the frontal prominence to the posterior tips of the mesencephalon and medulla. Twelve of these sections are illustrated in Plates 161AB to 171AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of C836’s brain and optic vesicle (Figure 12), and to show each illustrated section in situ (insets, Plates 161A to 171A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify non-neural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. The prosencephalon is considerably smaller than at GW4.5 (Part XI) and consists of a stockbuilding neuroepithelium surrounding a small prosencephalic superventricle with paired optic recesses. Anterior sections are tentatively identified as future telencephalic neuroepithelium and include the semicircular olfactory placodes at the embryonic surface. The diencephalic neuroepithelium is located in-between and posterior to the large pair of optic vesicles that form the most prominent prosencephalic feature. A preplacodal epithelium is in the head around the optic vesicles, but a definite lens placode cannot be identified. The preplacodal epithelium is continuous with the

thickened olfactory placode anterolaterally and the primordium of Rathke’s pouch in the ventral midline. The mesencephalon contains a stockbuilding neuroepithelium surrounding a small mesencephalic superventricle. A roof (tectum) and floor (tegmentum) can be differentiated in coronally cut anterior sections. It is difficult to distinguish neuroepithelial subdivisions in posterior sections that cut the mesencephalon horizontally. A few pioneer migrating cells are outside the presumptive tegmental and isthmal neuroepithelia posteriorly. The primordial plexiform layer at the brain surface is very thin throughout the entire mesencephalon. The most prominent neuroepithelial structures in the rhombencephalon are the rhombomeric evaginations. This specimen is one of the best to show the “rippled” neuroepithelium in Plates 167 to 170. As in M2300 (Part XI), the trigeminal ganglion (sensory axons of V) is attached to the brain surface at rhombomere 2. The vestibulocochlear ganglion (source of VIII axons) is attached to the rhombomere 4 brain surface. The otic vesicle touches the rhombomere 5 brain surface. A glossopharyngeal ganglion is lateral to the brain at rhombomere 6. The short nerve extending from the large vagal ganglion (sensory axons of X) touches the rhombomere 7 brain surface. The facial ganglion is tentatively identified adjacent to a placode in the hyoid arch that lies immediately ventral to the vestibulocochlear ganglion and posteroventral to rhombomere 3. Very few neurons are migrating from the rhombomeres. The small stockbuilding cerebellar neuroepithelium is only identifiable in the most posterior sections of the rhombencephalon and is difficult to distinguish from the mesencephalic tectum.

347

C836 Computer-aided 3-D Brain Reconstructions B. Side view

S thala u

P R O

S

E

Up

rm

ha

n

R3

o

Preoptic area

R4

Medullary velum

R5 R6

a

R7

BRAINSTEM FLEXURES

u ll a

1

a ull ed rm

med

Future telencephalon

we

R7

R2

Lo

er

R6

4 3

ll

L

ow

R5

e d u ll a

t

Upper rhombic lip

s

ed u

pe

o Hy p

mu

per m Up

P

sth

P

s

n

o

R4

mentu

s

N C E P S E H O

Su

bth ala m

H A L O N E P

Optic vesicle R

R3

eg

um

C

b- s mu

e ll

N

Epithalamu s

lamus Tha

reb Ce

R h o m b o m e r e s

Preoptic area

Optic vesicle

Tectum

I

s R2

t e l F uture e n c ep h alo n

O

Pretectum

m

m entu gm

Isth mu

us

Te

us

llum e be Ce r

s mu

Thal am

L

A

Epithala

N

T

Tectum

Pretectum

l a m us

Angled front view

P

A.

1. Medullary 3. Mesencephalic

Lower rhombic lip

4. Diencephalic

Spinal cord

Spinal cord

C.

Top view

Optic vesicle

C e r e b e l l u m

h

m

u s

T e c t u m

r e t e c t u m P

E p i t h a l a m u s

a l a m u T h s

lencephal ure te on Fut

P

R

O

I s P o

t

n

s

Medullary velum

N C E P H A L S E

O

N

R2

Upper rhombic lip

Figure 12. A, The left side of the 3–D model viewed from the front at a 45º heading; this view is used to "peel away" sections of each level in the following Plates. B, A straight view of the left side. C, A straight down view of the top. D, An upward view of the bottom, angled (120º) to look into the mesencephalic and diencephalic flexures.

D.

Bottom view Optic vesicle

R7 d

R6

r C o

i n a l

p

r medu pe ll

p

R5 a

medu er ll w a

s

n o

P s

n o

I s t h m u s

m

u g m e n t

e

o t m u s h a l a H y p

a r e a

re telencepha Futu lo

P r e o p t i c

P

N

P

R


T

R3 R4

U

E N C E P H A O S L

O

R2

o

n

L

Subthalamus

S

348

PLATE 161A


GW4 Coronal CR 4.0 mm C836

Level 1: Section 9 Preplacodal epithelium


Olfactory placode



Mesenchymal? densities near optic vesicle

Optic vesicle

Lens placode Preplacodal epithelium


Rathke's pouch epithelium

349

PLATE 161B


Level 1: Section 9 PROSENCEPHALON




Prosencephalic NEP


(future telencephalic)



DIENCEPHALON

Level 2: Section 27

THALAMUS




Thalamic NEP


SUBTHALAMUS

Subthalamic NEP Subthalamic primordial plexiform layer

Preoptic area NEP? Pigment epithelium


diencephalic superventricle optic recess


Preoptic primordial plexiform layer

Retinal NEP Anterior hypothalamic NEP Diencephalic floor plate


(future chiasmal GEP)

PREOPTIC AREA/ HYPOTHALAMUS FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold



350

PLATE 162A


GW4 Coronal CR 4.0 mm C836 Level 3: Section 36

Mesenchymal? densities near optic vesicle


Lens placode

Optic vesicle

Rathke's pouch epithelium Cephalic preplacode Lateral tongue primordia Part of the mandibular arch placodal epithelium gives rise to the thyroid gland.

Mandibular arch (I)

Arterial trunk

Liver

Peritoneal cavity





Hyo-mandibular cleft Hyoid arch (II)


Arches III and IV

351

PLATE 162B

Central neural structures labeled THALAMUS/EPITHALAMUS





pineal recess?



SUBTHALAMUS

Pigment epithelium

Subthalamic NEP

optic recess

Opthalmic germinal epithelia

Retinal NEP

Anterior/middle hypothalamic NEP





(future chiasmal GEP)

HYPOTHALAMUS Chiasmal glial channels?

DIENCEPHALON




352


PLATE 163A


GW4 Coronal CR 4.0 mm C836 Level 4: Section 48 Cell-sparse formative superarachnoid reticulum

Sphenoid primordium

Maxillary process Cephalic preplacode Rathke's pouch epithelium Future oral cavity

Migrating trigeminal ganglionic (V) neurons? Trigeminal ganglion (V) placode? Tongue primordia

Epithelium Lateral Medial

Mandibular arch (I)

Hyoid arch (II) Multiple loci in the branchial placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands.

Arterial trunk Arch III?

Arch IV? Pharynx Vagal ganglion (X) placode? Vagal ganglion (X)

Dorsal aorta



353


PLATE 163B

THALAMUS/EPITHALAMUS





Thalamic/epithalamic primordial plexiform layer pineal recess?

SUBTHALAMUS Subthalamic primordial plexiform layer

Subthalamic NEP


mammillary/ infundibular recesses

Middle/posterior hypothalamic NEP



(future median eminence and neurohypophyseal GEP)

HYPOTHALAMUS

DIENCEPHALON


Arrows indicate the presumed direction of neuron migration from germinal sources. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium


354

PLATE 164A GW4 Coronal CR 4.0 mm C836 Level 5: Section 63



Anterior cardinal vein Formative cell-sparse superarachnoid reticulum

Fused maxillary process and mandibular arch (I) Sphenoid primordium? Notochord


Oral placodes

Oral cavity Hyoid arch (II)

Multiple loci in the branchial placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands. Arch III


Pharynx Pharyngeal placodes

Arch IV?

Vagal ganglion (X) placode? Vagal ganglion (X)

Dorsal aorta Notochord


Somites Dorsal root ganglion boundary cap (Schwann cell GEP?)

355


PLATE 164B

Brain surface (heavier line) Tectal primordial plexiform layer

TECTUM



TEGMENTUM





(migrating peripheral ganglionic neurons from germinal sources in the branchial placodes)

MESENCEPHALON

Trigeminal ganglionic neurons (V)

Vagal ganglionic neurons (X)


Ventral gray


SPINAL CORD

Ventral funiculus

Spinal floor plate


Ventral NEP

Lateral funiculus Dorsal funiculus





356


PLATE 165A

Primordial mesenchymal brain case (skin, bone, and meninges) culum e r a r a c h n o i d r e ti



Trigeminal ganglion (V) placode? Anterior cardinal vein

Forma tive

cellspar s

e sup


Facial ganglion (VII) placode Facial ganglion (VII)

Hyoid arch (II)

Glossopharyngeal ganglion (IX) placode?


Arch III? Glossopharyngeal ganglion (IX) Vagal ganglion (X) placode? Vagal ganglion (X)

Arch IV?


Somites

Dorsal root ganglion boundary cap (Schwann cell GEP?)

357




PLATE 165B


Mesencephalic roof plate mesencephalic superventricle

Tectal NEP

(future aqueduct)

Tegmental/isthmal NEP Tegmental/isthmal primordial plexiform layer

isthmal canal

TEGMENTUM/ISTHMUS



PONS/MEDULLA Pontine primordial plexiform layer

R2 (trigeminal NEP)




Midline raphe glial structure GEP? Facial ganglionic neurons (VII)

Medial pontine NEP Pontine primordial plexiform layer PROPOSED RHOMBOMERE IDENTITY R2

Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus.

Glossopharyngeal ganglionic neurons (IX)

Rhombencephalic floor plate (midline raphe glial structure GEP?)

Midline raphe glial structure Vagal ganglionic neurons (X)

Medullary primordial plexiform layer?

Lower medullary NEP? (fuses with ventral spinal NEP)

RHOMBENCEPHALON

SPINAL CORD Ventral NEP

ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells.

Lateral funiculus

Dorsal funiculus


central canal



358

Fused maxillary process and mandibular arch (I) Trigeminal boundary cap* Trigeminal ganglion (V) Trigeminal ganglion (V) placode?


culum e r a r a c h n o i d r e ti


e sup


cellspar s



Forma tive

PLATE 166A

Facial ganglion (VII) placode Facial ganglion (VII)

Hyoid arch (II)

Glossopharyngeal ganglion (IX) placode? Glossopharyngeal ganglion (IX) Anterior cardinal vein Vagal ganglion (X)

Sympathetic trunk?

Somites

Dorsal root ganglion boundary cap*



PLATE 166B


MESENCEPHALON


TECTUM



Tectal NEP

(future aqueduct) Tegmental/isthmal primordial plexiform layer isthmal canal

TEGMENTUM/ISTHMUS

Tegmental/isthmal NEP Migrating tegmental/isthmal neurons

PONS Migrating trigeminal (V) neurons




R2 (trigeminal NEP) Central trigeminal tract



Medial pontine NEP

(abducens [VI], facial motor [VII]?) Migrating abducens (VI) and facial motor (VII) neurons?

Facial ganglionic neurons (VII)

R4 (vestibulo-auditory NEP) Migrating vestibulo-auditory neurons from R4 NEP

Rhombencephalic floor plate (midline raphe glial structure GEP?)

Glossopharyngeal ganglionic neurons (IX)



PROPOSED RHOMBOMERE IDENTITIES R2 Trigeminal NEP germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R4 Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.


(vagal motor [X], hypoglossal [XII]?, blends with ventral spinal NEP) Migrating hypoglossal (XII) and vagal motor (X) neurons?

SPINAL CORD

MEDULLA

Ventral NEP

RHOMBENCEPHALON ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Ventral gray

Intermediate NEP


central canal

Dorsal NEP Dorsal funiculus?

Spinal roof plate


360

PLATE 167A




Fused maxillary process and mandibular arch (I) Trigeminal boundary cap (Schwann cell GEP?)


Anterior cardinal vein Vestibulocochlear ganglion (VIII)


Glossopharyngeal ganglion (IX) placode? Glossopharyngeal ganglion (IX) Anterior cardinal vein Vagal ganglion (X) Sympathetic trunk?

Somites



361

PLATE 167B



Tectal primordial plexiform layer?

TECTUM?


ISTHMUS isthmal canal

Isthmal NEP



Early cell migration from rhombomeric NEPs

CEREBELLUM

Cerebellar NEP?

Fibrous layer in superficial cerebellum?

PONS

Migrating trigeminal (V)(V) neurons? Migrating trigeminal neuron

R2 (trigeminal NEP) PROPOSED RHOMBOMERE IDENTITIES

Migrating facial (VII) neurons

R3 (facial NEP)

R4 (vestibulo-auditory NEP) Migrating vestibulocochlear ganglionic neurons from germinal source in otic epithelium

R5 (vestibulo-auditory NEP Migrating vestibular and auditory neurons Migrating glossopharyngeal receptor neurons (solitary nucleus)

R6 (glossopharyngeal NEP) Migrating sensory vagal neurons

R7 (vagal sensory NEP) Lower intermediate medullary NEP (blends with intermediate spinal NEP)

MEDULLA

RHOMBENCEPHALON ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from germinal sources.

Intermediate gray

Lateral funiculus

Arrows indicate the regionally expanding shoreline of the superventricle with increase in stockbuilding NEP cells. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

Dorsal funiculus?

R3


Migrating vestibular and auditory neurons


R2

R4

R5

R6

R7

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

SPINAL CORD Intermediate NEP central canal

Spinal germinal Dorsal NEP zones Spinal roof plate

362

PLATE 168A





Trigeminal ganglion (V) Trigeminal boundary cap*


Nerve VII boundary cap* Nerve VIII boundary cap*


Otic vesicle Nerve V (trigeminal) Nerve VII (facial)

Lumen Epithelium Anterior cardinal vein

Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal)

Nerve IX boundary cap*

Nerve X (vagus) Nerve X boundary cap* Vagal ganglion (X)

Somites

Dorsal root ganglion boundary cap*


363


PLATE 168B

MESENCEPHALON

Brain surface (heavier line) Tectal primordial plexiform layer?

TECTUM?


ISTHMUS

isthmal canal

Isthmal NEP



CEREBELLUM

Cerebellar NEP Fibrous layer in superficial cerebellum


PONS

Migrating trigeminal (V) neurons

R2 (trigeminal NEP) Migrating facial sensory (VII) neurons





R3 (facial NEP)

R5 (vestibulo-auditory NEP Migrating glossopharyngeal receptor neurons (solitary nucleus)

R6 (glossopharyngeal NEP) Migrating sensory vagal neurons


Lower intermediate medullary NEP (blends with intermediate spinal NEP)

R2

R3

R4

R5

R6

R7

MEDULLA


Intermediate gray

SPINAL CORD

Lateral funiculus


Dorsal funiculus?



Spinal germinal Dorsal NEP zones Spinal roof plate

364

PLATE 169A


GW4 Coronal CR 4.0 mm C836 Level 10: Section 90 Primordial mesenchymal brain case (skin, bone, and meninges)


Trigeminal boundary cap Nerve VII boundary cap*


Nerve VIII boundary cap* Vestibulocochlear ganglion (VIII)

Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear)

Otic vesicle

Lumen Epithelium

Nerve IX (glossopharyngeal) Nerve X (vagus) Glossopharyngeal ganglion (IX) Nerve IX boundary cap* Nerve X boundary cap* Vagal ganglion (X)

Somites?

Dorsal root ganglion *boundary cap


365

PLATE 169B


Brain surface (heavier line) Tectal primordial plexiform layer?

TECTUM?


ISTHMUS

mesencephalic superventricle isthmal canal

Isthmal NEP?

(future aqueduct) Isthmal primordial plexiform layer?

CEREBELLUM


Cerebellar NEP Fibrous layer in superficial cerebellum

PONS

R2 (trigeminal NEP) Migrating trigeminal (V) neurons


R3 (facial NEP)

Migrating vestibular and auditory neurons

R5 (vestibulo-auditory NEP R6 (glossopharyngeal NEP) Migrating glossopharyngeal receptor neurons (solitary nucleus)

R7 (vagal sensory NEP) Migrating vagal sensory neurons

Lower intermediate medullary NEP (blends with intermediate spinal NEP)

R3




R2


R4

R5

R6

R7


MEDULLA


SPINAL CORD Intermediate NEP



Dorsal funiculus?

central canal

Dorsal NEP

Spinal roof plate


366

PLATE 170A



Trigeminal (V) boundary cap?*


Nerve VII boundary cap*

Nerve VIII boundary cap* Migrating vestibulocochlear ganglionic (VIII) neurons? Nerve V (trigeminal) Nerve VII (facial) Nerve VIII (vestibulocochlear)

Otic vesicle

Lumen Epithelium

Nerve IX (glossopharyngeal) Nerve X (vagus) Glossopharyngeal ganglion (IX)? Nerve IX boundary cap* Nerve X boundary cap* Nerve X (vagus)



367

PLATE 170B

Central neural structures labeled Metencephalic roof plate

CEREBELLUM


Vermis

Cerebellar NEP

Hemisphere


metencephalic pool

PONS



R5 (vestibuloauditory NEP

R6 (glossopharyngeal NEP) Migrating glossopharyngeal receptor neurons (solitary nucleus)

R7 (vagal sensory NEP) Migrating vagal sensory neurons




R2 (trigeminal NEP) R3 (facial NEP)


R2

R3

R4

R5

R6

R7



(gracile and cuneate nuclear NEP blends with dorsal spinal NEP)

myelencephalic pool


MEDULLA

RHOMBENCEPHALON ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold



368

PLATE 171A



Nerve VIII boundary cap* Migrating vestibulocochlear ganglionic (VIII) neurons? Nerve VIII (vestibulocochlear) Nerve IX (glossopharyngeal)

Otic vesicle

Lumen Epithelium

* Boundary caps are Schwann cell GEPs? Nerve IX boundary cap*


369

PLATE 171B

Central neural structures labeled Medial metencephalic roof plate CEREBELLUM Brain surface (heavier line)

Vermis

Cerebellar NEP

Hemisphere?

metencephalic pool

Lateral metencephalic roof plate

Future medullary velum

(lower rhombic lip)


R5 (vestibuloauditory NEP



(gracile and cuneate nuclear NEP blends with dorsal spinal NEP)


Lateral myelencephalic roof plate


(upper rhombic lip)


R6

Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

myelencephalic pool

MEDULLA

RHOMBENCEPHALON ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold



370

PART PARTXIV: XIV: GW3.8 GW3.8 SAGITTAL SAGITTAL

Carnegie Collection specimen #7724 (designated here as C7724) has a 4-mm crown-rump length (CR). However, at this early stage, CR length is an unreliable estimate of gestational age. The right side of the body has clearly separated 24 to 25 somites with both anterior and posterior neuropores closed. Using the timetables in Patten (1953) and Hamilton et al. (1959), we estimate that C7724 is at gestational week (GW) 3.8. C7724 was fixed in formalin, was embedded in a celloidin/paraffin mix, and was cut in 8-µm sagittal sections that were stained with hematoxylin and eosin. Various orientations of the computer-aided 3-D reconstruction of C836’s brain are used to show the gross external features of a GW4 brain (Figure 13). Like most sagittally cut specimens, C7724’s sections are not parallel to the midline; Figure 13 shows the approximate rotations in front (B) and back views (C). We photographed 24 sections at low magnification from the left to right sides of the body. Eight of the sections, mainly from the left side of the body, are illustrated in Plates 172AB to 179AB. Each illustrated section shows the entire embryo. Labels in A Plates (normal-contrast images) identify the approximate midline, non-neural structures, peripheral neural structures, and brain ventricular divisions; labels in B Plates (lowcontrast images) identify central neural structures. Plates 180AB to 184AB show high-magnification views of several parts of the brain. The prosencephalon is the smallest major brain structure with little distinction between a future telencephalon and diencephalon. The entire prosencephalic neuroepithelium is rapidly stockbuilding its various populations of neuronal and glial stem cells surrounding a small prosencephalic protoventricle. The ventral and lateral prosencephalon is surrounded by cephalic preplacodes at the surface (for example, the anterolateral olfactory placode) that are continuous with those extending into the roof of the developing oral cavity (for example, Rathke’s pouch).

The mesencephalon is smaller than at GW4 but has similar developmental features. The stockbuilding pretectal and tectal neuroepithelia have a relatively short anteroposterior length and blend with the presumptive cerebellar neuroepithelium in the dorsomedial rhombencephalon. The stockbuilding tegmental and isthmal neuroepithelia form a distinctive arch between the mesencephalic and diencephalic flexures. These neuroepithelia surround a small mesencephalic protoventricle. There is a very thin subpial fiber band in the tegmentum and isthmus. The rhombencephalon is the largest brain structure. Rhombomeres 2 through 7 form well-defined swellings in the lateral neuroepithelium (Plate 176). As in the GW4 specimens, most sensory cranial ganglia and the otic vesicle are located directly lateral to the rhombomeres with which they interact. The trigeminal ganglion (source of sensory V axons) appears in sections lateral to the last section that contains rhombomere 2. The vestibulocochlear ganglion (VIII afferents) and the otic vesicle are lateral to the last section that contains rhombomeres 4 and 5. The presumptive glossopharyngeal ganglion (IX afferents) is ventrolateral to the last section with rhombomere 6, and the presumptive vagal nerve (X afferents) is lateral to the last section with rhombomere 7. The presumptive facial ganglion (VII afferents) is near a branchial placode in the hyoid arch, slightly posterior and ventrolateral to rhombomere 3. Each rhombomere has a thin layer of pioneer migrating neurons that are only visible in most lateral sections, where the outer edges of the rhombomeric neuroepithelium are cut tangentially. Sections through the midline show a smooth neuroepithelium. Some migrating cells are outside the lower medullary neuroepithelium. The primordial white matter in the spinal cord extends into the lower medulla. The cerebellum stands out as the most immature and smallest rhombencephalic structure that blends with the isthmal neuroepithelium laterally and the presumptive tectal neuroepithelium medially.

371

EXTERNAL FEATURES OF THE GW4.0 BRAIN

mentu

R7

s

A perfect sagittal cut through the brain is parallel to the midline from anterior to posterior. Sections of C7724's brain rotate an estimated 6.6º counterclockwise from the midline, 3.3º to the left side of the anterior midline (B, front view), and 3.3º to the right side of the posterior midline (C, back view). In the sections illustrated on the following pages, anterior parts (top and left) are tilted toward the observer, while posterior parts (bottom and right) are tilted away from the observer. The lower part of the spinal cord (not shown in this reconstruction) has a concave curve, and the sacral tip is cut in the coronal plane.

P H A L O N

l a u l

1

R6

N C E

P

o

E Medullary velum

R5

e d

P R

O

S

MESENCEPHALON

R4

M

Anterior midline

Front view

Upper rhombic lip

Rhombomere (R) 2

R3

Figure 13. The 3-D reconstruction of the brain and upper cervical spinal cord in C836 is used to show the approximate external features of C7724, a 24-25 somite embryo estimated to be 25 (Patten, 1953) to 28 (Hamilton et al., 1959) days old (average age = GW3.8). A, The lateral view of the left side. External features are identified as in Figure 12B. The heavy numbered lines refer to brainstem flexures (boxed key); the pontine flexure (#2 in preceding specimens) is not present. B, Front view of the brain in A. The angled line shows how C7724's sections rotate left (arrow) from the anterior midline. C, Back view of the brain in A. The angled line shows how C7724's sections rotate right (arrow) from the posterior midline.

B.

us

n

on

e

3

thm

O M B

al die n c

ph

Is

H

4

C R

Ve n tr

u m ellu eb er

Optic vesicle

eg

ct

m

T

Te

m

C E P H N E e n c e p hal o l Te

Side

l n N view rsa O Do phalo L n ce A n d ie

al

A.

MESENCEPH AL ON Pretectum

BRAINSTEM FLEXURES 1. Medullary 3. Mesencephalic 4. Diencephalic

Lower rhombic lip

SPINAL CORD

C.

Pretectum

Back view

Tectum

Left side -3.3º

Right side

e e b l l

e

u

r

PROSENOptic CEPHALON vesicle

Optic vesicle

C

m

R4 R5 R6

Medulla

R7

RHOMBENCEPHALON

Left side

Pons Rhombic lip border

Posterior midline

Medullary velum

Right side

-3.3º SPINAL CORD

Spinal cord


Medulla

PLATE 172A

Medullary velum

GW3.8 Sagittal, CR 3.5 mm, 24-25 somites, C7724 Level 1: Slide 2, Section 30

entricle protov le) h a lic tric

h ven c ep en e f o u r t m b fu tu r o ( rh

e cl ri

lic protov ha ep ture aqueduct ent c ) en (fu es m


Oral-pharyngeal cavity

Hyoid arch (II) Branchial preplacodes

Mandibular arch (I)

Lung bud?

diencephalic pool


prosencephalic protoventricle

8?

Cephalic preplacodes

Arterial trunk

telencephalic pool

APPROXIMATE MIDLINE AREA RIGHT SIDE OF BRAIN AND SPINAL CORD


Foregut/midgut

Liver?

LEFT SIDE OF BRAIN

12?

Um

Heart in pericardial swelling

bi

l ic

al

in ve

13

14 s)

Umbilical cord

num

ber

15

ate

16

Mesonephric vesicles

19 20

ral Ventrta ao

Notochord

See Plates 180A and B for a higher magnification view of the prosencephalon, mesencephalon, and anterior rhombencephalon in a nearyby section.

21

central canal

22 23? 24? MIDLINE

(app

18

Hindgut

roxim

17

Mesonephric duct

ites

THIS PORTION OF THE BODY CURVES TOWARD THE OBSERVER AND IS CUT IN THE CORONAL PLANE.

Som

372

Anterior somites 8-12 are indistinct and blend together. Somites 1-7 are not in this section.

Po nt

t

nc

ALON

tral die

al/isthm ent m g

V e p h a li c ?

Primordial gray matter

P

lenc

Primordial white matter

H

c

lic ha ep nc die

Te

en

N

O T e

AL

ephalic NE enc PSs s e M

h

P

p

al rs Do

Prosencephalic NEPS NEPs

Migrating spinal neurons

e

Te

te

Ascending fiber tracts Migrating solitary nuclear neurons (IX glossopharyngeal receptors) from R6 NEP

al ic

re


Glial channels form superficial NEP border?

D C O R

al ct

te

A L

nt ra l

R2

I N

Sp Dor i n sal al te rm N e E di Ve a In

P

al

PLATE 172B

P

MESENCEPH

l

a

373

S

NCEPHALON RHOMBE Grac ile nucleand cu Upper rhombic lip IC NEPs L A ar N neate H P EP? E NC E N P y r a l l E u ed M B OM P R6 E RH R5 Lower R7 rhombic e N R4 n lip i R3

PROSEN

CE





R5

R6

R7

Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


Spinal surface (heavier line)

Spinal NEP

Ventral Intermediate Dorsal

Arrows indicate the presumed direction of axon growth in brain fiber tracts. Arrows indicate the presumed direction of neuron migration from neuroepithelial sources.

GW3.8 Sagittal, CR 3.5 mm 24-25 somites, C7724 o v e n t Level 2: Slide 2, t r o i c l e (futu ic pr re Medullary velum hal fo p e u c RI Section 24 rt h ben M I D L INE

e cl ri

m

ve

nt

cl

diencephalic pool

Mandibular arch (I)


Branchial preplacodes

Lung bud?

nk tru


ID

TS

F LE

diencephalic pool

(fu

en es m

AIN

Hyoid arch (II)

d or ch to

BR

Oral-p haryng eal cav ity

ial ter Ar

telencephalic pool?

e)

No


D NAL COR ND SPI IN A BRA OF DE SI

ri

)

rho

T H G

prot o alic ph re aqueduc ven e c tu t t

PLATE 173A

F EO

Liver?

Notochord

Foregut



ve in

Labeled on this page: Non-neural structures, brain ventricular divisions bi

lic

al

13? 14?

Um

Mid gut

Umbilical cord

15

16

19

Hindgut Notochord

a ort al a r t Ven

(a es it

22 23? 25?

24?

MID

E LIN

pp

21

m

See Plates 181A and B for a higher magnification view of the prosencephalon, mesencephalon, and anterior rhombencephalon in this section.

rox

20

central canal

te n umb

18

Mesonephric vesicle

ers)

17

Mesonephric duct

ima


Dermatome Myotome

So

374

Sclerotome

375 RHOMBENC

EPHALON

on ti

ephalic N enc EP es Ss

l/is enta th gm e

ha

di

ep

al D o rs

ALON

PH

lic

nc


Vent r a l die

M l Te

en


c ep

h a li c ?

PROS

EN

C

Spinal floor plate


Labeled on this page: Central neural structures Primordial white matter

PROPOSED RHOMBOMERE IDENTITIES R5 R6

Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion.

ABBREVIATIONS: NEP - Neuroepithelium R - Rhombomere



Spinal NEP Ventral

Intermediate Dorsal

C O R D

Prosencephalic NEPS NEPs

nc

lic ha ep

Spinal floor plate

E

th m al ?

Is

Pr


al Dors a te m edi

Reticular, raphe, oculomotor (III), NEP? and red nuclear NEPs?

A L I N S P

Reticular, raphe, abducens (VI), and facial motor (VII) NEPs? NEP?

o e ypog lossator (X), an, l (XII) d NEPs ?

ra l Vent

tal l/tec cta e t e

P

mal

In te r

SENCEPHALO

Gra c N C E P H A L I C nucile an E N B lea d cu M E r N ne O EP ate P s? R H s Medullary N E P N E P ne Re R5 vaga ticular, ra R6 lm ph h

Cerebellar NEP?

l NEP Spina

ME

PLATE 173B

Lower rhombic lip

Te

N

Upper rhombic lip

PLATE 174A Primordial mesenchymal brain case (skin, bone, and meninges)

Medullary velum

rhom

GW3.8 Sagittal, CR 3.5 mm 24-25 somites, C7724 m ye len cep Level 3: Slide 2, hal ic p ool ic protoventricle l a h Section 20 (fu ep tu benc re

fo

ur

th

n

i

e)

AI BR OF

cl

DE

tr

SI

ve

FT LE

metencephalic pool

l- p h

a ry n

Hyoid arch (II)

g ea l

cavi

AL CORD SPIN ND NA

O ra


ty

Mandibular arch (I)

nk tru

Laryngotracheal groove?

M I D LINE

Cephalic preplacdes

central canal

(future aqueduct)

Branchial preplacdes

ial ter Ar

mesencephalic protoventricle

pr pr osen oto cep ven ha tri lic cle

Notochord

Um

bili cal vein


RIGHT S IDE


14? Midgut

15? 16?

Umbilical cord

17

Dermatome

Mesonephric duct

18

u m be rs)

19


20

23? 25? 24?

MIDLINE

r ox i m

Som

See Plates 182A and B for a higher magnification view of the prosencephalon, mesencephalon, and anterior rhombencephalon in this section.

22

i tes

central canal

21

( ap p

Notochord

ate n

Hindgut Ve ao ntr rt al a

376

Myotome Sclerotome

R H O M B E N C E P H A L O

Upper rhombic lip

Cerebellar NEP

N

Po n ti n e R3 R2

Isthmal NEP

R4

R5

R6

Pioneer migrating hypothalamic, subthalamic, and tegmental neurons

ntal me eg

tal ec et

M B E N C E P H A L R H O I C M e d u llary EP

R7

PLATE 174B

N

N E NE P P

R va eticu hyp gal m lar, ra oglo otor phe ssa (X), , l (X II) Nand EPs ?

Lower rhombic lip

s


ha

nc

NC


E

li c

ie Dorsal d

EPHALO

Pr

alic ep

Ve n t r a

l

di

en cep h

? c e p h a li c

PR

O

S

S P I N A L

T

en

Spinal NEP

Prosencephalic NEPS NEPs el

N

T

ENCEPHALO M E S alic N N EP ceph S n e s al Tect Me


377


Ventral



R5

R6

R7



C O R D

Spinal floor plate

Dorsal


Intermediate




Spinal NEP

FONT KEY: Ventral ventricular divisions - capitals Germinal zone - Helvetica bold Intermediate Transient structure - Times bold italic Permanent structure - Times Roman or Bold Dorsal


PLATE 175A

GW3.8 Sagittal, CR 3.5 mm 24-25 somites, C7724 Level 4: Slide 2, LE FT Section 16 S

Medullary velum

le ntric otoicvlee) r p c i r phal h vent bence e fourt rhom (futur

ID E

F O

Oral-p haryng eal cav ity

Hyoid arch (II)

Mandibular arch (I)


ial ter ? Ar runk t


Liver? Umbilical vein?

ce nt ra l


Midgut


ca na l

prosencephalic protoventricle (optic recess)

MIDLINE


D COR NAL SPI ND NA AI BR


Umbilical cord

Notochord

RIGHT SID E

17?

Mesonephric duct?

19

22

central canal

23?


25?

24?

MIDLINE

ite s (

21

a p pro x i m ate

20

Som

Notochord

n u m be rs)

18

Ven tr a l ao r ta ?

378

Myotome and sclerotome Dermatome

ntal

e

e gm

ON

EPHAL

n

Migrating medullary (reticular formation?) neurons

Migrating solitary nuclear neurons (IX glossopharyngeal receptors) from R6 NEP Primordial gray matter

c germi

S

Tel

Prosencephalic NEPS

Spinal floor plate

R3 R4

R5

R6

R7



Ventral


Dorsal

Intermediate


R2

C O R D

N E P

O

p l a t e

E

h


NC

al

mi

cephalic? en

pt

PR


Migrating trigeminal neurons from R2 NEP

r o o f

O

Migrating vestibular and auditory neurons from R4 and R5 NEPs

S P I N A L

di D en or s a s c l e zone ph al al ic cephalic t r a l d i en Ve n

e Pr

t

? tal ec

Migrating hypothalamic, subthalamic, and tegmental neurons

T

Tectal?

Mesencephalic NEPS


i n a l S p

MES

PLATE 175B

S p i n a l

Migrating thalamic neurons?

379

P

I

ENCEPHALON

R H O M B E N C E P Migrating H A isthmal L O Upper rhombic lip neurons Dors Lower rhombic lip al lo N w er m B E N C E P H A M Cerebellar Trochlear Cerebellar O ed L ull nucleus (IV)? ("R1") NEP? NEP ary R H Medu I C NE l N P l a P E ry R6 R7 Ventr E N al low e P R5 N n i e t Po n R4 r me EP s al NEP dul R3 lary hm t R2 s NE

The dorsal lower medullary NEP contains cells that will expand later into a more defined mosaic including the cochlear nuclear NEP, the precerebellar NEP, and the gracile and cuneate nuclear NEPs.

Primordial white matter Primordial gray matter

The ventral lower medullary NEP contains cells that will expand later into a more defined mosaic including the following NEPs: raphe nuclear, vagal motor (X), hypoglossal (XII), prepositus nuclear, and reticular formation.


Spinal NEP Ventral Intermediate Dorsal


PLATE ?A 176A

GW3.8 Sagittal, CR 3.5 mm GW3.5 24-25 somites, C7724 Level 5: Slide 2, LE FT Section 12 SI

velum llary Medu rh

om

b

entricle roticolev) lich p ntr phfa t ve r e u c e n re o t (fu

u

DE

O

F

B

R AI

N D AN


RD CO AL IN

Mandibular arch (I)

SP



Arch III

Hyoid arch (II)

Optic vesicle ial ter k Ar run t



H

ea

Olfactory placode?

rt

in

pe

ri

Liver?

ca

rd

ia

ls

w

el

lin

Umbilical vein? MIDLINE

Midgut

g

RIGH T SI D


E

Notochord

oxim ate n umb ers)

17?

18

central canal

24?

ite s (

22


25?

21

INE DL MI

Som

Notochord

20

a p pr

r ta

19

Ven tr a l ao

380

Dermatome Myotome and sclerotome

R H O M B E N C E P Lower rhombic lip H

Upper rhombic lip

O M B E N C E P H A L R H I C M e d u

R2

EP

R4

R3

R5

R6

O

N

N E l l a r y P N E Dors al l o P wer me dul lar

s yN EP


Migrating solitary nuclear neurons (IX glossopharyngeal receptors) from R6 NEP Migrating vestibular and auditory neurons from R4 and R5 NEPs

i

germ

mic al


S P I N A L

Lateral prose cephalic NEP?

L




R7

PLATE 176B

A

n a l S p i

g tin l ra ta ig ec ns M pret uro ne

Migrating trigeminal nuclear complex (V) neurons from R2 NEP Migrating facial (VII) neurons from R3 NEP

l zones

SENCEPH

Pon

N ti n e

na

g r a tin Migtectal s r on n eu

ME


l ma IsthEP? N

AL

O

N

Cerebellar NEP

381

t

h

O

SENCEPHA

L


Spinal roof


R5

R6

R7


Dorsal

R4

The optic vesicle is an evagination of the prosencephalon in the developing brain. It contains the opthalmic germinal zones, cells that will generate parts of the eye: retinal NEP, optic nerve glioepithelium, and the pigment epithelium.

plat e

R3


Intermediate

R2

Spinal floor plate

Ventral


C O R D

RO

N E P

P

N

Op

Primordial gray matter Primordial white matter

The dorsal lower medullary NEP contains cells that will expand later into a more defined mosaic including the cochlear nuclear NEP, the precerebellar NEP, and the gracile and cuneate nuclear NEPs. Spinal surface (heavier line)

Spinal NEP Ventral

Intermediate Dorsal


Medullary velum

GW3.8 Sagittal, CR 3.5 mm GW3.5 24-25 somites, C7724 Level 6: Slide 2, Section 8

rhombencephalic protoventricle (future fourth ventricle)

BR AI N

PLATE 177A

LEFT SIDE OF

Formative cell-sparse superarachnoid reticulum

Oral cavity Oral-pharyngeal cavity

dis

tinc

Mandibular arch (I)

ite t som

Optic vesicle

Arch III

In

Hyoid arch (II)

s on left side of bod y

ial ter k Ar run t




Liver?


Umbilical vein?

MID LIN

See a high magnification view of the prosencephalon, mesencephalon, and rhombencephalon from the right side of the brain in Plates 184A and B.

E

central canal

THIS PORTION OF THE BODY FORMS A C-SHAPED CURVE, AND THE MIDLINE IS CUT IN TWO PLACES.

RIG

Notochord

HT SI

MIDLINE

P

24?

FS

ao r ta

DE O

25?

IN A L CO RD

central canal

Ve nt ra l

382

R H O M B E N C E P H A Lower rhombic lip L O M

Upper rhombic lip

EP P o n ti n e N

R2

Migrating isthmal neurons?

Neurons migrating from rhombomeres

R3

edulla ry N EP

R5

R4

383

PLATE 177B N

R6


R7

Trigeminal (V) Facial (VII) Vestibular and auditory (VIII)


Glossopharyngeal receptors (IX) Vagal sensory (X)

Nerve X (vagus)? Cephalic preplacodes Glial channels form superficial germinal zone border? i

n

germ

zones

m

ic

al

PROSENCEPHALON


O pthal

Olfactory placode

Labeled on this page: Central and peripheral neural structures Primordial white matter

SPINAL CORD

R7


Sp

in

R6

P

R5

NE

R4

Spinal roof plate

al

R3

Intermediate

R2

The optic vesicle is an Trigeminal NEP - germinal source of the central trigeminal nuclei except the evagination of the prosencephalon in the mesencephalic nucleus. Facial NEP - germinal source of facial developing brain. It motor and sensory receptor neurons of contains the opthalmic germinal zones, cells Spinal floor plate the facial (VII) ganglion. that will generate parts of Vestibulo-auditory NEP - germinal the eye: retinal NEP, source (with R5) of central auditory nuclei and vestibular nuclei, except the optic nerve glioepithelium, and the cochlear nuclei. pigment epithelium. Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal Spinal floor plate source of the dorsal sensory nucleus and other sensory vagal nuclei.

Ventral


Dorsal


Primordial white matter Primordial gray matter



Ventral Intermediate Dorsal

Spinal roof plate


384

PLATE 178A

GW3.8 Sagittal, CR 3.5 mm 24-25 somites, C7724 Level 7: Slide 1, Section 38 LE

1?

2?

FT

SI

DE

Arch III

Hyoid arch (II) Mandibular arch (I)

ter

io r

4? ca

Arch IV

rd

in

al

5?

ve in ?

DY BO

An

B

D AN

Oral-pharyngeal O ral cavity cavity

F

N AI R

3?


6?


7?

ial ter k Ar run t

8? 9?

H ea


O

rt in

10?

pe

11?

ri ca rd

12?

ia l


sw el

13?

li ng

14? 15?


MID

LIN

E

16?

See a high magnification view of the prosencephalon, mesencephalon, and rhombencephalon from the right side of the brain in Plates 184A and B.

TS IDE RI

GH

ce nt ra lc an al

Posterior cardinal vein?

22?

21?

E IN DL I M

Somites (approximate numbers)

385

PLATE 178B

RHOMBENCEPHALON (lateral edge of pons) Nerve V boundary cap (Schwann cell GEP?) Lumen

Trigeminal ganglion (V) Vestibulocochlear ganglion (VIII) Migrating vestibulocochlear (VIII) ganglionic neurons from otic vesicle epithelium

zo

nes

ger min

Nerve X (vagus)?

Otic vesicle

Oral and pharyngeal preplacodes


al

PROSENCEPHALON (optic vesicle)

Germinal epithelium


c

Optha l m i

Olfactory placode

Labeled on this page: Central and peripheral neural structures

The optic vesicle is an evagination of the prosencephalon in the developing brain. It contains the opthalmic germinal zones, cells that will generate parts of the eye: retinal NEP, optic nerve glioepithelium, and the pigment epithelium. The otic vesicle epithelium generates vestibular ganglionic neurons and spiral ganglionic neurons.


Spinal roof plate

Arrows indicate the presumed direction of neuron migration from neuroepithelial sources. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

al in

Do rsa l


Sp


NE

P

Ventral

Intermediate

Dorsal


Spinal roof plate

SPINAL CORD

386

PLATE 179A

GW3.8 Sagittal, CR 3.5 mm 24-25 somites, C7724 Level 8: Slide 1, Section 30

LEFT SIDE OF BRAIN AND B ODY

1? A nt er io rc ar

2?

di na n? ei lv


3? 4?


5?


6?


7? Mandibular arch (I) Maxillary process

Hyoid arch (II)

Arch III

Arch IV

8?

ea

ial ter k Ar run t

H

9?

rt in pe

10?

ri ca rd


ia

11?

l sw el

12 (missing)?

li ng

13?

14?

15?

16?


17? 18? 19?

THIS PORTION OF THE BODY IS CUT IN THE CORONAL PLANE.

20? 21? central canal

22? 24?

23? MIDLINE


387

PLATE 179B

Otic vesicle


Lumen

Facial ganglion (VII)

Germinal epithelium

Migrating Migrating facial facial ganglionic ganglion(VII) (VII) neurons from germinal source in placode?

Inferior glossopharyngeal ganglion (X) Glossopharyngeal placode?

Facial placode?

PROSENCEPHALON (optic vesicle)

At this level, branchial/pharyngeal placodes can be linked to presumptive sensory ganglia.

Vagal placodes?


Lens placode?

Inferior vagal ganglion (X) Cephalic and branchial placodes join (farther laterally, a similarly placed site is most likely the trigeminal placode) Olfactory placode

Labeled on this page: Central and peripheral neural structures The optic vesicle is an evagination of the prosencephalon. It contains the opthalmic germinal zones that will generate parts of the eye: retinal NEP, optic nerve glioepithelium, and the pigment epithelium. Some peripheral sensory neurons in the head are generated by the neural crest and some by the placodes. The olfactory placode generates primary olfactory sensory neurons (I). Branchial placodes on the pharyngeal arches generate some of the sensory neurons in the trigeminal ganglion (V), the facial ganglion (VII), the glossopharyngeal ganglia (IX), and the vagal ganglia (X). The otic vesicle epithelium generates some of the vestibular ganglionic neurons and spiral ganglionic neurons (both contribute axons to nerve VIII).

Spinal surface (heavier line) NEP - Neuroepithelium Arrows indicate the presumed direction of neuron migration from germinal sources.

Spinal NEP


SPINAL CORD (sacral tip)

This entire section is from the right side of the brain and body.


GW3.8 Sagittal, CR 3.5 mm, C7724 Near Level 1: Slide 2, Section 33 PROSENCEPHALON, MESENCEPHALON, AND ANTERIOR RHOMBENCEPHALON

388

PLATE 180A

PLATE 180B

Brain surface (heavier line) Epithalamic

a Th

la m

ic

alic

pro

to

ve

nt

e

(f

ic

e

EP

H

R3

facial NEP

rhombencephalic protoventricle


N

Pharyngeal preplacodes

Upper rhombic lip

AL O

Expanding superarachnoid reticulum


RHOMBEN C

R2

trigeminal NEP

NEP

Pros



Oral preplacodes

Pontin e


r

Cerebellar NEP?

y p o t h ala m

Rathke's Rathke'spouch? pouch (primordium epitheliumof anterior (primordium pituitary of anterior gland) pituitary gland)

tu

ct)

o

Middl e/po ste rior h

u

du

rio r h y p

th

l

Mesenc ephal ic te gme nta lN EP

lic

Mesenceph ali c isthmal N EP

/a nte

Mandibular arch (I)

R3

cl

q

p ti c

ic m a la

tra

pha


R2

ri

a

eo

n Ve

ce en di


ON

Me se nc ep ha

third ventricles)

ic thalam S ub

AL

ue

ephalic c n N e telencephalic pool E

mesenceph

diencephalic pool

prosencephalic prosencephalic protoventricle (future lateral and protoventricle

Pr

EPH

P NE al ct te

Telencephalic?

E PP

N NC

ENC

S

diencephalic pool


MES

lic

PPRROOSSEE

P

phalic p retecta l NEP

n ai ) br es al ing ym en ch m en d es an m e, al on di b or in, im sk Pr se ( ca

H HA AL

LOO

N

N

M e s e n ce

phalic dience l a s r Do


Medullary velum

389

LEFT SID EO FB R


N AI

PROSENCEPHALON, MESENCEPHALON, AND ANTERIOR RHOMBENCEPHALON


IN L ID M

EA R EA

390

PLATE 181A

H

A

(future aqueduct)


n

Mesencephalic tege men tal

ic i d d l e

r

s Po


Upper rhombic lip

eN tin

EP

Reticular, raphe, abducens (VI), and facial motor (VII) NEPs?


(future fourth ventricle) Medullary velum

CE


Medial po n


RHOM


BE N


t

isthmal canal

? r NEP

Mandibular arch (I)

NEP)

er io

(hyp otha lam M

lla ebe Cer

alic

ON

ph

P

ce

NE P Reticular, raphe, oculomotor (III), and red nuclear NEPs?

diencephalic pool

Mesence ph al i c is th m

r

EP


e

P

e

en

rio

e p halic pre tecta l/te cta lN E


diencephalic pool

NC

NEP

Pr

te

Mes en c


AL

telencephalic pool

h a li c N EP

SE

N

h a l i c NE

cep

ME

Isthmal

p ce

ie n


halamic Epit

O

Telencephalic N EP ?

ld

L

i ld ra n n t /a Ve optic


Do rs a

PS

P ROSE

PS

ro s

NC

E

Thalamic

H

A

N


P

H

LO


G supe lial cha rfici nne al N ls fo r EP bordm er?

P NE al

PLATE 181B

391


PROSENCEPHALON, MESENCEPHALON, AND ANTERIOR RHOMBENCEPHALON LEFT SIDE OF BRAIN


Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.


392

PLATE 182A


M


P

?

ic

Pioneer migrating hypothalamic, subthalamic, tegmental and isthmal neurons

N

a


d

Po ntine

R3

NEP

R4


a u (v e st d i t or ibulo yN EP )


(facialNEP)



h o t y p

Upper rhombic lip


Te

r

MBENCEPHALON

lencephalic

lla

m

O AL

be

RHO

th

n

Mandibular arch (I)

re

P

ub

P)

a

Ce

R2

PROSENCEPH

mental

E i n al N

P r e o p t i c


Te g

ma

(tr ig e m

h

Mese (future aqueduct) nce p h a li c NEPS

S




u

P N E la a S


l

isth

E

sence p Pro h a l i c

cta

lic h a le ) ep tricuct c en en ed es v qu m otore a r p tu (f

mic Thala

te re

Migrating

Isthmal

Ep it

ic lam a h

N

Te c t a l

s ron eu ln

rm s fo er? nel bord n a P h al c NE Gli ficial r e sup


NCEPHALON ESE

e fa c sur line) n i Bra avier (h e

i c


l a m

PLATE 182B

Medullary velum

393

LATERAL MESENCEPHALON AND RHOMBENCEPHALON LEFT SIDE OF BRAIN See Level 5 in Plates 176A and B.


Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. Vestibulo-auditory NEP - germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei.

R5 R6 R7

Vestibulo-auditory NEP - germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei Glossopharyngeal NEP - germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. Vagal (X) sensory NEP - germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

394


PLATE 183A

PLATE 183B FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold



RHOMBENCEPHALON


Lower rhombic lip


Upper rhombic lip

Cochlear nuclear NEP? Precerebellar NEP?

rhombencephalic protoventricle (future fourth ventricle)

Medullary NEP

Cerebellar NEP?

O

N

E ne N Ponti Isthmal NEP?

R2 (trigeminal NEP)


P

R4 (vestibuloauditory NEP)

R3 (facial NEP)

Migrating facial (VII) neurons from R3 NEP Migrating trigeminal nuclear complex (V) neurons from R2 NEP

g ra tin M ig cta l te n s ro neu

MESENCE

PH

AL


Gracile and cuneate nuclear NEP?

R5 (vestibuloauditory NEP)

R7 (vagal NEP)


Dorsal lower medullary NEP

Migrating vestibular and auditory (VIII) neurons Migrating solitary nucleus neurons from R4 and R5 NEPs (IX, glossopharyngeal receptors) from R6 NEP Migrating vagal sensory (X) neurons from R7 NEP


Oral preplacodes




ng ati al igr ct s M rete ron p eu n

Cephalic preplacodes Hyoid arch (II)

Arch III


Mandibular arch (I)

395

GW3.8 Sagittal, CR 3.5 mm, C7724 Similar to Levels 6 and 7 on the right side of brain: Slide 2, Section 42 LATERAL PROSENCEPHALON, MESENCEPHALON, AND RHOMBENCEPHALON

PROPOSED RHOMBOMERE IDENTITIES R2 R3


See Level 6 in Plates 177A and B, Level 7 in Plates 178A and B.

396

PLATE 184A

PLATE 184B Brain surface (heavier line)


RHOM BEN CEP HA LO N


Pontine NEP

Lumen


Migrating tegmental neurons

li u m


Nerve IX (glossopharyngeal)?

th e


Otic vesicle

R3 (facial NEP)

pi

ct)

Te g m e n t a l N E P

l N EP

Te c t a

me pr sence (fu otove phal tur e aqntric ic ued le u

Isthmal NEP?

R2 (trigeminal NEP)

G er min a

le


Nerve X (vagus)? Migrating vestibulocochlear ganglionic neurons (VIII) from otic vesicle epithelium

tal NEP

Migrating te

MESENCEP

HAL

ON

ctal n eur on s?

Isthmal NEP?

ec Pret


Cephalic preplacodes Oral preplacodes

Oral-pharyngeal cavity Migrating pretectal neurons


Branchial preplacodes Glial channels form superficial germinal zone border?



Mandibular arch (I)

Hyoid arch (II)

Arch III Arch IV

397

Optic vesicle PROSENCEPHALON


398

PART PARTXV: XV: GW3.2 GW3.2 CORONAL CORONAL

This specimen is embryo #714 in the Minot Collection, designated here as M714. The crown-rump length (CR) is 4-mm. CR length is an unreliable measure to estimate gestational age because this specimen is much less mature than C7724, which also has a 4-mm CR. Since the number of somites could not be counted accurately in transverse sections, age determination is based on the degree of maturation of the central nervous system. The anterior neuropore closes at the 20-somite stage (Patten, 1953; Hamilton et al., 1959); M714 has a large open anterior neuropore. Using the timetables in Patten (1953) and Hamilton et al. (1959), we estimate that M714 has approximately 17 to 18 somites and is at gestational week (GW) 3.2. M714’s prosencephalic and anterior mesencephalic sections are cut (8 µm) in the coronal plane, but the plane shifts to predominantly horizontal in the posterior mesencephalon, pons, and medulla. We photographed 21 sections at low magnification from the first section containing the head to the posterior tips of the rhombencephalon. Fifteen of these sections are illustrated in Plates 185AB to 197AB. All photographs were used to produce computer-aided 3-D reconstructions of the external features of M714’s brain and optic vesicle (Figure 14), and to show each illustrated section in situ (insets, Plates 185A to 197A). Each illustrated section shows the brain with all surrounding tissues. Labels in A Plates (normal-contrast images) identify nonneural and peripheral neural structures; labels in B Plates (low-contrast images) identify central neural structures. The prosencephalon is small and incomplete with a slit-like protoventricle. In front of the optic vesicles, the anterior neuropore is broadly open ventrally, and narrows dorsally. The most anterior sections have a continuum between the neuroepithelium (presumptively future telencephalic) and the cephalic preplacodal epithelium. The dorsal part of the anterior neuropore is closed in sections of the optic vesicle; these neuroepithelia are more clearly

identified as future diencephalic. The preplacodal epithelium lines the lateral and ventral surfaces of the head and roof of the oral cavity. The evaginated optic vesicle is nearly touching part of the preplacodal epithelium that may have something to do with induction of the lens placode later on. An olfactory placode is very difficult to identify. The mesencephalon contains a stockbuilding neuroepithelium surrounding a narrow keyhole-shaped protoventricle. Future tectal neuroepithelium is a small arch over the top, while the future tegmental and isthmal neuroepithelia form the slite-shaped bottom. There is a very thin cell-free primordial plexiform layer in future tegmental and isthmal areas. The most prominent neuroepithelial structures in the rhombencephalon are the rhombomeric evaginations. In this specimen, several sections show how closely rhombomeres are associated with sensory cranial ganglia. The trigeminal ganglion (source of V sensory axons) is nearly attached to the brain surface at rhombomere 2. The vestibulocochlear ganglion (source of VIII axons) is attached to the rhombomere 4 brain surface. The otic vesicle touches the rhombomere 5 brain surface. The presumptive glossopharyngeal ganglion (source of IX sensory axons) is lateral to rhombomere 6. The short nerve extending from the large vagal ganglion (source of X sensory axons) touches the rhombomere 7 brain surface. The presumptive facial ganglion (source of sensory VII axons) is near a placode in the hyoid arch, slightly posterior and ventrolateral to rhombomere 3. A very thin layer of migrating neurons lines the superficial border of some rhombomeres; for the most part, cell migration has not yet started. The small cerebellar neuroepithelium is barely identifiable in the most posterior sections where the dorsal rhombencephalic neuroepithelium blends with tectal/isthmal neuroepithelia.

399

M714 Computer-aided 3-D Brain Reconstructions MESE NC A. Angled front view B. Side view

P o n

s

Open anterior neuropore

Preoptic area Hypothalamus

Preoptic area Infundibulum Mammillary body

R3

R4+5

Upper medulla

RHOMBENCEPHALON

R6+7

Medullary velum

R2

BRAINSTEM FLEXURES

R4+5

R6+7

Lower medulla

Lower medulla

Cerebellum

R3

edulla Upper m

Ventral diencephalon

R2

3

H y p ot

RHOMBENCEPHALO N

Future telencephalon?


m

P o n s

Cerebellum

Optic vesicle

Optic vesicle

tu en

S u bthalamus Te g m la mu s

Teg me nt u

m

Upper rhombic lip

4

ha

Subthalamus

Thalamus

PROSENCEPHALON

Thalamus

Epithalamus

N

Epithalamus

Isthmus

Dorsal diencephalon

Isthmus

PROSENCEPHALON

Tectum

LO

M E S E N C E P H Pretectum A Tectum

EP H A L O N

Pretectum

1. Medullary 3. Mesencephalic

1

4. Diencephalic

Lower rhombic lip

R - Rhombomere Spinal cord Spinal cord

C. Top view

C

s

w

p

RHOMBENCEPHALON

d

nal Cor pi

a ll

me d er u

P s n o

R6+7

la

med er u

l

o

u m h t

Te

s

u gm e nt

u

s us

am

y

H

la m

I

tha

P


po

s

p

Roof of diencephalic protoventricle

Sub

th

al

n

U

m


ea c ar pti eo Pr

PROSENCEPHALON

s

R4+5

o

Open anterior neuropore

R3

L

R2

MESENCEPHALON

Optic vesicle

S

D. Bottom view

r C o

T

h m e r e u b e l l

m

e

Pr

c

t

u

etectum

Epithalamus

a lamu

Th

s

u

s

m

s

I

Medullary velum

i n a l

Upper rhombic lip

t

Figure 14. A, The left side of the 3–D model viewed from the front at a 45º heading; this view is used to "peel away" sections of each level in the following Plates. B, A straight view of the left side. C, A straight down view of the top. D, An upward view of the bottom, angled (120º) to look into the mesencephalic and diencephalic flexures. Arrows indicate the open anterior neuropore.

RHOMBENCEPHALON

Future telencephalon? Open anterior neuropore

n P o

PROSENCEPHALON

R2

d

MESENCEPHALON

Optic vesicle

S

p

400

PLATE 185A


GW3.2 Coronal 17-18 Somites? M714


Dorsal junction between cephalic preplacodal epithelium and prosencephalic NEP

Cephalic preplacode


Ventral junction between cephalic preplacodal epithelium and prosencephalic NEP

Level 2: Section 13 Cephalic preplacode

Optic vesicle

Lens placode?


Future olfactory placode? Ventral junction between cephalic preplacodal epithelium and prosencephalic NEP


401

PLATE 185B


PROSENCEPHALON

Level 1: Section 3 closing anterior neuropore (dorsal) Brain surface (heavier line)

Prosencephalic NEP

(future diencephalic?)


open prosencephalic protoventricle

Prosencephalic NEP


(future telencephalic?)

anterior neuropore (ventral)

Level 2: Section 13

PROSENCEPHALON THALAMUS



Thalamic NEP

Thalamic primordial plexiform layer Brain surface (heavier line)

SUBTHALAMUS

Subthalamic NEP



Hypothalamic primordial plexiform layer?

Future pigment epithelium?

Optic vesicle

diencephalic protoventricle

Future retinal NEP?

(future third ventricle) optic recess Preoptic primordial plexiform layer?

Preoptic area NEP? VENTRAL DIENCEPHALON

open prosencephalic protoventricle (future lateral and third ventricles)


closing anterior neuropore (ventral)


Arrows indicate the regionally expanding shoreline of the protoventricle with increase in stockbuilding NEP cells.

402

PLATE 186A


GW3.2 Coronal 17-18 Somites? M714 Level 3: Section 18


Optic vesicle

Cephalic preplacode

Future lens placode?

Future olfactory placode?

Branchial preplacode

Lateral tongue primordium?

Mandibular arch (I)





Hyo-mandibular cleft Hyoid arch (II)


Arches III and IV

403

PLATE 186B


DIENCEPHALON THALAMUS/EPITHALAMUS


(pineal gland primordium?)

Epihalamic NEP?

Thalamic/epithalamic primordial plexiform layer Brain surface (heavier line)

Thalamic NEP? SUBTHALAMUS

Subthalamic NEP


Middle hypothalamic NEP?



optic recess

Optic vesicle Future retinal NEP?



Preoptic primordial plexiform layer

Preoptic area NEP? Diencephalic floor plate VENTRAL DIENCEPHALON






404

PLATE 187A



Primordial mesenchymal brain case (skin, bone, and meninges) Maxillary process Optic vesicle Cephalic preplacode Cephalic preplacode Future Rathke's pouch?


Lateral tongue primordium Part of the mandibular arch placodal epithelium gives rise to the thyroid gland.

Arterial trunk Peritoneal cavity


Mandibular arch (I)

405

PLATE 187B


DIENCEPHALON THALAMUS/EPITHALAMUS

Diencephalic roof plate Epihalamic NEP?


Thalamic NEP SUBTHALAMUS Subthalamic primordial plexiform layer

Subthalamic NEP


Optic vesicle germinal zone


infundibular recess

Hypothalamic NEP



(primordium of median eminence and neurohypophysis?)

VENTRAL DIENCEPHALON




406

PLATE 188A


GW3.2 Coronal 17-18 Somites? M714 Level 5: Section 33 Primordial mesenchymal brain case (skin, bone, and meninges)


Posterior optic vesicle? Maxillary process Cephalic preplacode Trigeminal ganglion placode?

Future Rathke's pouch? Lateral tongue primordium Branchial/pharyngeal preplacodes

Mandibular arch (I)

Part of the mandibular arch placodal epithelium gives rise to the thyroid gland. Hyoid arch (II)

Medial tongue primordia Arterial trunk

Arch III?

Peritoneal cavity

Primitive gut


407

PLATE 188B


Diencephalic roof plate Epihalamic NEP


Thalamic NEP SUBTHALAMUS Subthalamic primordial plexiform layer

Subthalamic NEP


(future third ventricle) Hypothalamic primordial plexiform layer

Middle/posterior hypothalamic NEP?

infundibular and mammillary recesses


(primordium of median eminence and neurohypophysis?)

Migrating trigeminal ganglionic (V) neurons?


HYPOTHALAMUS





408


PLATE 189A GW3.2 Coronal 17-18 Somites? M714 Level 6: Section 38 Primordial mesenchymal brain case (skin, bone, and meninges)


Cephalic preplacode

Trigeminal ganglion (V) placode? Branchial/pharyngeal preplacodes

Rathke's pouch epithelium (primordium of anterior pituitary) Oralpharyngeal cavity


Maxillary process

Mandibular arch (I)

Hyoid arch (II)

Arch III?

Arterial trunk

Arch IV?

Peritoneal cavity

Primitive gut


409

PLATE 189B



mesencephalic protoventricle (future aqueduct)

Pretectal primordial plexiform layer

Pretectal NEP


SUBTHALAMUS?

Subthalamic NEP



(future third ventricle) Hypothalamic primordial plexiform layer

Posterior hypothalamic NEP

mammillary recess



Migrating trigeminal ganglionic neurons from the trigeminal placode in the fusing maxillary process and mandibular arch

HYPOTHALAMUS

DIENCEPHALON





PLATE 190A GW3.2 Coronal 17-18 Somites? M714 Level 7: Section 43

achnoid reticulum



ativ e su

per a r


For m

410

Cephalic preplacode

Migrating trigeminal ganglionic neurons? Trigeminal ganglion (V) placode? Branchial/pharyngeal preplacodes

Maxillary process

Rathke's pouch epithelium (primordium of anterior pituitary)


Mandibular arch (I)

Hyoid arch (II) Multiple loci in the placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands.

Arch III?


Arch IV? Primitive gut

Ganglion X?

Anterior cardinal vein Dorsal aorta Notochord Dorsal root ganglion


Somites

411


PLATE 190B Pretectal primordial plexiform layer

PRETECTUM

Mesencephalic roof plate Pretectal NEP

TEGMENTUM

Brain surface (heavier line) Tegmental primordial plexiform layer

Tegmental NEP



(future aqueduct)


Posterior hypothalamic NEP

Future mammillary body

HYPOTHALAMUS

DIENCEPHALON

Peripheral neural structure Migrating trigeminal ganglionic neurons from the trigeminal placode in the fusing maxillary process and mandibular arch

SPINAL CORD




Spinal floor plate


Ventral NEP



slit-shaped central canal





achnoid reticulum

PLATE 191A

per a r


ativ e su


For m

412

Trigeminal ganglion (V) Trigeminal ganglion (V) placode?

Anterior cardinal vein? Maxillary process

Notochord

Migrating trigeminal ganglionic neurons? Trigeminal ganglion (V) placode?

Internal carotid artery?

Mandibular arch (I)

Pharynx Hyoid arch (II) Multiple loci in the placodal epithelium of the arches gives rise to the thyroid, parathyroid, and thymus glands.

Vagal ganglion (X) placode? Migrating vagal ganglionic (X) neurons?

Branchial/ pharyngeal preplacodes

Arch III?

Arch IV?

Vagal ganglion (X)? Anterior cardinal vein Dorsal aorta

Notochord

Somites Dorsal root ganglion primordium




TEGMENTUM

Tegmental NEP

413


PLATE 191B


mesencephalic protoventricle (future aqueduct)




Migrating trigeminal ganglionic neurons from the trigeminal placode in the fused maxillary process and mandibular arch

Migrating vagal ganglionic neurons from the vagal placode in arch IV

SPINAL CORD

Spinal germinal zones Spinal floor plate


Ventral NEP Intermediate NEP Dorsal NEP Spinal roof plate



Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the protoventricle with increase in stockbuilding NEP cells. ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium


PLATE 192A



achnoid reticulum


ativ e su

per a r


For m

414



Notochord Internal carotid artery? Facial ganglion (VII) placode?

Hyoid arch (II)

Facial ganglion VII?

Glossopharyngeal ganglion IX? Glossopharyngeal ganglion (IX) placode? Arch III?

Dorsal aorta Pharyngeal preplacodes

Pharynx


Arch IV?

Vagal ganglion (X)?


Notochord

Somites Dorsal root ganglion


415

Central neural structures labeled Tectal primordial plexiform layer



PLATE 192B


Tectal NEP TEGMENTUM/ISTHMUS



(future aqueduct)


(raphe glial system GEP)

Isthmal NEP

Metencephalic floor plate


(raphe glial system GEP)

PONS

Migrating trigeminal ganglionic neurons from the trigeminal placode in the fused maxillary process and mandibular arch

RHOMBENCEPHALON

Migrating facial ganglionic neurons from the facial placode in the hyoid arch Migrating glossopharyngeal ganglionic neurons from the glossopharyngeal placode in arch III


SPINAL CORD



Ventral NEP



Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the protoventricle with increase in stockbuilding NEP cells.

ABBREVIATIONS: slit-shaped GEP - Glioepithelium central NEP - Neuroepithelium canal FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold

416

PLATE 193A



Facial ganglion (VII) placode? Facial ganglion (VII)?

Glossopharyngeal ganglion (IX)? Glossopharyngeal ganglion (IX) placode? Notochord Vagal ganglion X?


s u p e r a r a c h n o i d


F o r m a t i v e

Fused maxillary process and mandibular arch

r e t i c u l u m


Ganglion VIII

Hyoid arch (II)

Arch III?

Arch IV?

Somites Dorsal root ganglion primordium



417

PLATE 193B

Central neural structures labeled Brain surface (heavier line)




TEGMENTUM/ISTHMUS

Tegmental NEP

Isthmal NEP




(future aqueduct)

Medial pontine NEP Metencephalic floor plate

(midline raphe glial system GEP) Pontine primordial plexiform layer

PONS

Medial pontine NEP


RHOMBENCEPHALON

Migrating glossopharyngeal ganglionic neurons from the glossopharyngeal placode in arch III

Peripheral neural structure Migrating facial ganglionic neurons from the facial placode in the hyoid arch


SPINAL CORD



Ventral NEP

Ventral commissure



Intermediate NEP






418

PLATE 194A




Fused maxillary process and mandibular arch Trigeminal ganglion (V) Trigeminal ganglion boundary cap (Schwann cell GEP?)

Vestibulocochlear ganglion (VIII) Epithelium Lumen

Glossopharyngeal ganglion (IX)?


Vagal ganglion (X)?

Formative iculum superarachn oid ret

Otic vesicle

Somites



419

Central neural structures labeled MESENCEPHALON ISTHMUS

PLATE 194B PROPOSED RHOMBOMERE IDENTITIES



R2 Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R3 Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. R4 Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R5 Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R6 Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. R7 Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


Isthmal NEP mesencephalic protoventricle

(future aqueduct)

PONS/MEDULLA



Medial pontine NEP Migrating trigeminal (V) neurons?

R2 (trigeminal NEP) Migrating facial (VII) neurons R3 (facial NEP)

R4+5 (vestibulo-auditory NEP)

Migrating vestibulocochlear ganglionic neurons from the otic epithelium

Migrating vestibulo-auditory (VIII) neurons



Migrating glossopharyngeal receptor neurons (solitary nucleus)

R7 (vagal sensory NEP) Migrating vagal sensory (X) neurons Migrating vagal motor (X) and hypoglossal (XII) neurons?


(vagal motor [X] and hypoglossal [XII] NEPs blend with ventral spinal NEP)


Spinal germinal zones Ventral NEP

Migrating ventral gray neurons? Primordial white matter

Intermediate NEP


Migrating intermediate gray neurons?


ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium R - Rhombomere Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the protoventricle with increase in stockbuilding NEP cells.


420

PLATE 195A





Otic placode Otic vesicle

Epithelium Lumen


Vagal ganglion (X)

Somites



421

PLATE 195B



MESENCEPHALON ISTHMUS

R2 Trigeminal NEP - germinal source of the central trigeminal nuclei except the mesencephalic nucleus. R3 Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. R4 Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R5 Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R6 Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. R7 Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.


Mesencephalic roof plate Isthmal primordial plexiform layer

Isthmal NEP mesencephalic protoventricle

(future aqueduct)

PONS/MEDULLA

R2 (trigeminal NEP) rhombencephalic protoventricle R3 (facial NEP)


R4 (vestibulo-auditory NEP) Migrating vestibulo-auditory neurons

Migrating vestibulocochlear ganglionic neurons from the otic epithelium



Migrating glossopharyngeal receptor neurons (solitary nucleus) R6 (glossopharyngeal NEP)



Intermediate medullary NEP

(blends with intermediate spinal NEP)

Pioneer migrating medullary and spinal neurons



Intermediate NEP






422

PLATE 196A




Vestibulocochlear ganglion (VIII) Migrating vestibulocochlear ganglionic neurons from the otic epithelium Epithelium

Otic vesicle

Lumen

Glossopharyngeal ganglion boundary cap* Glossopharyngeal ganglion (IX) Vagal ganglion (X)

Vagal ganglion boundary cap*

Dorsal root ganglion boundary caps*

* Boundary caps are

Schwann cell GEPs? Somites


423

PLATE 196B



Midline rhombencephalic roof plate CEREBELLUM

Cerebellar NEP (vermis?)


Cerebellar NEP (hemisphere?) rhombencephalic protoventricle


Lateral rhombencephalic roof plate? PONS/MEDULLA

(future rhombic lips)

R3 (facial NEP)

metencephalic pool


Future medullary velum

R3 Facial NEP - germinal source of facial motor and sensory receptor neurons of the facial (VII) ganglion. R4 Vestibulo-auditory NEP germinal source (with R5) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R5 Vestibulo-auditory NEP germinal source (with R4) of central auditory nuclei and vestibular nuclei, except the cochlear nuclei. R6 Glossopharyngeal NEP germinal source of sensory neurons that receive input from the glossopharyngeal (IX) ganglion. R7 Vagal (X) sensory NEP germinal source of the dorsal sensory nucleus and other sensory vagal nuclei.

Migrating vestibulo-auditory neurons


myelencephalic pool

R5 (vestibulo-auditory NEP) Migrating glossopharyngeal receptor neurons (solitary nucleus) R6 (glossopharyngeal NEP)

R7 (vagal sensory NEP) Migrating vagal sensory (X) neurons

Dorsomedial lower medullary NEP



Pioneer migrating medullary and spinal neurons

Spinal germinal zones Dorsal NEP



Spinal roof plate



424

PLATE 197A GW3.2 Coronal 17-18 Somites? M714


Level 14: Section 88







425 Anteromedial myelencephalic roof plate

PLATE 197B


MEDULLA Lateral myelencephalic roof plate (ventral rhombic lip)

Medullary velum

Future precerebellar and auditory

(cochlear nuclear)

NEPs?


(future fourth ventricle, myelencephalic pool)

Dorsomedial lower medullary NEP (gracile and cuneate nuclei?)


Level 15: Section 93 Posteromedial myelencephalic roof plate

MEDULLA

Anteromedial myelencephalic roof plate

RHOMBENCEPHALON Medullary velum

Dorsomedial lower medullary NEP (gracile and cuneate nuclei?)



Arrows indicate the presumed direction of neuron migration from germinal sources. Arrows indicate the regionally expanding shoreline of the protoventricle with increase in stockbuilding NEP cells. FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Roman or Bold


(future fourth ventricle, myelencephalic pool) Posteromedial myelencephalic roof plate

RHOMBENCEPHALON

426

PART PARTXVI XVI

CONCLUDING CONCLUDING ESSAY ESSAY JOSEPH ALTMAN and SHIRLEY A. BAYER

We began to work on this project over a decade ago to produce a comprehensive, multi-volume Atlas of Human Central Nervous System Development (CNS). Our aim in starting this project was to try to interpret normal human CNS development in light of the understanding we have gained in the preceding three decades from an experimental analysis of the prenatal and postnatal development of the rat CNS. In that extensive work, we injected 3H-thymidine at daily intervals to groups of pregnant rats. Those injections labeled DNA in the proliferating progenitors of neurons and neuroglia in the rat embryos and fetuses. We also injected 3H-thymidine at varied intervals to groups of infant, juvenile, and adult rats to study the postnatal course of cell proliferation during late CNS development. By varying survival times after administration of the radiochemical from hours, to days and months, we used the techniques of short-survival, sequential-survival, and long-survival autoradiography to achieve the following. (1) Determine the proliferation dynamics of progenitor cells in the various compartments (mosaics) of the primary neuroepithelium (NEP) and in the various secondary germinal matrices—cortical and striatal subventricular zones (SVZ), cerebellar external germinal layer (EGL), hippocampal subgranular zone (SGZ)—as a function of prenatal and postnatal age. (2) Track the migratory routes of different populations of young neurons, their sojourn in transitional fields, and their final settling in the developing CNS. (3) Construct quantitative timetables of the birth dates of different classes of mature neurons in different components of the adult rat CNS. The results of these studies were published in a series of journal articles (see Introduction, Part ID), and were reviewed in chapters contributed to edited books (Altman, 1992; Altman and Bayer, 1975, 2004; Bayer and Altman, 1995a, 1995b, 2004b). We embarked on this ambitious project for two reasons. First, to fill a gaping void in the literature on this important subject. There is currently no comprehensive atlas available that covers the entire time span of prenatal human CNS development from the time when the neural tube and the brain vesicles close (approximately GW3) until birth at the end of the third trimester (approximately GW37). Although there are many published accounts of certain facets of the early development of the human CNS (mostly in volumes of the Carnegie Institution of Washington, Contributions to Embryology), and a few published overviews of human brain development (e.g., Sidman and Rakic,

1982), as well as books on its early phases (e.g., Gasser, 1975; O’Rahilly and Müller, 1994), this five-volume atlas is the first comprehensive source that provides a detailed description of the entire course of this momentous morphogenetic event. There are obvious needs for a detailed description of the prenatal development of the human brain and spinal cord: (i) as an aid to medical practitioners, (ii) as a reference work for molecular, physiological, and behavioral neurobiologists, and (iii) as an empirical foundation for the ethical, legal, and psychological assessments of the putative mental status of the human embryo and fetus. Second, we hoped that by extrapolating from the experimental data obtained in animals, we could go beyond a mere narrative account of developmental landmarks in human CNS development to a dynamic analysis of some of the morphogenetic processes involved. What we were surprised to find is that our detailed examination of the full course of CNS development in normal human embryos and fetuses has come to shed new light on some of the basic mechanisms involved in the production, migration, differentiation, and assembly of CNS neurons, and some aspects of its afferent and efferent wiring and circuitry formation. We begin here with a brief overview of these insights and provide some of the details with summary illustrations and documentation in the succeeding sections.

A. Overview The Beginning of CNS Development: Stockbuilding NEP Cells and the NEP Matrix. An examination of the CNS of an older embryo or fetus may give the impression that the germinal matrix lining the ventricle (the ependymal layer of classical neuroanatomists and the ventricular zone of modern ones) is merely one of its laminar constituents, one that generates neurons and neuroglia for its more mature strata. In fact, for several weeks after closure of the neural tube, the human CNS consists only of a proliferative matrix of NEP cells and is devoid of differentiating neurons and neuroglia. The proliferating pluripotent or fate-restricted stockbuilding progenitor cells that compose the mosaic compartments of the NEP matrix are the sole constituents of the early-embryonic CNS. As development proceeds, the stockbuilding NEP cells give rise, at different rates in different NEP compartments, to differentiating (postmitotic) daughter cells. The latter exit the proliferative NEP matrix and start to form the differentiating elements of the brain parenchyma.

427 The Superventricles and their Variegated Shorelines. We offer evidence that, after closure of the neural tube, the mitotic division of stockbuilding NEP cells is promoted by the hypertrophied telencephalic, diencephalic, mesencephalic, and rhombencephalic superventricles. Because stem cell nuclei of the pseudostratified NEP matrix have to shuttle to the ventricular lumen to undergo mitosis, the areal extent and configuration (protuberances, eminences, invaginations, etc.) of the variegated superventricular shorelines, and their persistence over time, are a limiting factor in determining the population size of the neurons generated at different NEP locations. For instance, the immense growth of the cerebral and cerebellar cortices in the human CNS is dependent upon the immense expansion and long endurance of the telencephalic and rhombencephalic superventricles, respectively. Moreover, the cerebrospinal fluid of the superventricles and the hypertrophied embryonic telencephalic and rhombencephalic choroid plexuses may contain trophic factors that promote NEP cell proliferation.

an important role in the early evolution of the invertebrate and protochordate CNS, the overriding principle of CNS development in lower and higher vertebrates (including humans) is NEP mosaicism, the progressive diversification of progenitor compartments to generate neural systems with distinctive functions.

The Superarachnoid Reticulum as a Parenchymal Expansion Field. As first identified in the present volume, the developing human brain is encased during the first trimester in the superarachnoid reticulum, a transiently inflated and spongy meningeal tissue sandwiched between the pial membrane, adjacent to the brain, and the formative dural membrane, adjacent to the mesenchymal tissue that will form the skull. The initial enlargement of the superarachnoid reticulum antedates the onset of neuronal migration and differentiation, providing expansion space for the settling neurons, their afferent and efferent fibers, and other differentiating components that form the developing brain parenchyma. Accordingly, we propose that during the embryonic and early fetal periods, components of the hypertrophied superarachnoid reticulum serve as regional parenchymal expansion fields. The superarachnoid fluids may contain trophic factors that promote NEP cell differentiation.

Neuronal Migration, Sojourning, and Transitional Fields. Although the fate-specification of neurons begins before they leave their NEP compartments, their ongoing diversification is dependent on subsequent events, beginning with their migration, sojourning, and interactions with other neural elements along their trajectory. Instead of a uniform principle of cell migration, the available evidence suggests great diversity in the patterns and mechanisms used by different translocating neuronal populations and their dependence on different guideposts and signaling agents. Some classes of neurons migrate a short-distance, others follow a long course; some neurons migrate singly or in small groups, others form chains or large streams; some neurons move radially, others take a tangential or a tortuous path; some neurons move directly to their final destination, others sojourn for a shorter or longer period in transitional fields where they are subjected to different influences and where they may establish transient or enduring connections with other neuronal systems.

Metamerism versus Functional Mosaicism as Principles of CNS Development. According to the popular metameric hypothesis, the mammalian neuraxis is composed of a large number of reiterated transverse blocks or segments. However, the evidence we present indicates that the diverse NEP components of the dorsal and ventral mesencephalon, diencephalon, and telencephalon give no hint of a metameric organization at any phase of their development. While the trunk region and the spinal cord are distinguished by peripheral segmentation (the reiterated somites and the dorsal and ventral roots), the central gray matter of the spinal cord has a longitudinal (columnar) rather than a segmental organization. And while the rhombencephalon shows distinctive central neuromerism at a certain stage of its development, the different rhombomeres are not reiterated units but highly diversified NEP mosaics (see below). Although peripheral metamerism probably played

The Rhombomeres as NEP Mosaics. Contrary to the widely held view that the head-related rhombomeres are reiterated segmental units, analogous to the trunk-related peripheral somites and spinal ganglia, we present morphogenetic evidence that rhombomeres 2 to 7, are transient NEP mosaics that are directly linked with different cranial ganglia—the trigeminal, facial, vestibulocochlear, glossopharyngeal, and vagal ganglia—and morphogenetically related to the branchial and epibranchial placodes that, together with neural crest cells and some mesenchymal elements, give rise to such diverse structures as the face, the jaws, the palate, the inner ear, the upper gut, and several visceral organs.

The Secondary Germinal Matrices. Proliferative NEP cells in some regions of the CNS generate not only differentiating neurons but also fate-restricted cells that retain their proliferative potency after they have left the ventricular lumen. These secondary matrices include the SVZ of the neocortex and the basal ganglia, the interstitial subgranular zone (SGZ) of the hippocampal dentate gyrus, and the subpial EGL of the cerebellar cortex and the cochlear nuclei. The secondary germinal matrices persist in many regions for a long time after the primary NEP matrix has disappeared. The neurogenic secondary matrices produce microneurons (like granule cells) with locally arborizing axons that become interdigitated with the earlier-generated larger neurons with long axons. Dispersed, fate-restricted glial progenitor cells in the CNS produce neuroglia that support neuronal growth and repair throughout life.

428 Periphero-Central Induction and Signaling. Functional mosaicism is a product of reciprocal induction and signaling between diversifying NEP matrix compartments and the peripheral or central structures with which they are fated to interact. At the beginning of development, NEP matrix compartmentation has to be coordinated with different peripheral structures: the sensory neurons of the spinal and cranial ganglia, the sense organs they serve, and the different peripheral muscle groups they will innervate. In the trunk region, this periphero-central coordination is accomplished by reciprocal induction and signaling between the spinal NEP and regional neural crest cell populations, the somites, the notochord, and their derivatives (the spinal ganglia, the cutaneous receptors, the developing axial and limb muscles, etc.). In the head region, this coordination is accomplished by reciprocal induction and signaling between the cephalic NEP and the peripheral cranial, branchial, and epibranchial placodes and their derivatives (the olfactory epithelium, the lens, the cranial sensory ganglia, the inner ear, etc.). This coordination occurs during the early first trimester in humans. Current research in experimental animals, which we will briefly review, implicates specific genes, transcription factors, and signaling molecules are involved in this morphogenetic transaction. Centro-Central Induction and Signaling. The higherorder components of the CNS—such as the thalamic relay nuclei, the neocortical sensory and motor projection areas, the feedback loops of the cerebellum and the basal ganglia—have no direct connections with peripheral sense organs and effectors. However, they are intimately associated with one another through large fiber tracts and elaborate regional networks of axon terminals, dendrites and synaptic junctions. The establishment of topographically organized projection systems, and the development of serial or hierarchic interconnections among them, is dependent on centro-central induction and signaling. Current research in animals focuses on the identification of signaling mechanisms involved in the guided pathfinding of axons and the choreographed migration of neurons. In the human CNS, long-range axonal connections are established by the late first trimester but the establishment of the fine circuitry of many brain regions through interdigitation of interneurons and microneurons is a lengthy process that extends beyond the second and third trimesters and continues through the postnatal period of brain development.

B. The NEP Matrix: Stockbuilding NEP Cells, and Differentiating NEP Cells The NEP Matrix. For weeks after uterine implantation, the human embryo lacks a functional CNS with differentiated neurons furnished with dendrites, axons, and intercellular connections that make possible the gathering, conveying, processing, and storing of sensory information and the generation of responses to them by overt movements and

actions. It is only about one month following conception (approximately GW4-4.5) that differentiating neurons start to aggregate in the earliest-maturing regions of the spinal cord (Altman and Bayer, 2001). In the late-developing cerebral cortex, the rudiment of the gray matter, the cortical plate, does not begin to form until approximately GW8-8.5 (Volume 4 of this Atlas). Instead of differentiating neurons, the early embryonic CNS consists of an expanding proliferative matrix of neural stem cells and precursor cells, the neuroepithelium (NEP). The life-career of the NEP matrix begins in the form of a superficial, flat ectodermal sheet, the neural plate, which is recognizable in the human embryo by approximately GW2.5 (O’Rahilly and Müller, 1994). This open NEP matrix is devoid of its own distinctive fluid environment. The ventricular system filled with cerebrospinal fluid (CSF) begins to form at approximately GW3-3.5 when the open neural plate changes into a closed vessel. This takes place as two other ectodermal derivatives, the neural crest and the placodes, differentiate lateral to the folding neural plate. The highly motile neural crest cells give rise to the PNS of the trunk region, including the neurons of the dorsal root ganglia, the sympathetic and parasympathetic ganglia, and enteric nervous system, as well as some non-neuronal elements (Weston, 1970; LeDouarin, 1982). In the head region, neural crest cells are believed to produce some neuronal elements and the facial skeleton, but the peripheral cranial nerve ganglia and components of the head sensors (olfactory, visual, auditory) originate from a distinctive ectodermal matrix, the placodes (Knouff, 1935; Jacobson, 1963; Noden 1993). After separation of the neural crest and the placodes, the neural folds fuse dorsally and that leads, in the trunk region, to the formation of the closed NEP matrix of the neural tube (future spinal cord) and, in the head region, to the formation of the cranial vesicles (future brain). There are initially three cranial vesicles: the prosencephalic NEP (forebrain primordium), the mesencephalic NEP (midbrain primordium), and the rhombencephalic NEP (hindbrain primordium). Then, as a portion of the prosencephalic NEP evaginates and expands laterally, the forebrain rudiment is transformed into a medial diencephalic NEP (future thalamus, subthalamus, and hypothalamus) and a bilateral telencephalic NEP (future cerebral cortex, basal ganglia, and olfactory bulb). Stockbuilding and Differentiating NEP Cells. There are several controversial issues regarding the features and properties of NEP cells that constitute the NEP matrix. One concerns the differences in the cleavage orientation and fate of NEP cells that undergo mitosis near the lumen, another concerns the variable morphology of NEP cells, and still another the typology of NEP cells. It has been noted in the past (e.g., Bayer and Altman, 1991a) that the cleavage plane of NEP cells varies from vertical (perpendicular to the ventricular lining) to horizontal (parallel to the ventricular lining). It has been hypothesized that vertical cleavage results in symmetrical cell division, and hori-

429 zontal cleavage in asymmetric cell division (e.g., Chenn and McConnell, 1995). Symmetric NEP cell division is assumed to produce two neural precursor cells, or what we call stockbuilding cells. Asymmetric cell division is presumed to produce at least one differentiating (postmitotic) neural cell. Presumably the cell located farther from the ventricular lumen withdraws from the mitotic cell cycle, leaves the NEP matrix, and starts the long process of differentiation. From the perspective of the areal extent of the ventricular shoreline, vertically cleaving cells take up twice as much NEP/CSF interface space as the horizontally cleaving cells. Hence, a high rate of symmetric divisions should result in expansion of the ventricular shoreline whereas high rate of asymmetric divisions should result in its shrinkage. Indeed, some evidence has been presented that there is an increase in the asymmetric division of cortical NEP cells in mice as a function of increasing fetal age (Estivill-Torrus et al., 2002). Currently there is considerable interest in the molecular mechanisms that affect the switch from stockbuilding to neurogenic NEP cell division. Notch signaling has been reported to foster expansionary (stockbuilding) symmetric division of NEP cells (Ishibashi et al., 1994; Artavanis-Tsakonas et al., 1999; Alexson et al., 2006). In the absence of repressor type-bHLH (basic helix-loop-helix) genes, which are essential for Notch signaling, NEP cells prematurely differentiate into neurons and neuroglia (Nakamura et al., 2000; Hatakeyama et al., 2004). Transient misexpression in mice of the repressor -type bHLH genes, Hes1 and Hes5, known Notch effectors, results in the expansion of the stockbuilding population of telencephalic NEP cells, but once Hes expression starts to decrease, the NEP cells differentiate into neurons and neuroglia (Ohtsuka et al., 2001). Another consequence of the depletion of repressor type Hes genes is that the premature differentiation of NEP cells prevents the formation of lategenerated astrocytes and ependymal cells (Kageyama et al., 2005). Other molecular factors that appear to promote stockbuilding cortical NEP cell division include C3G, a guanine nucleotide exchange factor (Voss et al., 2006), and ß-Catenin, a protein enriched in adherens junctions at the ventricular lining, since its over-expression produces enlarged cortices in transgenic mice (Chenn and Walsh, 2003). Some stockbuilding genes and factors antagonize genes, while other factors promote cell differentiation. Among the latter are the activator type bHLH genes, Mash1, Math, and neurogenin (Kageyama et al., 2005). Another factor, Aspm, a protein whose mutation is associated with microcephaly (reduced neuron populations) in humans, is down-regulated in NEP cells as they switch from stockbuilding to neurogenic cell division (Fish et al., 2006). It is important to emphasize that the NEP cells which line the ventricles have neither the morphological nor the physiological features of their progeny, the distinctive neurons, neuroglia, ependymal cells, tanycytes, and a few other neural elements of the developing and mature CNS. Dif-

ferentiated neurons reside outside the NEP matrix, have axons, dendrites and synaptic vesicles, and conduct generator and action potentials. In higher vertebrates and humans, the NEP matrix does not contain differentiated neurons. Differentiated neuroglia, likewise, have clear distinguishing anatomical and physiological characteristics. For instance, oligodendrocyte lamellae produce the myelin sheath of axons, astrocyte processes nourish neurons, and the radial glia (like the Bergmann glia of the cerebellar cortex) form compartmental palisades. However, we cannot flatly assert that the NEP matrix does not contain neuroglia because that contradicts the popular current view that some of the cells of the ventricular matrix are “radial glia” (e.g., Malatesta et al., 2000; Hartfuss et al., 2001). However, that designation is not justified and has only led to conceptual confusion. Observations made over a century with the Golgi technique has revealed three types of NEP cells: (1) a round globular cell with its endfoot attached to the ventricular lumen; (2) an oval fibrous cell with its endfoot contacting the ventricle and its thin radial fiber reaching the pial surface; and (3) a detached cell with its leading fiber approaching or reaching the pial surface. (Illustrations of these cell types were provided by Morest, 1970, and Morest and Silver, 2003.) Most (though not all) anatomists have assumed that these NEP cells are neural progenitor cells but differed in viewing them as either different types of precursor cells or different stages of the same cell. The popular notion that the fibrous NEP cells are “radial glia,” a separate cell lineage that guides migrating young cortical neurons toward their targets, was advocated by Rakic (1971). The subsequent demonstration that many of these ventricular cells express the glial marker GFAP (Levitt et al., 1981) reinforced this identification. However, the expression of a marker that in the mature nervous system specifically reacts with glial filaments, does not rule out the possibility that the same marker reacts with transient filamentous elements in non-glial embryonic cells (Bennett, 1987). Indeed, the current demonstration that the majority of mitotic NEP cells with radial fibers are neuron progenitors (Malatesta et al., 2000; Miyata et al., 2001; Tamamaki et al., 2001; Noctor et al., 2002) definitively establishes that the fibrous NEP cells are not specialized glia (like the Bergmann radial glia) but are prototypical NEP cells. In addition to neurons, the fibrous NEP cells may also generate other neural elements, such as astrocytes (Misson et al., 1991). Whether there is any direct relationship between symmetrically and asymmetrically dividing NEP cells, on the one hand, and the globular and fibrous NEP cells, on the other, remains to be determined. In light of our observations in the developing human brain, the fibrous NEP cell may be viewed as a neural progenitor that is in contact with the aqueous medium of the superventricle with its endfoot, and approximates the spongy medium of the pia and superarachnoid reticulum with its radial fiber.

430 C. The Superventricles and the Superarachnoid Reticulum The Superventricles. While the telencephalic, diencephalic, mesencephalic and rhombencephalic ventricles are quite narrow and quasi-tubular at GW3.5 (Figure 15A), they expand considerably during the rest of the first trimester (Figure 15B to 15G). We have named these ballooning embryonic cisterns superventricles (Volume 4: Bayer and Altman, 2006). There is a caudal-to-rostral gradient in the time course of superventricular expansion and shrinkage, with the rhombencephalic superventricle leading and the telencephalic superventricle trailing. There are also differences in the magnitude of superventricular expansion, being less pronounced for the mesencephalic and diencephalic superventricles associated with midline nuclear structures (midbrain, thalamus, hypothalamus) and the rhombencephalic and telencephalic superventricles associated with hemispheric cortical structures (cerebellum and cerebrum). We attribute great significance to the expansion, configurational changes, and shrinkage of the superventricles because the ventricular shorelines provide the substratum for NEP cell mitotic division and because proximity to the ventricular CSF appears essential for that division. In our earlier study of human spinal cord development (Altman and Bayer, 2001; Volume 1: Bayer and Altman, 2002), we documented the relationship between ventral-to-dorsal expansion and shrinkage of the central canal with ventral-to-dorsal thickening and thinning of the NEP matrix. That relationship resulted in early-to-late neurogenetic gradients between ventral horn motor neurons and dorsal horn sensory-relay neurons. We inferred that increased proliferation in a NEP compartment is associated with the expansion of its shoreline in the ventricular lumen. As a NEP compartment produces neurons that leave the matrix, it thins and its ventricular shoreline shrinks. Supporting that inference is the relationship we document in this Atlas between the temporal order in the expansion and shrinkage of different components of the NEP matrix—the lengthening and shortening of their shorelines in the superventricles—and the rise and fall in the number of neurons they generate. The first ventricle that commences to expand to form a cavernous cistern is the rhombencephalic superventricle. As seen in sagittal sections, the rhombencephalic superventricle first forms a dome dorsally as the medullary velum expands (Figure 15A, B). Then, as the medullary flexure forms ventrally it also expands in that direction to form a triangular cavity (Figure 15C, D). The medullary velum then invaginates to produce the expanding rhombencephalic choroid plexus, and the superventricle becomes divided into a metencephalic (cerebello-pontine) pool and a myelencephalic (medullary) pool. Thereafter, the rhombencephalic superventricle begins its gradual shrinkage and eventually assumes the size and form of the familiar fouth ventricle. The expansion of the mesencephalic superventri-

cle (the future aqueduct) and that of the diencephalic superventricle (the future third ventricle) is not as pronounced as that of the rhombencephalic superventricle but is evident when the parenchyma of the mesencephalic tectum and tegmentum, and the diencephalic thalamus and hypothalamus, start to expand. The expansion of the telencephalic superventricles does not commence until approximately GW5.5, when two symmetrical, balloon-like fluid compartments start their lateral outpouching from the midline prosencephalic ventricle. The great expansion of the telencephalic superventricles continues up to approximately GW11. Then, in correlation with expansion of the cerebral cortex and the basal ganglia, the telencephalic superventricles begin to shrink during the second and third trimesters and are eventually transformed into the enduring lateral ventricles. The initial expansion, of the midline prosencephalic superventricle, and then of the bilateral telencephalic superventricles are illustrated in coronal sections in Figure 16A to 16I. How is overall superventricular expansion, in general, and regional differences in the configuration and magnitude of that expansion, in particular, related to neurogenesis? We propose that the area of the variegated ventricular NEP lining, in combination with such important factors as cell cycle speed and the number of mitotic divisions prior to differentiation, determines the size of the neuron population generated at a particular site. This is so because the nuclei of the pseudostratified NEP cells have to undergo “interkinetic nuclear migration” (Sauer, 1936), i.e., shuttle to the ventricular lumen, to undergo mitotic division. That is, the number of NEP cells produced is limited by the surface area (length and width) of the ventricular shoreline because the elongated cells inside the NEP matrix cannot undergo mitosis unless there is room for them to descend to the NEP/CSF interface. The overall extent of the shoreline of a particular superventricle provides the required space for the shuttling NEP cells to continue their cycle of divisions until a specified number of neurons are generated for that brain system; the variegated regional size or configuration of the shoreline—in the form of expanding and shrinking eminences, protuberances, evaginations and invaginations into the ventricle—provide the required space for the production of the right number of neurons for a specific brain region or structure within that brain system. In general, NEPs lining ventricular surfaces that produce large cortical structures composed of similar neuronal populations (e.g., the neocortical, tectal, and cerebellar NEPs) tend to be extensive and smooth, whereas the NEPs lining ventricular surfaces that produce a multitude of distinctive but smaller neuronal populations (e.g., the NEP mosaics that produce the various thalamic and hypothalamic nuclei) tend to be small and corrugated. The correlation between ventricular size and the magnitude of the generated neuronal population is illustrated in a comparison of the area of the telencephalic superventricle (specifically, its cortical pool) between the rat that develops a small neoText continues on page 435

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Anchoring point of pontine flexure

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Medullary velum

Figure 15. Expansion of the superventricles (light and dark blue) in embryos of estimated ages between GW3.8 and GW7.7 (A-D). Note the temporal difference between the expansion of the rhombencephalic and telencephalic superventricles. Sagittal sections.

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GW3.2 Cephalic preplacode open anterior neuropore

Stockbuilding closed prosencephalic NEP

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Figure 16. Summary of ventricular expansion in coronal sections of the forebrain on this and the following two Open prosen- pages. The open prosencephalic NEP in cephalic a GW3.2 embryo (A). Expansion of the NEP medial prosencephalic superventricle (blue) between GW4.0 and GW5.0 (BD), and of the paired lateral telencephalic superventricles between GW5.5 and GW8.3 (E-I). Note the direct relationship between the expanding telencephalic superventricles and the NEP/placodal expanding fetal choroid plexus (green). junction

expanding prosencephalic superventricle

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Primordial mesenchymal brain case (skin, bone, meninges)


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433 Choroid plexus M1000 stem cells GW5.5 Expanding cell-sparse superarachnoid reticulum Stockbuilding dorsal telencephalic (cortical) NEP

Figure 16 continued. Pia roof of diencephalic superventricle foramen of monro


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Thick cortical plate in lateral limbic and neocortex

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Continuity between diencephalic and telencephalic choroid plexus

Insular cortex

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435 cortex and the human that develops a very large neocortex (Figure 17). In addition to the ventricular shoreline providing the space for the mitotic division of NEP cells, there is emerging evidence that the embryonic CSF contains trophic molecules that promote NEP cell division. Since we know that the fate-restricted progenitor cells of the secondary germinal matrices—like those of the subpial external germinal layer of the cerebellum (Altman and Bayer, 1982a) and the interstitial subgranular zone of the hippocampus (Altman and Bayer, 1975)—undergo mitosis some distance from the ventricles, the presence of CSF is obviously not required for the proliferation of late-generated microneuronal progenitors (Altman and Das, 1965b). However, proximity to the embryonic CSF may be a prerequisite for the production of the early, pluripotent NEP cells that generate the large projection neurons. This assumption is supported by reports that the embryonic CSF contains a variety of gene products, proteins, and growth factors (Parada et al., 2006), some of which promote NEP cell proliferation and neurogenesis in experimental animals (Gato et al., 2005; Martin et al., 2006; Mashayekhi and Salehi, 2006). What is the origin of the embryonic CSF? In light of the fact that in the mature brain CSF production is dependent on the choroid plexus (CP), it is important to note that the initial great expansion of both the rhombencephalic and telencephalic superventricles antedates by several weeks the formation of the CP (Figure 15 and Figure 16). Moreover, the great flowering of the fetal CP is limited to two sites, the anterior (cerebellar) pool of the rhombencephalic superventricle and the dorsal (neocortical) pool of the telencephalic superventricle. It is significant that in humans, an immensely enlarged CP fills the ballooning telencephalic superventricle as the neocortical NEP vastly expands to produce a large neocortex; compare that to rats where a small CP is in the more flattened telencephalic ventricle as the rat neocortical NEP minimally expands to produce a small neocortex (Figure 17). This suggests that there may be two phases in CSF production in the human CNS: an early phase that depends on a “budding” CP, and a late phase that depends on a “blooming” CP, one that sustains the prolonged production of cortical and cerebellar NEP cells. It should be noted finally that the fetal CP has a different cellular organization than the mature CP (Kappers, 1958; Tennyson and Pappas, 1964; Shuangshoti and Netsky, 1966; Dohrmann, 1970; Dziegielewska et al., 2001; Johansson et al., 2005). The adult CP is a distinctive frond-like tissue composed of a monolayer of differentiated cuboidal cells that surround a capillary core. The exposed surface of these cuboidal cells is covered by a rich meshwork of microvilli and some cilia, and the cell interior is filled with mitochondria. In contrast, the fetal CP is a smooth, multilayered (pseudostratified) epithelium composed of spindle-shaped cells that have a simple exposed surface and contain few mitochondria. Unlike the mature CP cells, these fetal CP cells are full of glycogen. Hence,

it may be that one of the functions of the embryonic CP is the glycolytic (anaerobic) support of NEP cell proliferation and neurogenesis. The Superarachnoid Reticulum. The mature brain is surrounded by a tripartite membranous envelope, the meninges, composed of the fine pia mater abutting the brain parenchyma, the tough dura in contact with the bony skull, and the spongy arachnoid sandwiched between the two. The pia is composed of a network of reticular and elastic fibers that adhere to the underlying neural tissue. The dura is formed of connective tissue that serves both as a protective envelope of the brain and as a periosteum of the skull. The interlacing cobweb-like processes of the arachnoid form the spongy subarachnoid space filled with CSF, containing granulations, villi, septa and other regional modifications in relation to the local distribution of blood vessels, perivascular spaces, and venous sinuses. Little information is currently available about the features and properties of the embryonic meninges (Angelov and Vasilev, 1989; Kamiryo et al., 1990; Sturrock, 1990). A study of the development of the human optic nerve emphasized the presence of glycogen-rich cells in the dura and arachnoid (Sturrock, 1987). Observations we present in this Volume indicate that of the three components of the meninges, the pia emerges early during human brain development but the dura much later. Before differentiating neurons start to leave the NEP matrix, a thin tissue layer separates the brain primordium from the surrounding celldense (darkly staining) mesenchymal tissue that will later form the skin and bone of the skull. Associated with the development of this formative pia, a cell-sparse (lightly staining) field emerges between it and the cell-dense mesenchyme. This cell-sparse field expands enormously as the brain parenchyma grows through the first trimester, and has variably sized divisions in relation to the different developmental dynamics of components of the telencephalon, diencephalon, mesencephalon and rhombencephalon. We call this embryonic meningeal field the superarachnoid reticulum (Figure 18A to 18D). (We must note here that we have failed to recognize this unique feature of the developing brain in the GW7.5 specimen illustrated in Plates 186 to 206 in Volume 4 of the Atlas [Bayer and Altman, 2006], where we simply referred to the region as the “meninges.”) As the dura gradually becomes recognizable, the superarachnoid reticulum is seen to become enclosed by an internal and external network of blood vessels associated with the pia and the dura, respectively. By about GW9, the superarachnoid greatly diminishes in size (Figure 18E) and the meninges gradually assume their mature form. Because the expansion of the superarachnoid reticulum antedates the growth of the brain parenchyma (Figure 18A to 18C), and the brain parenchyma subsequently expands into the space formed by it (Figure 18D, E), we postulate that the superarachnoid reticulum constitutes a parenchymal expansion field, a site that is being readied for (and possibly promoting) the entry of differentiText continues on page 441

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Figure 17. Comparison of the relative size of the cortical division of the telencephalic superventricle (dark blue) in the rat, which develops a small neocortex and humans, which develops a large neocortex, in sagittal (A and B, this page) and coronal (C and D, facing page) sections. Note the correlated difference in the size of the choroid plexus (green) in the two species.

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C. Rat, embryonic day 18

lateral ventricle

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Choroid plexus does not "bloom" into ventricle

BASAL GANGLIA THALAMUS

AMYGDALA

SUBTHALAMUS

third ventricle

HYPOTHALAMUS

1 mm

D. Human, M841, GW9.2 telencephalic superventricle

Choroid plexus "blooms" into ventricle

BASAL GANGLIA

THALAMUS?

SUBTHALAMUS AMYGDALA

HYPOTHALAMUS


Cerebral cortical NEP (long linear extent)

1 mm

438 mesencephalic superventricle with surrounding mesencephalic NEPs

ISTHMUS

GW4

GW5.5


Medullary velum

prosencephalic superventricle rhombencephalic superventricle

closed anterior neuropore


(first appears at base of brain and around isthmus)

Surrounding rhombencephalic NEPs

halic diencep tricle en superv

Surrounding prosencephalic NEPs

B. C6516

t su elen pe ce rv ph en al tr ic icl e

A. M9297

c li ha le ep ic nc tr be ven om er rhsup


(expansion extends into telencephalon and diencephalon prior to large ventricular expansions)

Medullary velum

C. 1390

GW7

al s p i nr a l t cen nal a c

su di pe en rv ce en ph tr ali ic c le


m su es pe en rv ce en ph tr al ic ic le

Budding telencephalic choroid plexus

Medullary velum


Incipient invaginating rhombencephalic choroid plexus

Superarachnoid reticulum (continues to expand around the entire brain in advance of parenchymal growth)

Ventricle Superarachnoid

1 mm

Choroid plexus

Figure 18. Expansion of the superarachnoid reticulum (yellow) between the pia and the formative dura between GW4.0 and GW8.0 (A-D, this and the facing page) followed by its shrinkage by GW9 (E). Sagittal sections.

439

D. C145

GW8 PRETECTUM

TEGMENTUM

TH

CEREBELLUM ISTHMUS

AM

te suplencep erv h ent alic ric le

AL

Becomes thinner over the rapidly expanding cerebral cortex

TE

L LO SA HA BA EP C N LE

N

US LAM


PONS

HYP

BASAL GANGLIA

Medullary velum

OTH A

SU

BT

HA

LA

MU

S

US

Budding telencephalic choroid plexus


m su es pe en rv ce en ph tr al ic ic le

Budding rhombencephalic choroid plexus

MEDULLA Superarachnoid reticulum Continues to be large at base of brain SPINAL CORD

E. C6658 GW9

Superarachnoid reticulum (shrinks in all areas as the parenchyma expands)



nc "b eph lo a su om lic c pe s" ho rv to ro en fi id tri ll t p c l e h e l ex

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PRETECTUM

TH

A AL

MU

S

m su es pe en rv ce en ph tr al ic ic le TEGMENTUM

Te le

CEREBELLUM

BASAL GANGLIA

TE

Ventricle Superarachnoid Choroid plexus

1 mm

L

L LO SA HA BA EP C EN

S

N

H

T UB

HA

H OT YP

LA

AL

S MU

AM

US


C E R E B R A L

C

O

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T

E

X


Medullary velum

440

TELENCEPHALIC CHOROID PLEXUS RHOMBENCEPHALIC CHOROID PLEXUS c en

eph alon

u alam s Th

Te

ct

u

SUPERARACHNOID RETICULUM (parenchymal expansion field)

Cor

tic

m

al

te

l

Expanding proliferative shoreline

superventricles Bas al tel

superventricles

Tegm en po t h

al

Hy

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Olfactory bulb

Cerebellum

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STOCKBUILDING NEPs Figure 19. Schematic drawing of the embryonic human CNS with the expanding superventricles (blue) that provide the extended shoreline for the mitotic division of stockbuilding NEP cells, and the expanding superarachnoid reticulum (yellow with blue) that serves as the parenchymal expansion field for the migrating and settling neurons.

© 2008 by Taylor & Francis Group, LLC

ombenceph

alo

n

rd Dorsal spinal co al central can cord Ventral spinal

441 ating neurons and their processes into the developing brain tissue. We speculate that the two fluid-filled spaces, the expanding superventricles and the superarachnoid reticulum—interconnected by the rhombencephalic and telencephalic tela choroidea (Figure 19)—play a complementary role in CNS development, trophic factors in the superventricular CSF promoting expansion of the stockbuilding NEP cell population, and trophic factors in the superarachnoid CSF promoting neuronal differentiation and migration.

D. Metamerism or Mosaicism as Principles of CNS Development From Metamerism to Functional Compartmentation. It is a popular current hypothesis that the neuraxis that extends from the caudal spinal cord to the rostral forebrain develops from a series of reiterated segments or metameric units. This idea goes back to pre-Darwinian evolutionary speculations of early 19th-century thinkers, such as Oken and Goethe, who argued that the vertebrate skull is composed of a number of transformed vertebrae (de Beer, 1937; Starck, 1963). That is, the head is but a modified rostral portion of the trunk. An extension of this idea was that, much as the axial spinal cord is divided into so many transverse segments by the entering and exiting dorsal and ventral spinal nerves, so also the brain is built from a series of transverse units, the neuromeres. This view was supported by the identification of a set of 7 or 8 protuberances, the rhombomeres, in the developing vertebrate hindbrain in association with some of the cranial nerves (Orr, 1887; Vaage, 1969). This view has currently found some support from studies that use genetic markers and mutants to analyze the features of the different rhombomeres (Lumsden and Krumlauf, 1994; Gavalas et al., 1998, 2003) and metamerism has been extended to the midbrain and forebrain by postulating the existence of the so-called prosomeres (Rubinstein and Puelles, 1994). However, an analysis of what is meant by “segmentation” or “metamerism “ when applied to different components of the body and the CNS reveals conceptual inconsistencies. Segmentation in the trunk (somitomerism) is principally a peripheral phenomenon. It is manifested during early development by the presence of reiterated morphogenic blocks, the somites, and as the PNS develops later, by the formation of the segmental dorsal root ganglia and dorsal and ventral spinal nerves in relation to reiterated dermatomal and myotomal compartments. This peripheral segmentation, which is an ancient chordate legacy (Romer, 1970) persists throughout life with profound topological transformations in all vertebrates, including humans. Significantly, however, there is no central segmentation (transverse partitioning) in the vertebrate spinal cord. On the contrary, the vertebrate spinal cord (unlike the neuraxis of many invertebrates) is a longitudinally organized morphogenetic and functional system. The spinal NEP is a con-

tinuous proliferative matrix through its entire length. And as differentiation begins and progresses, the motor neurons of the ventral horn form longitudinal columns, and the sensory neurons of the dorsal horn extend from lower lumbar levels to upper cervical levels without any transverse partitioning (e.g., Altman and Bayer, 2001). In contrast to the enduring peripheral metamerism in the spinal cord, the metamerism in the rhombencephalon, the reiterated NEP bulges known as rhombomeres, is an instance of transient central segmentation. Although there are indications for some peripheral metamerism here, too, in the form of vestigial gill arches (branchiomerism), these reiterated structures of ancestral protochordates have become transformed into structurally and functionally diversified cranial organs in vertebrates (Shimeld and Holland, 2000). In contrast to the reiterated trunk somites, the cranial primordia and placodes (see below) give rise to such diverse specialized structures as the jaws, various oro-facial organs, the inner ear, the pharynx, and certain viscera that are absent in the trunk region. Moreover, the cranial ganglia associated with these various organ systems—trigeminal, facial, vestibular, spiral, glossopharyngeal, and vagal ganglia—are not reiterated structures like the spinal ganglia but functionally diversified sensory systems. There is no empirical evidence to support the idea of a metameric organization either in the skull or the rostral CNS (mesencephalon, diencephalon, telencephalon). Even in the most primitive extant vertebrate, the lamprey, the mesoderm is segmented along the trunk but not in the head region (Kuratani et al., 1999). Nor is there evidence that the different brain vesicles are reiterated metameric units. For instance, the two transverse “segments” of the tectum in the dorsal mesencephalon (the inferior and superior colliculi) are structurally and functionally quite dissimilar structures, one being the target of auditory fibers from the medulla, the other of optic fibers from the retina. The multifarious structures in the tegmentum of the ventral mesencephalon, the red nucleus, the oculomotor nuclear complex, the periaqueductal gray, the substantia nigra, the ventral tegmental area, the interpeduncular nucleus, etc. provide not even a hint of reiterated transverse organization. The same applies to the diencephalon and the telencephalon. Compartmentation in components of diencephalon (optic vesicle, thalamus, subthalamus, preoptic area, hypothalamus, etc.) and compartmentation in the telencephalon (olfactory bulb, neocortex, hippocampus, striatum, amygdala, etc.) cannot be fitted, by any stretch of the imagination, into a reductionistic metameric framework. We do not deny metamerism is a facet in the early development of body and brain in vertebrates, man included, but we question its significance as an overriding morphogenetic principle of CNS development. Specifically, we look upon metamerism as a protochordate and protovertebrate legacy, a phylogenetic burden that was gradually overcome as the segmented, limbless, and headless worm-like ances-

442 tral lines acquired a far more complex body structure with a functionally integrated CNS in the course of vertebrate evolution. The story may have begun with an ancestral protochordate, not unlike the extant Amphioxus. Amphioxus spends most of its time buried in gravel or sand and uses its oral cilia and gills to filter nutrients suspended in the water (Buchsbaum, 1948). The mesoderm of Amphioxus is segmentally organized, with bilaterally arranged muscle blocks along its elongated body which resembles that of a fish. The wave of alternate contractions of these muscle blocks produces the undulatory swimming motion that allows Amphioxus to flee when disturbed. The spinal cord of Amphioxus is situated above the notochord (the phylogenetic and ontogenetic precursor of the vertebral column of vertebrates), and it shares many features with the spinal cord of vertebrates, such as the dorsal position of the sensory nerves and the ventral position of the motor nerves (Bone, 1960). However, the bipolar sensory neurons of Amphioxus are located in the spinal cord centrally rather than peripherally in the spinal ganglia (as in vertebrates), and its motor neurons are located near the spinal canal rather than in the ventral horn. Moreover, Amphioxus lacks a true brain. As a sedentary filter feeder, Amphioxus survives without having a head furnished with specialized sense organs and a specialized muscular oral apparatus. Instead of eyes, it has a few pigmented photosensitive cells near the anterior tip of the spinal cord, and its “cerebral vesicle,” marked by Otx gene expression (Shimeld and Holland, 2000), is composed of little more than a few dopaminergic and serotoninergic cells (Lacalli, 2001; Ekhart et al., 2003; Moret et al., 2004). It has been argued that Amphioxus is a degenerate chordate. Another possibility is that a closely related filter-feeder species was the ancestor of more advanced chordates who, by acquiring cranial sense organs and a brain, became enabled to cruise freely in the open waters and turn into successful scavengers, grazers, or predators. The recent discovery of fossil chordates from 520 million-year-old Lower Cambrian beds, named Haikouella and Yunnanozoon, may bridge the gap between these two lines of chordates (Holland and Chen, 2001; Mallatt and Chen, 2003). Haikouella had a body similar to Amphioxus and a head furnished with some sense organs and a small bilobed brain. If this scenario is correct, the quasi-segmental spinal cord is a protochordate legacy, with the proviso that it became further elaborated as the notochord became transformed into an articulated cartilaginous or bony vertebral column and a rib cage, and as the neural crest-derived dorsal root ganglia became exteriorized in more advanced fish. The transformation of sedentary chordates into mobile fish required the development of a head with specialized information-gathering sense organs—the olfactory, visual, auditory, vestibular, lateral line, and gustatory systems— and such specialized motor mechanisms as the muscular mouth of jawless fish and the skeletomuscular jaw of more advanced fish. The trunk also became modified as the

segmentally mediated undulatory swimming was supplemented by fin action for better postural control and maneuverability. As the extremities evolved in tetrapods, body and head organization became more complex, requiring modification of existing neural systems and the formation of new ones. Contrary to the idea that the skull and face are modified vertebral segments, and the brain a modified spinal cord, it appears more likely that most cranial organs and the brain mechanisms serving them evolved de novo. This is supported by the accumulating evidence that different morphogenetic mechanisms are involved in the regulation of spinal cord development and brain development. The spinal NEP surrounding the slit-shaped spinal ventricle is initially uniform in its appearance both in rat (Altman and Bayer, 1984) and humans (Altman and Bayer, 2001; Volume 1 of this Atlas). But then the spinal ventricle changes its shape and a ventrodorsal compartmentation becomes apparent, with the ventral NEP starting to generate the motor neurons of the expanding ventral horn and, later, the dorsal NEP starting to generate the sensory neurons of the dorsal horn. The ventrodorsal compartmentation in the spinal cord is under the inductive influence of the neural crest and two peripheral structures, the somites and the notochord. The sheet of crest cells that spins off the dorsal neural fold and migrates toward the discrete somites, give rise to the neurons of the segmented dorsal root ganglia, and these exert a “dorsalizing” influence upon the dorsal spinal NEP. The unsegmented notochord, in contrast, exerts a “ventralizing” influence on the ventral spinal NEP to generate the motor neurons of the ventral horn. The early differentiation of motor neurons depends on Shh (sonic hedgehog) signaling that is transmitted from the notochord and the floor plate to the ventral NEP (Briscoe, 2000), and retinoids and FGFs (fibroblast growth factors) have been implicated in the rostrocaudal axial specification of spinal motor neurons (Liu et al., 2001). The morphogenesis of the brain obeys different principles than the spinal cord. The somites are absent and the notochord is often indistinct. There is no evidence for comparable dorsalizing and ventralizing influences upon the development of the rhombencephalic, mesencephalic, and telencephalic NEPs. Here the principal inductive influence emanates from two altogether different cranial structures that are absent in the trunk region, the gill arches and the cranial and branchial placodes. The Rhombomeres as NEP Mosaics. As illustrated in this volume of the Atlas, the rhombencephalic NEP has several components. (i) The upper rhombic lip region and (ii) the lower rhombic lip region dorsally form the bridgeheads of the membranous medullary velum that covers the rhombencephalic superventricle. (iii) The less clearly defined ventromedial NEPs are the source of several pontine and medullary motor nuclei. (iv) The ventrolateral NEPs form distinct rhombomeres. The region of the upper rhombic lip NEP is the direct or indirect source of neurons of the cerebellum. The region of the lower rhombic lip

443 NEP is the source of the neurons of the precerebellar nuclei (inferior olive, basal pontine gray, etc.) and the cochlear nuclei. The rhombomeres, a set of conspicuous semicircular NEP evaginations, are the most often cited example of brain metamerism. The exact number of rhombomeres has been controversial (the old literature is reviewed by Vaage, 1969); in the currently popular numerical designation, there are seven rhombomeres (R1-R7), where R1 refers to the cerebellar NEP. This is unfortunate because the cerebellar NEP has little in common either structurally or functionally with rhombomeres R2-R7, which do share some common properties. As we shall describe later, the expanding and long-enduring cerebellar NEP is largely connected with second- and higher-order central afferent systems (some first-order vestibular input excepted), in contrast to the transient rhombomeres that are intimately associated with the first-order peripheral afferents of cranial sensory ganglia. According to Bartelmez and Evans’ (1925) observations in human embryos, R1 and R2 originate from prorhombomere A, R3 and R4 from prorhombomere B, and R5-R7 from prorhombomere C. A similar pattern has been described in mouse embryos (Osumi-Yamashita et al., 1996). According to the latter study, crest cells from prorhombomere A (R2?) migrate into the first branchial arch and produce the trigeminal (V) ganglion; crest cells from prorhombere B (R3 and R4) migrate into second arch and produce the facial and vestibulocochlear (VII-VIII) ganglia; crest cells from the anterior portion of prorhombomere C (R5 and R6) migrate into the third arch and produce the glossopharyngeal (IX) ganglia; and crest cells from the posterior portion of prorhombere C supply cells to the fourth arch and the vagal (X) ganglia. Our observations, which start at a later developmental stage (about 17 to 18 pairs of somites) than those described by Bartelmez and Evans (2 to 16 pairs of somites), indicate a close association between the rhombomeres and the branchial placodes, and the following developmental pattern. In the youngest embryo (estimated age GW3.2), in which the rhombencephalic superventricle is just beginning to expand, three evaginating bulges are evident caudal to the cerebellar NEP (Figure 20A). The first is contiguous with the condensing, small trigeminal ganglion inside the maxillary process. We designate this germinal mosaic as the trigeminal NEP (R2), the target of future cranial nerve V fibers. The affiliation of the second rhombomere (presumably R3) is unclear; we hypothesize that it is the facial (nerve VII) NEP. According to a recent study, R2-derived neuronal progeny contribute to the lower jaw somatosensory representation, and R3 progeny to whisker representation in the barrel fields of mice (Oury et al., 2006). It is possible that the facial NEP is mostly a source of efferents that start to sprout after the facial motor neurons leave R3. (The roundabout migration of facial motor neurons, whose trailing axons form the loop [genu] of the facial nerve, is well known [Altman and Bayer, 1982b]). The third evagi-

nating early rhombomere is aligned with the condensing vestibulocochlear ganglion (nerve VIII) and the otic vesicle. We presume this rhombomere will become subdivided into R4-R5, as seen in a slightly older embryo (estimated age GW4.0) with a greatly expanded rhombencephalic superventricle (Figure 20B). As we documented over two decades ago in the rat (Altman and Bayer, 1982b), the otic vesicle is the source not only of the non-neural elements of the inner ear but also of a large contingent of delaminating neurons, either the neurons of the spiral ganglion and/or some components of the vestibular ganglion. R6 and R7 are not evident in the GW3.2 embryo (Figure 20A) but are beginning to evaginate in the GW4.0 embryo (Figure 20B) and are pronounced in a somewhat older embryo (Figure 20C). In the latter, R6 is aligned with and is in direct contact with afferents of superior glossopharyngeal ganglion, the source of the sensory fibers of cranial nerve IX, and R7 with superior vagal ganglion, the sensory component of nerve X. Accordingly, we designate R6 as the glossopharyngeal NEP, and R7 as the vagal NEP. If this interpretation is correct, the oro-facial peripheral and central systems innervated by the future cranial nerves V and VII, and the vestibulocochlear systems innervated by the future cranial nerve VIII are developing before the pharyngeal-visceral systems innervated by the future cranial nerves IX and X. By GW5, the rhombomeres begin to shrink (Figure 21A); by GW7, they are no longer recognizable in the developing pons and medulla (Figure 21B). Although we stress here the sensory affiliations of the rhombomeres, there is ample evidence from animal studies that the rhombomeres are also a source of motor neurons (e.g., Lumsden and Keynes, 1989). According to a recent study in larval and adult frogs (Straka et al., 2006), trigeminal efferents are derived from R2-R3; facial and vestibular efferents from R4-R5; glossopharyngeal efferents from R6; and vagal efferents from R7 and the rostral medulla and the spinal cord. However, in the human specimens we have examined we find only cranial afferents entering the rhombomeres and no evidence of exiting motor fibers. This may be because either axonal outgrowth occurs after the transient NEP rhombomeres are no longer recognizable as such or axonal sprouting commences, as is well known in the case of facial nerve, after the migrating motor neurons have left the rhombomere NEPs. Timetables of the Development of Placodes, Cranial Ganglia, and Rhombomeres. Whereas in the trunk region, crest cells generate the peripheral neurons of the spinal (dorsal root) ganglia (Weston, 1970; LeDouarin, 1982), the principal source of neurons of the cranial ganglia in the head region are the placodes (Knouff, 1935) or, more accurately, the cephalic and branchial preplacodes that later become divided into specialized placodes (Jacobson, 1963; Noden 1993). The cephalic and branchial preplacodes, formed by pluripotent stem cells, are thickened epithelia that extend from the vicinity of the forebrain NEP to the Text continues on page 446

444

A.

B.

C.

A few rhombomeres are definite in the rhombencephalic NEP.

Each rhombomere is well defined as a small convex NEP evagination.

The rhombomeres reach their peak definition from each other.

M714 GW3.2

C836 GW4

M2321 GW4.3

Mesencephalic NEP

Mesencephalic NEP

Cerebellar NEP?

Trigeminal ganglion near R2 NEP

R2 Trigeminal ganglion (V)

R2

R2 R3

Trigeminal boundary cap

R3 Vestibulocochlear ganglion (VIII)

R4/5

Epithelium R6?

R3

R4

R4

R5

Lumen

Otic vesicle

Vestibulocochlear ganglion near R4 NEP

R5

R6

R7? Vagal ganglion (X) R7

Glossopharyngeal ganglion (IX, near R6 NEP)

R6

Otic vesicle near R5 NEP

Spinal NEP R7 Vagal ganglion (X, near R7 NEP)

Somites

Spinal NEP

Figure 20. Formation of the rhombencephalic neuroepithelial bulges (rhombomeres) between GW3.2 and GW4.3 (A-C) in relation to the sensory ganglia of the cranial nerves. Coronal sections. (Serial sagittal sections from a GW5 embryo are shown in Figure 27).

Spinal NEP

Somites

A. C8966, GW5

B. C1390, GW7

isthmal canal



Cerebellar NEP


US HM IST

ISTHMUS

Mesencephalic (isthmal) NEP

Mesencephalic (isthmal) NEP

Upper rhombic lip

Pontine NEP

PONS

Cerebellar NEP



Medullary velum

R2 R3


Facial nerve genu (VII)

Pontomedullary trench (site of the earlier R3?)

Medullary NEP

R4


R5 Rhombomeres are in the medullary and pontine NEP.

Lower rhombic lip

R6

RHOMBENCEPHALON

R7

Rhombomeres have disappeared.

MEDULLA

Lower rhombic lip

Solitary tract

Figure 21. Rhombomeres present in a GW5 embryo (A) disappear by GW7 (B). Sagittal sections.

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446 border of the trunk region. The different hypothetical origins of neurons of the spinal and cranial nerve ganglia, prior to fusion of the neural tube and the brain vesicles, is illustrated in Figure 22. As seen in a coronal section of a GW3.2 embryo, the cephalic preplacode is continuous with the prosencephalic NEP prior to the fusion of the anterior neuropore (Figure 23A). It abuts the ventral and ventrolateral prosencephalon but is notably absent over the dorsal prosencephalon and farther caudally (Figure 23A, B). The cephalic preplacode is continuous, by way of the oral epithelium, with the branchial preplacode that covers the brachial arches I-IV some distance from the rhombencephalon (Figure 23B). Experiments in animals have shown that, during early embryonic development, the cells of the pluripotent preplacodes are competent to form different special placodes (Jacobson, 1963; Groves and BronnerFraser, 2000). This pluripotency is lost when the preplacodes become partitioned into special placodes that acquire diverse structural and functional properties. The cephalic preplacode divides into the profoundly different olfactory, optic, and pituitary placodes; the branchial preplacode divides to form the diverse domains of the trigeminal, facial, vestibulocochlear, glossopharyngeal, and vagal ganglia. The human embryos we have analyzed in this volume provide chronological information about the developmental course of the peripheral cephalic and branchial placodes, the cranial nerve ganglia they generate, and the emergence and dissolution of the rhombomere NEPs with which they are associated. Thickening of the olfactory placode anteromedially is evident in the GW3.8 and GW4.0 embryos (Figure 24A, B). The formation of the nasal olfactory epithelium and the outgrowth of olfactory nerve fibers is in progress in the GW5.0 and GW5.5 embryos (Figure 24C, D). The vesiculation of the olfactory placode (formation of the nasal cavity) is evident in the GW5.5 embryo (Figure 24D), which also shows the first hint of the evagination of the olfactory NEP. The evagination of the olfactory NEP becomes more and more pronounced between GW6.5 and GW7.5 (Figure 24E to 24I). The presumptive optic lens placode, which surrounds the bulging optic vesicle NEP in the GW3.2 and GW4.0 embryos (Figure 25A, B) thickens in the GW4.5 embryo (Figure 25C) as the invaginating optic vesicle NEP becomes partitioned into a thin pigment epithelium and a thicker retinal NEP. Contrary to the classical view that the evaginating optic vesicle NEP induces placodal lens formation (Spemann, 1938), it has recently been argued that an earlier interaction between the anterior neural plate NEP and a portion of the preplacode establishes a lens-forming bias in the latter (Grainger et al., 1997). The lens placode assumes a spherical configuration in the GW5.0 embryo (Figure 25D) and the onset of the cytological differentiation of the crystalline lens is evident in the GW6 embryo (Figure 25E). The exodus of ganglion cells from the retinal NEP and the sprouting of optic nerve fibers is in progress in the GW7 embryo (Figure

25F). Finally, the formation of the pituitary gland begins in GW3.8-GW5.0 embryos with the folding of the posterior tip of the cephalic preplacode (Figure 26A to 26C). The fusion of this placode between GW5.5-GW6.5 produces Rathke’s pouch, the primordium of the anterior pituitary gland or adenohypophysis (Figure 26D, E). Contiguity between Rathke’s pouch and the posterior pituitary GEP, the neurohypophysis derived from the hypothalamic NEP, is evident in the GW6.0-GW7.0 embryos (Figure 26E to 26H). As noted earlier, the caudal branchial preplacode has two components: a thinner epithelium covering the roof of the oral cavity, which is continuous with the cephalic preplacode, and a thicker portion covering the oral cavity floor (Figure 23B). The latter has several components: the discrete placodes that surround the maxillary process, the mandibular and the hyoid arches (I, II), and visceral arches III and IV (Figure 23B and Figure 27). As it is currently understood, mesenchymal elements of the maxillary process and the mandibular arch are the source of various orofacial structures, such as the jaws, and the visceral arches generate components of the tongue, the pharynx, and the upper gut. The placodes surrounding these diverse structures, together with neural crest cells, are the precursors of sensory cells and of neurons in the cranial ganglia. The latter are the trigeminal (V), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), and vagal (X) ganglia. The afferents of these ganglia link various orofacial, inner ear, and gut organs derived from the branchial arches with different divisions of the rhombencephalic NEP. The neurons of the trigeminal and facial ganglia link the orofacial region with rhombomeres R2-R3. As early as GW3.2, a small, spherical trigeminal ganglion is visible near R2 (Figure 20A), and by GW5.0 there is direct continuity between the large trigeminal ganglion and R2 (Figure 27A, B). By that age, three cell-dense arms of ganglion V are identifiable (Figure 28A), and by GW5.5 the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve approximate the eye region and penetrate the maxillary process and the mandibular arch, respectively (Figure 28B). With respect to the central projections of the trigeminal nerve, by GW5.5 the penetrating fibers start to form a bulge at the knee of the pons (Figure 29A, B), and trigeminal fibers clearly intermingle with other fibers of the pontine white matter by GW6.5 (Figure 30A). As early as GW4.0, the vestibulocochlear ganglion is aligned with R4, and the otic vesicle with R5 (Figure 20B). The two are also aligned peripherally with the hyoid arch (Figure 27) but their association is problematic. The afferents of the vestibulocochlear ganglion reach R4 by GW5.0 (Figure 27B), and they penetrate the white matter of R4 and R5 between GW5.5 (Figure 29A, B) and GW6.5 (Figure 30A). The formation of the glossopharyngeal and vagal ganglia, and of R6 and R7 with which they are aligned, is not evident until about GW4.0-GW4.3 (Figure 20B, C). The two are aligned peripherally with the visceral arches III and

447 IV (Figure 27A, B, Figure 29B) but their exact relationship remains to be elucidated. Glossopharyngeal afferents approximate R6, and vagal fibers approximate R7 by GW5.0 (Figure 27B, C), and they penetrate the white matter of R6 and R7, respectively, by GW5.5 and GW6.5 (Figure 25B, Figure 30A, B).

E. Exogenous and Endogenous Mechanisms of NEP Cell Diversification The Role of Periphero-Central Signaling in Placodal and NEP Cell Diversification. An important first step in mosaicism of the NEP matrix is synchronizing its diversification with both somatic and neural development in the body periphery, including the placodes and the peripheral nervous system (PNS). There is emerging experimental evidence that periphero-central coordination is aided by induction and signaling molecules (Baker and Bronner-Fraser, 2001; Streit, 2004). For instance, the rostral cephalic preplacode expresses Six, Eya, and Dach protein markers (Ikeda et al., 2002; Brugmann et al., 2004; Schlosser and Ahrens, 2004; Litsiou et al., 2005) and it has been found that in the presence of both cranial mesoderm and rostral neural plate NEP, excessive Six1 expression expands the preplacode at the expense of the epidermis, whereas Six1 depletion results in a reduction of the preplacodal domain (Brugmann et al., 2004; Ahrens and Schlosser, 2005). The next step in the diversification of the rostral cephalic preplacode is the formation of the olfactory and optic placodes, each with profoundly different structural and functional fates. Whereas the olfactory placode gives rise to the specialized bipolar sensory neurons of the olfactory epithelium, which send fibers to the evaginating olfactory bulb NEP, the optic placode has no neurogenic potential but gives rise to the crystalline lens of the eye. Mutant mice lacking functional Pax6 proteins fail to develop eyes and nasal cavities (Stoykova et al., 1996), and Pax6 and D1x5 are initially expressed in both future olfactory cells and lens-forming cells of the eye (Bhattacharyya et al., 2004). However, as the presumptive lens cells acquire a columnar morphology, D1x expression is reduced in the optic placode whereas Pax6 is lost in the olfactory placode (Bhattacharyya et al., 2004). The authors concluded that loss of D1x is required for the proliferative cells to adopt a lens fate and that the balance between Pax6 and D1x expression regulates cell sorting in the segregating placodes. With reference to periphero-central signaling, it has been reported that extraocular signals affect optic vesicle NEP development (Kagiyama et al., 2005), and that retinoic acid (Matt et al., 2005) and Vax2 signaling are involved in the fate-modification of retinal NEP cells and pigment epithelium cells (Kim and Lemke, 2006). It has also been reported that Mash1 promotes the development of retinal bipolar cells, and Math3 and NeuroD that of amacrine cells (Morrow et al., 1999; Inoue et al., 2002). There is little information about the molecular mechanisms involved in the specification of the third component of

the cephalic preplacode, the pituitary placode, except for the report that targeted disruption of the homeobox genes Nkx2.1, Ttf1, and Titf1 results in the disruption of pituitary gland development (Takuma et al., 1998). A different set of genetic factors and periphero-central signaling molecules appear to be involved in the development of the hindbrain region associated with the branchial placodes. The different rhombomeres express a distinctive combination of Hox genes (Krumlauf, 1993; Wilkinson, 1993; Gavalas et al., 2003; McNulty et al., 2005). Hoxa2 is involved in R2 and R3 specification (Gavalas et al., 1998; Gaufo et al., 2004), and Hoxa1 and Hoxb1 play a role in R4 and R5 specification and the growth of the cranial nerves associated with them (Mark et al., 1993; Arenkiel et al., 2004). Differences have also been noted in the expression of transcription factors Krox20 and Kreisler in the different rhombomeres (Sham et al., 1993; McKay et al., 1994; Schneider-Maunoury et al., 1997; Chomette et al., 2006), and retinoic acid also appears to be involved in rhombomere specification (Niederreither et al., 2000; Dupé and Lumsden, 2001). Members of the Fgf family of signaling proteins, mesoderm-derived Fgf3 and Fgf8, and Ngn2 (Fode et al., 1998; Begbie et al., 2002; Holzschuh et al., 2005; Nikaido et al., 2006; Sun et al., 2007; Nechiporuk et al., 2007) appear to play a role in the early determination and diversification of the epibranchial placodes and their derivatives, the glossopharyngeal and vagal ganglia, and the homeobox genes Phox2a and Phox2b in its later stages (Morin et al., 1997). Among the signaling molecules implicated in the specification of the otic placode and the formation of the inner ear are certain members of the Fgf family (Vendrell et al., 2000; Adamska et al., 2001; Ladher et al., 2005; Nechiporuk et al., 2005; Martin and Groves, 2006) and Ngn1 (Ma et al., 2000). It is noteworthy that gene expression is different in the rhombomeres, where sensory and motor neuron precursors are not obviously segregated, than in the spinal cord where the ventral NEP cells generate motor neurons and the dorsal NEP cells generate sensory interneurons. Mash1 and Math3 are necessary for the development of facial and trigeminal motor neurons (Ohsawa et al., 2005). The genes Frizzled3a and Celsr2 are necessary for cell polarization and the guidance of the roundabout migration of facial motor neurons in the brainstem (Wada et al., 2006), and their migration is also dependent on Phox2b signaling (Coppola et al., 2005). The genes implicated in dorsoventral patterning in the spinal cord are the class II Ssh-promoting Nkx2.2, Nkx2.9, Nkx6.1, and Olig2 genes, and the class I Ssh-repressing Dbx1 and Dbx2 genes (Briscoe et al., 1999; Kessaris et al., 2001; Marquardt and Pfaff, 2001). In light of the profound structural and functional heterogeneity of the cephalic and branchial placodes, we propose that unlike the segmented somites of the trunk, the placodes of vertebrates are not reiterated metameric units but an altogether different kind of progressively diversiText continues on page 451

448

SPINAL CORD 1.

Neural crest

Neural groove

3.

Epidermis

Neural plate

2.

BRAIN

Neural fold

Fusing neural tube

4.

1.

Preplacode

2.

3.

4.

Cephalic placodes protoventricle

Spinal ganglion

central canal Neuroepithelium (NEP)

Branchial placodes

Cranial ganglion Dispersing neural crest

Figure 22. Hypothetical differences in the organization of the neural plate of the spinal cord (1-4, left) and forebrain (1-4, right). Neurons of the dorsal root ganglia derive from delaminating neural crest cells, whereas neurons of the cranial nerve ganglia derive from preplacodal stem cells. Cephalic placodes are close to the neuroepithelium, while branchial placodes (on the arches) are farther from the neuroepithelium.

A. GW3.2 Coronal, M714 (Section 18)

B. GW3.8 Sagittal, C7724 (Slide2, Section 30)

Prosencephalic NEP (dorsal)

M

e M

ic r ece s

s

pretecta

l/te

ct


al

NE

Ps

Cerebellar NEP?

Rathke's pouch epithelium

R2

nc ep

lic

R3

NEP

s

ha

Prosencephalic NEP (ventral)

Oral epithelium


Mandibular arch (I)

avity Oral c

Mandibular arch (I)

R4

(farther from central nervous system)

/phary

Hyoid arch (II)

c ali eph le enc ric mb ent rhorotov p


Optic vesicle

Prose

opt

lic

tegmental/isthma alic lN ph EP ce n se

p pr ro o t s en o v ce en p h tr al ic ic le

(nearer central nervous system)

ha

s


es

en

p ce


Medullary velum

R5

R6

nx

h III? Arc

Figure 23. A. Continuity between the prosencephalic NEP and the cephalic preplacodes in a GW3.2 embryo. Coronal section. B. Continuity between cephalic preplacodes and branchial preplacodes, by way of the oral epithelium, in a GW3.8 embryo. Sagittal section.

R7?

Arch IV?

449

450 NEP - Neuroepithelium

optic recess

A. GW3.8, C7724

E. GW6.5, C9247 Optic germinal zones

Olfactory placode

Neocortical NEP

Nerve I (olfactory)

Cephalic preplacode

Limbic cortical NEPs Olfactory NEP

See I

B. GW4, C9297



Basal ganglionic NEP

Optic germinal zones

Olfactory placode Cephalic preplacode

C. GW5, C8966

F. GW7, C1390

prosencephalic superventricle Prosencephalic NEPs

Olfactory NEP

Dorsal Diencephalic NEPs Ventral


Olfactory nerve reaches prosencephalon optic recess

Olfactory placode


G. GW7.5, C6202 dorsal pool

D. GW5.5, C6516

re

l N E


P

Ce

bral cortica


Olfactory NEP?

Olfactory NEP


See J ventral pool

Formative nasal cavity


foramen of monro

optic recess

Nasal cavity


Scale bars A-D

Scale bars E-G


451

H. GW6.5, C9247


Limbic cortical NEP Olfactory NEP

Limbic cortical NEP


FORMATIVE OLFACTORY BULB

I. GW7.5, C6202

OLFACTORY BULB Olfactory NEP

Nerve I (olfactory)



Multiple contact points between olfactory nerve and basal telencephalon



Nerve I (olfactory)


Multiple contact points between olfactory nerve and basal telencephalon

Figure 24. The time course of the thickening and invagination of the olfactory placode, the formation of the olfactory epithelium (pink), the outgrowth of the olfactory nerve fibers (yellow), and the evagination of the olfactory NEP, at lower (A-G, facing page) and higher magnification (H-I, this page). Sagittal sections.

fying germinal system. Starting as a pluripotent preplacodal germinal matrix, this system gives rise to several faterestricted placodes that, in turn, contribute to the generation of various non-neural components of different cranial organs (eyes, nose, jaws, ears, palate, etc.) as well as to the neurons of the various cranial ganglia that innervate these organs and connect them with specific NEP compartments. This coordinated diversification of the peripheral elements of the head region with complimentary NEP mosaics is exemplified by the development of the rhombomeres, which are distinguished from one another by forming different peripheral and central connections and serving different functions. In this view, neither the different cranial ganglia nor the rhombomere NEPs are reiterated metameric units but are more like the other diversifying NEP mosaics of the CNS, except that they are more conspicuous and short-lived than many other NEP mosaics.

This may be due to a transient role of the rhombomeres in the morphogenesis of the cranial nerve system and their drastic reorganization as the hindbrain develops in higher vertebrates and humans. We speculate that the fleeting presence of rhombomeres 2-7 in higher vertebrates constitutes a recapitulation of a stage of hindbrain evolution in ancestral fish. The rhombencephalic NEP mosaics of fishes, as seen in extant piscine species, produce such hypertrophied structures as the facial lobe, the target of nerve V and VII afferents, the octavolateral lobe, the target of nerve VIII afferents, and the vagal lobe, the target of nerve IX and X afferents (Evans, 1952; Wagner, 2001). However, these paleocephalic central sensory structures, much like the large optic lobe, became reorganized during the phylogeny of higher vertebrates as new neencephalic (forebrain) circuits evolved to process sensory input. Correspondingly, the sensory systems of higher vertebrates and Text continues on page 461

452 C. GW4.5, M2300

Optic vesicle

Cephalic preplacode

Prosen- optic cephalic NEPs

re c


Developing eye


Prosencephalic NEPs


open prosencephalic protoventricle

A. GW3.2, M714

ess

Pigment epithelium

Ventral diencephalic NEPs

Lens placode optic recess


anterior neuropore

Retinal NEP

Pituitary placode

(future Rathke's pouch)

B. GW4, C836

Cephalic preplacode

D. GW5, C8314


Developing eye




closed prosencephalic superventricle

Optic vesicle

Pigment epithelium

optic recess Ventral diencephalic NEPs

Future retinal NEP?

Prosencephalic NEPs

Retinal NEP

Lens placode Pituitary placode


All scale bars = 0.25 mm

Cephalic preplacode

optic recess

Pituitary placode

Maxillary process


Maxillary placode

Invaginating lens placode

453 ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium

E. GW6, M2161

Developing eye

Migrating lateral preoptic neurons Preoptic NEP

Sclera/cornea


Anterior hypothalamic NEP

Lens Optic nerve GEP Vi

Choroid fissure

t re

ous

body

Migrating anterobasal nuclear neurons Pigment epithelium Retinal NEP intraretinal space (optic recess)

F. GW7, M2155 (serial sections of eye only) (1) Approximate section 190

Retinal ganglion cells absent (most distant from exiting optic nerve)

(3) Section 164

Lens

(equatorial cut)

ou

(most plentiful near sprouting optic nerve)

Retinal NEP

sb

Choroid fissure

Epithelial cells Fiber cells

Vi t r e

Pioneer retinal ganglion cells

od

y

Crystallin protein

(site of optic nerve sprouting)

(2) Section 178 Retinal ganglion cells

intraretinal space (optic recess) Pigment epithelium

Glial channels adjacent to Retinal NEP?

(fewer due to greater distance from optic nerve exit)

Figure 25. Time course of the development of some components of the eye. Already evaginated when the anterior neuropore closes at GW3.2 (A, facing page), the optic vesicle interacts with the cephalic prepladode to differentiate a lens placode opposite the future retinal NEP on GW4 and GW4.5 (B, C, facing page); the lens placode invaginates on GW5 (D, facing page). Development of the crystalline lens is well underway by GW6 (E, this page) as well as the differentiation of the optic germinal zones into the retinal NEP, the pigment epithelium, and the optic nerve GEP. Pioneer ganglion cells and optic nerve fibers emerge by GW7 (F, this page). Coronal sections.


454 A. GW3.8, C7724


D. GW5.5, C6516 diencephalic superventricle


Hypothalamic NEPs

Prosencephalic NEPs

Lamina terminalis?

Pituitary placode

Lamina terminalis Rathke's pouch (primordium of anterior pituitary)


Cephalic preplacode

Oral cavity

B. GW4, C9297

E. GW6.5, C9247 diencephalic superventricle

Posterior pituitary GEP

Hypothalamic NEPs


Anterior

Hypothalamic NEPs

Pituitary placode

Lamina terminalis Preoptic NEP


Lamina terminalis

Middle

Rathke's pouch (primordium of anterior pituitary)

Oral cavity

Cephalic preplacode

C. GW5, C8966

F. GW7, C1390 diencephalic superventricle Hypothalamic NEPs Preoptic NEP


Middle

Posterior pituitary GEP

Hypothalamic NEPs

Anterior

Preoptic NEP Lamina terminalis

Lamina terminalis



Oral cavity


455 G.


GW6 M2161 (anterior)

Pia

(hypothalamic pool)

Lateral

Hypothalamic neuroepithelium

Middle

Subpial fiber layer Migrating hypothalamic neurons

Arcuate nuclear?

Median eminence

Anterior pituitary epithelium

Pituitary gland primordium

intraglandu

la

r cl eft

Sella turcica of sphenoid bone

Intermediate pituitary epithelium

H.


GW6 M2161 (posterior)

Pia

(hypothalamic pool)

Subpial fiber layer

Lateral

Hypothalamic neuroepithelium Middle


Arcuate nuclear?

Pituitary gland primordium Anterior pituitary epithelium

Sella turcica of sphenoid bone Posterior pituitary glioepithelium (pituicyte precursors)

Figure 26. Time course of the development of the anterior and posterior parts of the pituitary gland. In GW3.8, GW4.0, and GW5.0 embryos (A-C, facing page), the pituitary placode folds in and fuses to form Rathke’s pouch on GW5.5 (D, facing page), the primordium of the anterior pituitary gland. Continuity between the pituitary placode-derived anterior pituitary gland and the hypothalamic NEP-derived posterior pituitary gland is established in the GW6.5 and GW7.0 embryos (E, F, facing page). Sagittal sections. The contiguity between the anterior and posterior pituitary glands is illustrated at a higher magnification in a GW6.0 embryo (G, H, this page). Coronal sections.

456

GW5 Sagittal, CR 7.1 mm, C8966 A.

Cerebellar NEP

Slide 8, Section 2

Upper rhombic lip

Lower rhombic lip Medullary velum


R2

R4

R3

R5

R7

R6 Otic vesicle

Boundary caps Ganglion VIII (vestibulocochlear)

Epithelium Lumen Arch IV?

Branchial placodes

Developing eye

Lower medullary NEP

Spinal NEP

intraretinal space

Retinal NEP Lens Pigment epithelium

Hyoid arch (II)

Ganglion X (inferior)

Arch III

Anteri o

Mandibular arch (I)

Choroid fissure

r c ar

Ganglion X placode?

Sympathetic trunk ganglia Dorsal root ganglia

dina l vei n


Heart primordium

Olfactory (cephalic) placode

Spinal nerve (dorsal root)

Parasympathetic intramural (cardiac) ganglion? Spinal nerve (ventral root)

Slide 8, Section 8

Cerebellar NEP

B.

Pontine and medullary NEPs

R2 R3


Ganglion V (trigeminal)

Boundary caps

Ganglion X (superior)? Nerve IX (glossopharyngeal) Nerve X (vagal)?

Ganglion IX (inferior)

Ganglion VIII (vestibulocochlear)

Migrating ganglionic neurons?

X

th

Heart primordium

Parasympathetic intramural (cardiac) ganglion?

nglia nk ga

Ganglion X placode in Arch IV

ru c t


eti

Hyoid arch (II)

pa

Mandibular arch (I)

m

IX

Ganglion IX placode in Arch III Lateral nasal process

Nerve XI (spinal accessory)?

Sy

Maxillary/ mandibular placode Maxillary process

R7

R6

Ganglion VII (facial)?

Nerve V (sprouting opthalmic branch) Nerve V (sprouting maxillary branch)

Developing eye

R4

R5

Lower medullary NEP

Dorsal root ganglia

457

C.

Upper rhombic lip

Medullary velum

Lower rhombic lip

Cerebella r NE P

Slide 8, Section 14

FONT KEY: ABBREVIATIONS: ventricular divisions - capitals NEP - Neuroepithelium Germinal zone - Helvetica bold R - Rhombomere Transient structure - Times bold italic Permanent structure - Times Roman or Bold PROPOSED RHOMBOMERE IDENTITIES R2

Pontine and medullary NEPs

R2

R3

R5

R4

R3

R6

R7

Nerve X boundary cap and nerve roots

Ganglion VIII (vestibulocochlear) Ganglion V (trigeminal)

Nerve V (sprouting maxillary branch)

Otic vesicle

R4

Ganglion IX (superior) Ganglion X (superior)

Ganglion VII (facial)

R5

Nerve X (vagal) Ganglion IX (inferior)

Sy

m

pa

R6

t

h

Maxillary/ mandibular placode

Ganglion IX placode Branchial placodes in arch III?

runk ganglia ic t

The placodal epithelium separated from the mesenchyme is an artifact of histological processing.

et

Hyoid arch (II)

Maxillary process


Heart primordium

D.

RHOMBOMERE/GANGLION RELATIONSHIPS R2

Upper rhombic lip

Slide 8, Section 20

Lower rhombic lip

R3

R2 Ganglion V (trigeminal)

R4 Otic canal (marks the initial invagination from the embryonic surface)


R5

Otic vesicle Ganglion VII (facial)

R6

Sy

Heart primordium

R7 t


runk ganglia ic t

Hyoid arch (II)

Ganglion V placode?

pa

Axons from the trigeminal ganglion enter the brain here. Axons from the facial ganglion (VII) enter the brain at the junction between R3 and R4. Vestibulocochlear ganglionic (VIII) axons enter the brain here. The otic vesicle touches this part of the brain. Axons from the glossopharyngeal ganglia (IX superior and inferior) enter the brain here. Axons from the vagal ganglia (X superior and inferior) enter the brain here.

et

Mandibular arch (I)

Maxillary process

m

h

Ganglion VII placode?

Nerve V (sprouting mandibular branch)

R7


Figure 27. Serial sagittal sections, from medial (A) to lateral (D), through the head and neck region of a GW5.0 embryo (facing page and this page). They illustrate the vertical alignment and contiguity between cranial ganglia V, VII, VIII, IX, and X, and rhombomeres R2, R3, R4, R5, R6, and R7, respectively. The spatial relationship of the cranial ganglia and the maxillary process, the mandibular arch (I), the hyoid arch (II), and arches III and IV are also visible.

GANGLION/PLACODE RELATIONSHIPS V

Derived from a placode at the junction of the maxillary process and mandibular arch. VII Derived from a placode in the hyoid arch. VIII Both the vestibular and spiral ganglia are derived from the otic vesicle epithelium. IX The inferior and possibly most of the superior ganglionic neurons are derived from a placode in arch III. X The inferior and possibly most of the superior ganglionic neurons are derived from a placode in arch IV.



A.

GW5 Sagittal, C8966 Slide 8, Section 2

Developing eye Lateral nasal process intraretinal space

Pigment epithelium

Retinal NEP

Nerve I (sprouting)

Nerve V (sprouting opthalmic branch)

Lens

Olfactory placode


Choroid fissure


Maxillary process

B.

GW5.5 Sagittal, C6516 Slide 20, Section 14



Nerve V (sprouting mandibular branch) Mandibular arch (I)


Developing eye intraretinal space

Basal ganglionic NEP

Retinal NEP

Pigment epithelium

Nerve V (sprouting opthalmic branch)


ar NEP rebell Ce

Choroid fissure

Dorsal rhombic lip



R2 Pontine NEP

Maxillary process

Medullary velum


Nerve V (sprouting mandibular branch)

Hyoid arch (II)


Mandibular arch (I) Figure 28. A. Far-lateral section from the opposite side of the preceding GW5.0 embryo, to show the relationship between the three prongs of the trigeminal ganglion and the developing eye, the maxillary process, and the mandibular arch. B. Sprouting fibers of the ophthalmic, maxillary, and mandibular branches of the distal trigeminal nerve in a GW5.5 embryo. Sagittal sections.


Ventral rhombic lip


Otic vesicle Lumen

Epithelium

459 GW5.5 Sagittal, C6516 A. Slide 6, Section 11

R2


R4

Tr i



R5

ge mi

Pigment epithelium

na

ga

l

Intraretinal space

EYE



nglion (V)

Retinal NEP


VIII afferent fibers



Anterior cardinal vein Facial ganglion (VII)?

Choroid fissure

Superior glossopharyngeal ganglion (IX)

Lumen Epithelium

Otic vesicle

B.

Slide 7, Section 6

R2

Nerve V boundary cap Tr g a ig e m ng i (V lionnal )

Nerve V


R3

R4

R5

R6

R7

VII + VIII afferent fibers Glossopharyngeal afferent fibers


Vagal afferent fibers

rv

e I X

Maxillary process

Ne


X Ne rv e

Figure 29. A. Entry of afferents of the trigeminal ganglion into R2, and of the vestibulocochlear ganglion into R4 and R5. B. Entry of afferents of the glossopharyngeal ganglion into R6 and of the vagal ganglion into R7. Sagittal sections from a GW5.5 embryo.

Mandibular arch (I)

Inferior glossopharyngeal ganglion (IX)

Migrating IX ganglionic neurons? Hyoid arch (II)


Arch III Inferior vagal ganglion (X)

460 GW6.5 Sagittal, C9247 A. Slide 16, Section 13

Pontine NEP VIII afferent fibers

V afferent fibers Nerve V boundary cap

Nerve VIII boundary cap Otic vesicle


epithelium

Nerve VIII (vestibulocochlear)



B. Slide 18, Section 13

Medullary NEP

X afferent fibers

IX afferent fibers




FONT KEY: Germinal zone - Helvetica bold Transient structure - Times bold italic Permanent structure - Times Bold

Nerve X (vagus)

Nerve X boundary cap

Figure 30. A. Penetration of trigeminal and vestibulocochlear afferents into the pons (former R2 and R4-R5). B. Penetration of glossopharyngeal and vagal afferents into the medulla (former R6 and R7). Sagittal sections from a GW6.5 embryo.

461 humans become reorganized during ontogeny. An example may be the necessary reorganization of the octaval system of aquatic fish, which have a lateral line organ but lack a cochlea, into the cochlear system of land vertebrates. The innervation of this complex tonotopically organized auditory system required profound reorganization of the auditory system in terrestrial vertebrates. Indeed, the neurons of the dorsal and ventral cochlear nuclei do not derive from the rhombomeres but from a new germinal system, the cochlear NEP and EGL located in the region of the lower rhombic lip. Passing from hypothetical phylogeny to empirically based observations, we may summarize the differences between the development of the spinal cord and the brain as follows. First, unlike the segmented mesodermal somites in the trunk region, the head mesoderm is unsegmented. Second, whereas the neural crest-derived and somitedependent spinal ganglia are reiterated structures, the branchial arches, the diversifying olfactory, optic lens, and pituitary placodes, and the trigeminal-facial, otic, and glossopharyngeal-vagal placodes and ganglia derived from them are not reiterated structures but functionally diverse systems. Third, while there is no hint of any longitudinal (rostrocaudal) metameric compartmentation either in the spinal NEP or the differentiating components of the spinal cord gray matter (although there is a pronounced dorsoventral compartmentation), there is, in contrast, a marked compartmental heterogeneity in the brain vesicles. This is marked at the outset by the presence of variegated NEP mosaics of different sizes and shapes both in the hindbrain and the forebrain. Finally, since these NEP regions—e.g., the NEPs of the different thalamic nuclei and the different areas of the cortex—become linked to the periphery much later (after synaptic connections are established with the primary afferents and the lower motor neurons), their diversification cannot be attributed to periphero-central signaling mechanisms but must be due to either endogenous regulation and/or centro-central signaling processes. Endogenous Genetic Regulation in NEP Cell Diversification. While in conventional histological preparations individual NEP cells look alike throughout the neuraxis, the NEP matrix consists of morphologically dissimilar NEP compartments from the beginning. The configuration and cellular dynamics of these NEP compartments— those of the rhombencephalon, mesencephalon, diencephalon and telencephalon—are quite different from the outset, and so are the different classes and varieties of neurons and brain structures they generate. While the dome-like expanse of dorsal telencephalic NEP matrix, for instance, generates an immense number of relatively homogeneous classes of neocortical neurons, the variegated ventral telencephalic NEP matrix produces a much greater variety of neurons for such diverse brain structures as the septum, the basal telencephalic nuclei, the basal ganglia, the amygdala, etc.—each with its own neurogenetic timetable, pop-

ulation size, and cell composition. The same applies to the variegated diencephalic and mesencephalic NEP compartments. And as development progresses, the larger NEP matrix divisions become subdivided into a series of bilaterally symmetrical smaller protuberances, cavities, and discontinuous stretches and patches marked by different cell depth and cell packing density. In our earlier experimental studies in rats, we used 3H-thymidine autoradiography to date the changing spatial and temporal dynamics of cell proliferation in many of these NEP mosaics, track the migratory paths and settling patterns of the tagged neurons, and correlate these data with the chronology of neurogenesis in various structures of the mature brain. Based on that information, we named the identified NEP mosaics by their putative target structures. Thus, we have such divisions as the neocortical, limbic-cortical, and basal ganglionic NEPs in the early telencephalon; the thalamic, subthalamic, and hypothalamic NEPs in the early diencephalon; and the tectal and tegmental NEPs in the early mesencephalon. And as embryonic development progresses, most of these early NEP compartments become partitioned into smaller components, such as the tectal NEP into the superior collicular and inferior collicular NEPs, the tegmental NEP into the NEPs of the red nucleus, oculomotor nuclei, substantia nigra, etc. We illustrate this progressive NEP matrix compartmentation at a select coronal level of the human prosencephalon which begins with three diencephalic divisions—the all-thalamic, all-subthalamic and allhypothalamic—and each of which becomes subsequently divided into smaller NEP mosaics (Figure 31). The fact that dissimilar proliferative and differentiation dynamics are expressed by many NEP compartments—e.g. cortical, basal ganglionic, hippocampal, thalamic, etc.— that have no direct peripheral connections suggests that their diversification is due not to exogenous signaling but to endogenous signaling. (Although we postulate that the superventricles and the superarachnoid reticulum influence NEP cell proliferation and differentiation, respectively, their diffuse or global influences cannot explain the local diversification of NEP cells.) The inference that the germinal cells forming certain NEP compartments are intrinsically different is supported by genetic studies that are currently carried out in mice and other experimental animals. For instance, it has been reported that Pax6 is expressed by NEP cells throughout the prosencephalic neural plate before it folds and fuses (Inoue et al., 2000); thereafter, Pax6 expression persists in the dorsal telencephalic NEP but vanishes from the ventral telencephalic NEP (Stoykova et al., 2000). Six3 (Oliver et al., 1995) and retinoic acid expression (Mic et al., 2004; Halilagic et al., 2006; Ribes et al., 2006) have been implicated in the early phases of prosencephalic development. The dorsal telencephalic NEP cells that are destined to generate cortical neurons also express the transcription factors Emx1 and Emx2, whereas the ventral NEP cells destined to generate basal ganglionic neurons express Nkx2.1, Dlx1, Dlx2, and Gsh2 (Torreson et Text continues on page 465

462 A. GW3.2, M714 diencephalic protoventricle

0.25 mm

B. GW4.5, M2300 expanding diencephalic superventricle

Optic germinal zones optic recess

Lens placode Optic nerve GEP? Cephalic preplacode

C. GW5.5, M1000 Pretectal (mesencephalic) NEP

Stockbuilding diencephalic NEPs

Allhypothalamic NEP

Cephalic preplacode

Retinal NEP


Allsubthalamic NEP

Optic germinal zone

Pigment epithelium

FONT KEY: ventricular divisions - capitals Germinal zone - Helvetica bold Transient structure - Times bold italic

Allthalamic NEP


thalamic pool

Migrations

Thalamic Subthalamic Hypothalamic

Stockbuilding diencephalic NEPs begin to partition

Thalamic NEP (evaginating)

Figure 31. Progressive NEP matrix compartmentation, cell migration, and cell differentiation in the developing human diencephalon between GW3.2 (A) and GW8.0 (G) on this page, the facing page, and the following overleaf. The original NEP divisions – the Subthalamic all-thalamic, all-subthalamic and NEP all-hypothalamic – become (thickening) subdivided into smaller, faterestricted mosaics (marked by Hypothalamic asterisks) that generate NEP neurons for different nuclei (evaginating) (most of them not identified). The process is temporally associated with 0.25 mm the expansion and shrinkage of the diencephalic superventricle (blue) and the superarachnoid reticulum (yellow). Coronal sections.

Thalamic NEPs (evaginating and stockbuilding)

expanding diencephalic superventricle

Thalamic NEPs

(thinning by unloading neurons)

Superarachnoid reticulum Optic germinal zones

subthalamic pool

by unloading neurons)

Pigment epithelium intraretinal space (optic recess)

Retinal NEP

Subthalamic NEPs (thinning

hypothalamic pool

Hypothalamic NEPs (thinning by

unloading neurons)

Onset of subthalamic and hypothalamic parenchymal expansion 0.25 mm

Partitioning of diencephalic NEPs continues

463 D. GW6.5, M2051 Pretectal (mesencephalic) NEP



Thalamic NEPs expanding thalamic pool

diencephalic superventricle Cerebral cortical NEP

(stockbuilding, some neuronal unloading)


Migrations

Thalamic NEP





shrinking subthalamic pool

Amygdaloid NEP

shrinking hypothalamic pool


All diencephalic NEPs continue to partition

Subthalamic NEPs (thinning by unloading neurons)

Optic nerve GEP

Hypothalamic NEPs

Pigment epithelium


Retinal NEP Optic tract and chiasm

optic recess

Optic nerve GEP

pineal recess

E. GW7, M2155

Partitioning thalamic NEPs

(stockbuilding, more neuronal unloading)

diencephalic superventricle Cerebral cortical NEP

0.5 mm

Thalamic NEP expanding thalamic pool


Onset of thalamic parenchymal expansion


shrinking subthalamic pool

Amygdaloid NEP

Subthalamic NEPs shrinking hypothalamic pool


Hypothalamic NEPs (thinning by unloading neurons) 0.5 mm

Figure 31 continues.

464 F. GW7.5, C8553


Choroid plexus


ABBREVIATIONS: GEP - Glioepithelium NEP - Neuroepithelium SVZ - Subventricular zone expanding telencephalic superventricle

Migrations

Thalamic NEPs



Fornical GEP Choroid plexus

Subthalamic NEPs (thinning

by unloading neurons)

Amygdaloid NEP

shrinking diencephalic superventricle

Hypothalamic NEPs (thinning by unloading neurons) Superarachnoid reticulum


Parenchymal expansion due to extensive neuronal migration from the NEPs continues in the diencephalon

1 mm

G. GW8, C609


Choroid plexus

Fornical GEP

Migrating dorsal complex and periventricular complex neurons?

Ganglionic NEP and SVZ Cerebral cortical NEP

C aud

THALAMUS

shrinking diencephalic superventricle

AMYGDALA

SUBTHALAMUS

a te

Thalamic NEPs

(depleted of most neuronal stem cells)

Internal capsule (contains axons from thalamic neurons entering cerebral cortex)

Subthalamic NEPs (depleted of most neuronal stem cells)

HYPOTHALAMUS Amygdaloid NEP

Diencephalic parenchymal expansion is primarily due to axonogenesis and dendrogenesis rather than to the addition of new neurons from the NEP

Hypothalamic NEPs (depleted of most neuronal stem cells)

Migrating mammillary body neurons

1 mm

Figure 31 concludes.

465 al., 2000; Wilson and Rubenstein, 2000; Yun et al., 2001; Corbin et al., 2003; Muzio and Mallamaci, 2003). Similarly, Ngn1 and Ngn2 are expressed in the dorsal telencephalic NEP, whereas Mash1 is expressed in the ventral telencephalic NEP (Ma et al., 1997; Fode et al., 2000; Parras et al., 2002). It has also been reported that Emx2 expression in the rodent cortical NEP shows a high-caudal to low-rostral gradient, and a high-medial to low-lateral gradient (Leingartner et al., 2003). Since these gradients are the opposite of cortical maturation gradients, not only in rodents (Bayer and Altman, 1991a) but also in humans (as illustrated in the preceding Volumes of this Atlas), it may be inferred that Emx2 expression declines as cortical neurogenesis and differentiation progresses. The importance of Emx signaling in dorsal telencephalic development is indicated by observations that cortical development, including that of the hippocampus, is greatly retarded in Emx mutants (Bishop et al., 2003). In light of the importance we attribute to the growth of the fetal telencephalic choroid plexus in cortical development, an interesting aspect of Emx gene action is its suppression of telencephalic choroid plexus development (von Frowein et al., 2006). The production of choroid plexus progenitor cells is apparently associated with Otx2 and BMP7 expression. (The cells of the non-neuronal rhombencephalic choroid plexus are reported to express Lmx1a and Gdf7; Landsberg et al., 2005). Finally, according to a study (Kimura et al., 2005) that has used a different nomenclature from the one we use in this Atlas, Wnt8b expression demarcates the hippocampal primordium in the medial cortex as well as other components of the limbic circuit, including the diencephalic epithalamus and mammillary body. Within the hippocampus, Ephb1 demarcates the ammonic NEP that generates pyramidal cells, Wnt3a the dentate NEP that generates granule cells, and TTR the primordium of the nonneuronal telencephalic choroid plexus. There are currently few studies available regarding the genetics of diencephalic and mesencephalic NEP cell specification. Pax6 is expressed in the diencephalic NEP (Warren and Price, 1997) but not in the mesencephalic NEP where, instead, Pax3 and Pax7 are expressed (Kammermeier and Reichert, 2001). It has been reported that Msx1 genes induce Wnt1 expression at the dorsal midline region of the mesencephalic and diencephalic junction, and in homozygous Msx1 mutant mice several structures fail to develop at this site (Bach et al., 2003). Mash1 is required for the specification of NEP cells that produce the neuroendocrine neurons of the ventral, arcuate, and ventromedial hypothalamic nuclei (McNay et al., 2006). The bHLH transcription factor SCL is expressed in pretectal, midbrain, and hindbrain NEPs but not elsewhere in the CNS (van Eekelen et al., 2003), and the transcription factor Otx2 appears to control neurogenesis and progenitor identity in the midbrain tectum and tegmentum (Vernay et al., 2005). There are more data regarding the genetics of cerebellar development. FGF and Wnt expression are early

events in cerebellar morphogenesis (Morales and Hatten, 2006; Waters and Lewandoski, 2006). Pax6, Math1, Tbr1, and Tbr2 have been implicated in the generation of the NEP cell line that produces cerebellar deep nuclear neurons, and these factors are expressed sequentially as the young deep neurons enter the nuclear transition zone (Fink et al., 2006). Among other transcription factors expressed in this lineage are Otx3-Dmbx1 (Kimura et al., 2005). The Purkinje cell lineage expresses Math1 (Jensen et al., 2004; Wang et al., 2005), Ptf1a (Hoshino, 2006), and Ebf2 (Croci et al., 2006). The cell lineage of the EGL, which gives rise to cerebellar microneurons (see below) and can be visualized with the marker RU49 (Alder et al., 1996), expresses Math1 (Machold and Fischell, 2005) and Zic1 (Aruga et al., 1998). Integrin-linked kinase (ILK) is reported to be a critical agent in the proliferation of granule cell precursors (Mills et al., 2006) and Bergmann glial cell differentiation (Belvindrah et al., 2006). Finally, the transcription factor Zpf423 has been implicated not only in the production of cerebellar granule cells but also in the proliferation of other late-generated precursors in the cerebral cortex and the hippocampus (Alcaraz et al., 2006). In conclusion, there is growing evidence for genetically controlled diversification of some of the NEP mosaics that are not direct targets of exogenous (peripheral) influences. We return later to a third source of morphogenetic regulation, i.e., the centrocentral signaling between developing components of the CNS.

F. Timetables of Neurogenesis There is no direct way to determine the time of origin of neurons in the human CNS. But that can be done indirectly by using the quantitative data obtained in rats with 3 H-thymidine autoradiogaphy. Notwithstanding the great difference in the speed of their development, embryonic (E) days versus gestational weeks (GW), there is a close morphological correpondence between prenatal rat CNS development (E11 to E21) and human CNS development during the first trimester (GW3-GW12). This matching relationship is shown in sagittal sections (Figure 17A, B), anterior coronal sections (Figure 17C, D), and posterior coronal sections (Figure 36A, B). We re-examined this relationship in the material presented in this atlas series by matching sections from the spinal cord and the brain in the two species. Tables of the estimated chronological eqivalence of human CNS neurogenesis with the empriciallybased rat CNS neurogenesis are summarized in the Appendix (page 490). We emphasize that the correpondence is limited to the first trimester and early second trimester in humans. Rats are born at a time equivalent to the early second trimester in humans. After birth on E22/P1, the rat CNS rapidly matures and has an adult appearance by the time of weaning (approximately P21). In contrast, human CNS maturation is stretched out over a long time span through the remaining second trimester, the third trimester, and even into the early postnatal years.

466 G. Cell Migration, Sojourn Zones, Secondary Germinal Matrices, and Fate-Restricted Glioepithelia Cell Migration and Migratory Streams. Following the production of neurons, cell migration is one of the most important mechanisms in the morphogenetic organization of the developing and maturing CNS. Cell migration is a very complex and regionally diversified process. There are small cohorts and large streams of migrating neurons; short-distance and long-distance migrations; and migrations with a straight path or a tortuous route. Some young neurons migrate through interstitial tissue, like the precerebellar neurons that form the inferior olive (Altman and Bayer, 1987b); others migrate beneath the pia, like the neurons of the posterior extramural migratory stream that produce the neurons of the pontine gray (Altman and Bayer, 1987d); still others migrate in a subventricular position, like the rostral migratory stream (RMS) of the telencephalon that conveys neurons to the olfactory bulb (Altman, 1969; Luskin, 1993). While most migrations are ipsilateral, there are also some that are contralateral, i.e., the neurons of the precerebellar extramural migratory stream that form the lateral reticular and external cuneate nuclei (Altman and Bayer, 1987c). The simplest form of cell migration is the short-distance radial translocation of a cohort of young neurons from a NEP mosaic to a nearby parenchymal destination. This mode of migration, often at a right angle to the NEP matrix, is known as radial migration. It is a widespread phenomenon during early embryonic development throughout the CNS before the expanding parenchyma becomes filled with aggregates of cell bodies and crisscrossing fiber tracts that obstruct the path of migrating neurons toward their final destination. For instance, in the developing cerebral cortex, the earliest Cajal-Retzius neurons migrate radially to the primordial plexiform layer (Figure 32A, B) before the cortical plate begins to form (Figure 32C). As Golgi studies have shown, these cells have a trailing process in the NEP and a leading process approaching the pial surface (Morest, 1970). Although radial migration has been attributed to guidance by “radial glia” (see Section B, page 428), a far more likely or prevalent mechanism is perikaryal (or somal) translocation (Berry and Rogers, 1965; Morest, 1970; Nadarajah et al., 2003; Hatanaka et al., 2004). As we noted earlier, nuclear translocation within the spindleshaped cytoplasm is a fundamental property of NEP cells that shuttle to and from the ventricular lumen to undergo mitotic division. The nuclei of young neurons leaving the NEP matrix may similarly translocate inside the cells’ radially extending neurite. (Such a process of nuclear translocation has been well documented for granule cells in the cerebellar cortex; e.g., Altman and Bayer, 1997). However, radial migration is only one of the many forms of neuronal locomotion. For instance in the developing cerebral cortex, the translocating young neurons interrupt their radial migration and interact with various fiber systems.

The cells and fibers undertake a choreographed series of movements to form different layers in what we have called the stratified transitional field or STF (Figure 32D). The evidence that clonally related cells disperse widely in the developing cerebral cortex (Walsh and Cepko, 1992; Mathis and Nicolas, 2006) indicates that cells may migrate both radially and non-radially (tangentially) within the same brain region (Bayer et al., 1991a, b). Tangential migration is better accounted for by an amoeboid form of locomotion rather than perikaryal translocation, with the philopodia of a neuron’s leading process sampling the local milieu or responding to distal signals, and determining the direction of cell progression in one or another direction. Radial translocation at a right angle from the NEP matrix to the surface of the cortex is obviously a simpler task than the guidance of tangential migration to some distant site that may require multiple navigation cues. Cell polarization, guidance by attractive and repulsive forces in the immediate vicinity of the moving cell, and long-range signaling by molecular diffusion gradients have been postulated to act as directional biases, signposts, and beacons. However, little is currently known about their exact nature. Filamin-A has been implicated in the control of the shape of migrating cortical neurons and their direction of migration (Sato and Nagano, 2005). The subpial Cajal-Retzius cells of the primordial plexiform layer secrete reelin, a large extracellular matrix protein. Reelin has been implicated as a signaling factor in the columnar (vertical) and laminar (horizontal) organization of cortical cells (Ogawa et al., 1995; Nishikawa et al., 2002). The absence of reelin in mutant mice results in abnormal cell migration and cell lamination not only in the cerebral cortex but also in the hippocampus, the cerebellum (D’Arcangelo et al., 1995), the olfactory bulb (Hack et al., 2002), and some hindbrain nuclei (Rossel et al., 2005). In the developing human cerebral cortex, reelin expression is present in the primordial plexiform layer by GW7 to GW8 (Zecevic et al., 1999) and somewhat later in the hippocampus (Abraham et al., 2004). Among other factors that appear to play a role in the migration and settling of cortical neurons is Cdk5 (Hammond et al., 2004), presenilin-1 (Louvi et al., 2004), COUP-TF nuclear receptors (Tripodi et al., 2004), and GABA(B) receptors (Lopez-Bendito et al., 2003). The properties of the rostral migratory stream (RMS) associated with the forebrain subventricular zone (SVZ), which forms prenatally in animals (Pencea and Luskin, 2003) and humans (Volume 4 of this Atlas), but persists through adulthood, has received considerable experimental scrutiny recently (though mostly in postnatal animals). In the human forebrain, the RMS is recognizable as a distinct entity by GW11, and it expands greatly during the second and third trimesters (Volumes 2-3 of this Atlas). In adult rats, progenitor cells stream in this glia-encased tube (Peretto et al., 1997) and supply not only microneurons (granule cells) to the olfactory bulb but also neuroglia Text continues on page 468

467

A.


GW6.5 M2155

Primordial plexiform layer Cerebral cortical NEP

Settling Cajal-Retzius cells Migrating subplate neurons

B.

GW7.5 M2042


Horizontally-aligned Cajal-Retzius cells

C.

Vertically-aligned subplate neurons

NEP Layer I Cortical plate

GW8 C9226

(STF 1) (STF 5)

NEP Cajal-Retzius cells

D.

GW8.5 M2050 Delaminating subplate neurons

Layer I Cortical plate

STF 1

STF 5

NEP Figure 32. Onset of the parenchymal development of the neocortex. The first differentiating neurons to leave the neocortical NEP, at about GW6.5, are the Cajal-Retzius cells (A), followed by the subplate neurons at about GW7.5 (B). The cortical plate begins to form, at the site illustrated, at about GW8.0 (C), and the first two layers of the stratified transitional field (STF) at about GW8.5 (D). (Modified Figure 9 from Altman and Bayer, 2002.)

468 (Aguirre et al., 2002; Fukushima et al., 2002). The proliferating SVZ cells and the migrating RMS cells express the microtubule associated protein, doublecortin (Yang et al., 2004) and stathmin (Jin et al., 2004). The homeobox gene Vax1 (Soria et al., 2004) and nitric oxide (Moreno-Lopez et al., 2004) were found to exert an inhibitory influence on SVZ and RMS cell proliferation, and integrin and laminin have been implicated in the migration of RMS cells (Emsley and Hagg, (2003). Slit1 and Slit2, repellents of neurite growth secreted by cells of the septum bordering the RMS stream, exert a guiding influence on migrating RMS cells (Nguyen-Ba-Charvet et al., 2004). In contrast to the RMS, relatively little is known about the guidance mechanisms of the unique migratory stream of neurons that form the precerebellar nuclei (Altman and Bayer, 1997). The progenitors of olivary neurons (the source of climbing fibers) that migrate in the posterior intramural migratory stream express Math1. Netrin1 is involved in the migration of inferior olivary neurons (Bloch-Gallego et al., 1999; Alcántara et al., 2000; de Diego et al., 2002). The pontine neurons (a source of mossy fibers) that migrate in the anterior extramural migratory stream express Ngn1, and their migration is abnormal in mutant mice lacking the chemokine receptor CXCR4 (Vilz et al., 2005). The precerebellar nuclei are absent or disorganized in netrin1 homozygous mutant mice (Kubota et al., 2004). As in the rat, the precerebellar migratory streams are also prominent in the developing human CNS. The posterior intramural migratory stream begins to form by GW6.5-GW7.0 (this Volume) and the expanding inferior olive is evident by GW7.5 (Volume 4 of this Atlas; Bayer and Altman, 2006). The anterior extramural migratory stream is not recognizable until GW9 and the pontine gray nucleus starts to form about GW11, at about the same time that the earliest descending corticofugal fibers begin to traverse it. Transitional Fields and Sojourn Zones. We have illustrated during the second trimester (Volume 3 of this Atlas; Bayer and Altman, 2005) the prominence of the cortical transitional stratified field (STF) situated between the NEP

and the expanding cortical plate, the future gray matter. We identified six cellular and fibrous layers within the STF, distinct strata where cortical neurons sojourn for some time and mingle with afferent, efferent, and commissural fibers before they resume their migration and settle in the cortical plate. We postulated that the STF is a staging area where connections form between the topographically unspecified sojourning cortical neurons and the somatotopically, tonotopically and retinotopically specified thalamocortical afferents that provide input to them. The STF begins to form in the earlier-maturing anterolateral cortical region, and it spreads slowly dorsally and medially. Where present, the STF consists initially of two layers, the fibrous STF1, and the cellular STF5 (Figure 32D). By GW10, the bilayered STF is present throughout the anterior cortex, and it is evident that STF5 is composed of sojourning young cortical neurons that have left the NEP, and STF1 is the target of thalamocortical afferents that have crossed over from the diencephalon into the telencephalon in the internal capsule (Figure 33A). Between GW9 and GW11 an additional layer, STF4, begins to emerge slowly and uncertainly in the earlier maturing lateral aspect of the neocortex. The emergence of STF4 may be associated with the onset of the descent of corticofugal fibers. The other STF layers (STF3, STF2, and STF6) begin to form thereafter and all six of the STF layers are present by GW13.5 (Figure 33B). By the latter age, there is also a clear difference in the organization of the STF in the future motor cortex anteriorly and the future sensory cortex posteriorly. STF5 is best developed in the motor areas (Figure 34A) and STF3 is a unique feature of the sensory areas (Figure 34B). The different cytological organization of STF in the motor cortex and visual cortex is illustrated at higher magnification in a GW20 fetus (Figure 35). In the visual cortex, STF3 is composed of three sublayers (the honeycomb matrix). We hypothesize that the trilaminar STF3 contains sojourning neurons that will form the granular layer (layer IV), the principal target of thalamocortical afferents, and the STF5 contains sojourning neurons that will form the pyramidal layer (layer V), the source of corticospinal efferents.

Text continues on page 472

469

Layers of the stratified transitional field

B. GW13.5, Y144-63

E AL N P+SVZ IC

CO R

L CA TI

COR T

L PLATE ICA T R

E AT PL AL IC ATE RT BPL CO SU

CO

A. GW10, Y1-59

Z

5

Choroid plexus

1

SV P+ NE

Choroid plexus

Ammon's horn Dentate gyrus

Hippocampus (ammon's horn)

6

Hippocampus

ate ud a C

THALAMUS n

bus Glo idus l l a p

ps

STRIATAL NEP+SVZ

THALAMUS

n

I

er

ca al

2

e ul

Putamen

Int

ral reb e Ce duncl pe

ix Forn

ix rn Fo

STRIATAL NEP+SVZ

5

4

1

Amygdala

ral reb e Ce duncl pe

Optic tract

te

Caudate

rn

al

ca

ps

ul

e

Amygdala

Figure 33. Increase of the STF from two layers in the GW10 fetus (A) to six layers in the GW13.5 fetus (B). Coronal sections. (Modified Figure 8 in Altman and Bayer, 2002.)

470

GW13.5 CORONAL, Y144-63 Cort

A.

ical plate

Subplate

Anterior section of motor cortex

CORTIC

PARACENTRAL LOBULE

AL

1 NE P

+S

2

4 5

VZ

Choroid plexus

Lateral ventricle um

NE

llos

TR

Hippocampal commissure

S

In

Fornix

u Ca

da

te

rn al ca ps ul e

f c o rp u s c a

te

rs o

Z SV + P

IA TA L

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6

Putamen

SEPTUM

B.

1

Posterior section of sensory cortex

3

4 5

rti

6

Co

PARIETAL LOBE

ca

SUPERIOR COLLICULUS

MIDBRAIN TEGMENTUM

OCCIPITAL LOBE

e lat l p

SVZ P+ NE AL us IC plex RT roid Cho CO

Lateral ventricle

5 4

31

PONTINE GRAY

Figure 34. Regional differences in STF organization in the future motor cortex anteriorly (A) and the future sensory cortex posteriorly (B) in a GW13.5 fetus. Coronal sections. (Modified Figure 11 in Altman and Bayer, 2002.)

471

A. Motor cortex

GW20 Sagittal, Y27-60

Cortical plate

Layer I

B. Visual cortex

Cortical plate

Layer I

Subplate

Subplate

STF

1

STF

STF 2

(STF 2)

1

a STF b c 3

STF

4

STF 4

STF 5 STF 5

STF 6

STF 6

CORTICAL NEP+SVZ

CORTICAL NEP+SVZ

Figure 35. Regional differences in STF organization in the motor cortex (A) and the visual cortex (B) in a GW20 fetus (specimen #5, Volume 4, Bayer and Altman, 2005). Note the thickness of STF4 in the developing motor cortex and the complex organization of STF3, with the honeycomb matrix, in the developing visual cortex. Sagittal sections. (Note the scale differences between A and B; the motor cortex is much thicker than the visual cortex at this stage.)

472 Like the developing cerebral cortex, the developing cerebellar cortex has transitional fields and sojourn zones. In contrast to the cerebellar NEP of the rat, which has only two divisions, C1 and C2 (Figure 36A), the primordia of the medial vermis and a single (“intermediate”) hemisphere, the human cerebellar NEP also has a true “lateral” or neocerebellar division, identified as C3 (Figure 36B). Surrounding these three NEP divisions are complex migratory and sojourn zones, the cerebellar transitional field (CTF). As seen in a GW6.5 embryo (Figure 37), CTF1 is a superficial fibrous layer, CTF2 consists mostly of tangentially migrating early generated neurons, CTF3 is an inner fibrous layer, and CTF4-5 consists mostly of radially-migrating and sojourning younger neurons that have recently left the cerebellar NEP. On the basis of neurogenetic dating studies in the rat, we identify the first wave of cells in CTF2 as the early-generated deep nuclear neurons which form the nuclear transition zone or NTZ (Altman and Bayer, 1997). The human NTZ has three components, the presumed migrating and sojourning neurons of the future fastigial nucleus (NTZ1), interpositus nucleus (NTZ2), and the late-developing, and as yet small, dentate nucleus (NTZ3). The cerebellar NEP may begin to form as early as GW4.0 (Figure 20B) and it is clearly recognizable as a distinct entity by GW5.0 (Figure 21A). By GW5.5, the cerebellar NEP has two components, the vermal C1 and intermediate C2 posteriorly (Figure 38A, top), and an additional division, the lateral C3, anteriorly (Figure 38A, bottom). The cells of NTZ1 and NTZ2 that uniformly abut C1 and C2 posteriorly are presumed to be young neurons that have recently radially migrated a short distance from the NEP matrices. The presumably earlier generated neurons situated superficially in the NTZ of the anterior cerebellum appear to migrate tangentially toward the midline. In older embryos—GW6.5 (Figure 38B), GW7.0 (Figure 38C) and GW7.5 (Figure 38D)—the tangentially migrating cells of the NTZ (particularly those of NTZ1 in the anterior cerebellum) begin to form a growing superficial mass of cells, which we identify as the sojourning neurons of the fastigial nucleus. As the vermis fuses in the GW8.5 embryo (Figure 38E), the axons of these neurons cross the midline to form the hook bundle. As the cells of the cerebellar NTZ migrate tangentially, new waves of radially migrating cells (CFT4+5) appear to leave the cerebellar NEP in the GW 6.5 and GW7.0 embryos (Figure 38B, C). These may be straggling deep neurons and/or the earliest complement of Purkinje cells. The radial migration of the bulk of Purkinje cells appears to reach its peak in the GW7.5 embryo when they form a crescent-shaped mass of densely packed and darkly staining cells outside the NEP both in the posterior cerebellum (Figure 38D, top) and the anterior cerebellum (Figure 38D, bottom). The packing density of these cells decreases in the GW8.5 (Figure 38E) and GW9 (Figure 39A) specimens, suggesting that the Purkinje cells begin to disperse as they ascend toward

the surface. By GW11, the ascending Purkinje cells form a crescent-shaped mass superficially, and the dentate, interpositus and fastigial nuclei are now settling in the depth of the cerebellum (Figure 39B). We have suggested earlier that these elaborate migratory movements, and some others that we describe below, are part of a choreographed morphogenetic process responsible for the wiring of the complex circuitry of the maturing cerebellum (Altman and Bayer, 1997). The first step in this process, the upward migration of Purkinje cells, is associated with the spreading of a secondary germinal matrix, the external germinal layer, over the surface of the formative cerebellar cortex, as we describe below. Secondary Germinal Matrices. Proliferative NEP cells in some regions of the CNS generate not only differentiating neurons but also fate-restricted neurogenic precursor cells that retain their proliferative potency after they have left the ventricular shoreline. These secondary matrices include the subventricular zone (SVZ) of the neocortex and the basal ganglia, the subgranular zone (SGZ) of the hippocampal dentate gyrus, and the external germinal layers (EGL) of the cerebellar cortex and cochlear nuclei. We proposed some time ago (Altman and Das, 1965b) that these secondary germinal matrices share two properties. First, they generate microneurons, neurons that develop locally arborizing short axons that form the fine (local) circuitry of specific brain regions. This is in contrast to the primary NEP matrix that generates macroneurons, a variety of large neurons that sprout long axons that interconnect distant brain regions and form the gross (global) circuitry of the CNS. Second, the microneurons of a particular brain region are generated after its macroneurons have been produced. This occurs in some brain regions during the late gestational period, in other regions postnatally during infancy, and in a few of them through adulthood. For instance, the subpial EGL of the cerebellum, which spins off a component of the dorsal rhombic lip (the germinal trigone) and spreads over the surface of the cerebellum, begins to produce its microneurons (granule, basket, and stellate cells) after the cerebellar NEP has generated its macroneurons, the deep neurons and the Purkinje cells (Altman and Bayer, 1997). The EGL begins to form in the human cerebellum between GW7.5 and GW8.5 (Figure 38D, E) and by GW11 it forms a subpial canopy over the entire formative cerebellar cortex (Figure 39B). Significantly, the EGL that persists in rats until about postnatal day 21, the age they are weaned (Altman and Bayer, 1997), is still present in human cerebellum through the second year of postnatal life (Raaf and Kernohan, 1944; our unpublished observations). The descent of cerebellar granule cells into the formative cerebellar cortex to form the internal granular layer, leaving behind their axons, the parallel fibers, in the molecular layer, begins after the ascending Purkinje cells have commenced to sprout dendrites. The outcome of these choreographed movements of macroneurons and microneurons, such as the ascent of Text continues on page 481

473

A. E15 Rat


Pretectal NEP

ABBREVIATIONS:


MESENCEPHALON

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP Tegmental EGL - External germinal layer NEP NTZ - Nuclear transitional zone NTZ1 - Medial (putative fastigial neurons) NTZ2 - Intermediate (putative interpositus neurons) NTZ3 - Lateral (putative dentate neurons) isthmal canal

ISTHMUS

Isthmal NEP NTZ1

FUTURE VERMIS

FUTURE INTERMEDIATE HEMISPHERE

NTZ2 rhombencephalic superventricle

C1 Upper rhombic lip


C2


Medullary velum

B. GW7 Human, M2155 Figure 36. The configuration of the anterior cerebellar NEP in rat (A) and human (B) at comparable stages of brain development. In the rat, the cerebellar NEP has two divisions, C1, the primordium of the vermis, and C2, the primordium of the phylogenetically older ("intermediate") hemisphere. In humans, it has three divisions with the added C3, the primordium of the neocerebellar lateral hemisphere. Coronal sections.

ANTERIOR CEREBELLUM

0.5 mm

Pretectal NEP


Tegmental NEP

MESENCEPHALON

isthmal canal

ISTHMUS

Isthmal NEP NTZ1

FUTURE VERMIS

ANTERIOR CEREBELLUM FUTURE INTERMEDIATE HEMISPHERE

NTZ2

FUTURE LATERAL HEMISPHERE

NTZ3

C1 C2


C3




0.5 mm

474 isthmal canal

GW6.5 Coronal, M2051, Anterior Cerebellum NTZ1

ABBREVIATIONS:

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP CTF - Cerebellar transitional field NTZ - Nuclear transitional zone NTZ1 - Medial (putative fastigial neurons) NTZ2 - Intermediate (putative interpositus neurons) NTZ3 - Lateral (putative dentate neurons)

Isthmal NEP NTZ2

Tangentially migrating deep neurons in the superficial NTZ (CTF2)

CTF1 CTF2 CTF3 CTF4+5


C2

NTZ3

Future intermediate hemisphere

Future vermis rhombencephalic superventricle

Radially migrating deep neurons


C1


Future lateral hemisphere

Figure 37. Three components of the human cerebellar NEP (C1, C2, C3), the presumed primordia of the vermis, intermediate hemisphere, and the lateral hemisphere, and the organization of the cerebellar transitional field in a GW6.5 embryo. CTF1 is a superficial fibrous layer; CTF2 consists mostly of tangentially migrating, earlier-generated (older) deep nuclear neurons (NTZ1, NTZ2, NTZ3; superficial arrows); and CTF4+5 consist mostly of radially migrating, later-generated (younger) neurons (double-headed arrows). Coronal section.

Figure 38A. GW5.5 Coronal, M2161 ABBREVIATIONS:

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP GEP - Glioepithelium NTZ - Nuclear transitional zone NTZ1 - Medial (putative fastigial neurons) NTZ2 - Intermediate (putative interpositus neurons) NTZ3 - Lateral (putative dentate neurons)


Future vermis


NTZ1

C1

Isthmal NEP

C2

isthmal canal

Upper rhombic lip

Posterior cerebellum NTZ1

NTZ2

NTZ2


Upper rhombic lip

Trochlear nerve (IV) decussation

Trochlear nerve GEP

Figure 38 (this page and the following four pages). Coronal sections of the developing posterior cerebellum (top) and anterior cerebellum (bottom) from embryos of the following ages: GW5.5 (A), GW6.5 (B), GW7.0 (C), GW7.5 (D), and GW8.5 (E). The vermal NEP (C1) develops earlier and is more prominent posteriorly; the hemispheric NEP (C3) develops more slowly and is prominent anteriorly. The tangentially migrating deep neurons (NTZ) accumulate superficially in the GW7.5 embryo and the axons of the putative fastigial nucleus neurons cross to the opposite side as the vermis fuses medially in the GW8.5 embryo. Concurrently, the radially migrating and sojourning Purkinje cells form a large crescent-shaped mass in the depth of the formative cerebellum. The subsequent ascent of Purkinje cells toward the surface is associated with the formation and spread of the EGL beneath the pia.

C2


C1 rhombencephalic superventricle


Anterior cerebellum

C3

Tangentially migrating deep neurons NTZ3

Lateral cerebellar notch?

Medullary velum

475

476

Figure 38B. GW6.5 Coronal, M2051 ABBREVIATIONS:



Posterior cerebellum

Future vermis

C1

NTZ1


Trochlear nerve (IV) decussation

Trochlear nerve GEP

isthmal canal

Tangentially migrating deep neurons

NTZ2

C2 Medial cerebellar notch

Upper rhombic lip


NTZ1


NTZ2


Isthmal NEP

NTZ3


C3 Lateral cerebellar notch


C1


Anterior cerebellum

Figure 38C. GW7 Coronal, M2246


ABBREVIATIONS:


Trochlear nerve (IV) decussation Trochlear nerve GEP

Future vermis

C1

NTZ1


Posterior cerebellum Tangentially migrating deep neurons

NTZ2

Radially migrating deep neurons and earliest Purkinje cells

C2

isthmal canal

Upper rhombic lip

NTZ1

NTZ1

Isthmal NEP

NTZ2


Upper rhombic lip

NTZ3


C2

Anterior cerebellum

C1 rhombencephalic superventricle

C3


477

478

Figure 38D. GW7.5 Coronal, M2248 ABBREVIATIONS:

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP EGL - External germinal layer NTZ - Nuclear transitional zone NTZ1 - Medial (putative fastigial neurons) NTZ2 - Intermediate (putative interpositus neurons)

Inferior colliculus

Future vermis


NTZ1


Upper rhombic lip

NTZ2

Radially migrating Purkinje cells

C1


isthmal canal

Sojourning Purkinje cells

C2 Pioneer EGL Budding germinal trigone

Isthmal NEP

Dentate and interpositus nuclei in superficial position

PONS

NTZ3

Future intermediate and lateral hemisphere

Anterior cerebellum


C2/3

C2

Pontine NEP


Figure 38E. GW8.5 Coronal, M2050


ABBREVIATIONS:

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP EGL - External germinal layer


Vermal fusion field

Inferior colliculus

Future vermis and intermediate hemisphere

Transversely oriented fastigial neurons sprouting decussating fibers of the hook bundle (before descent)

Spreading EGL

Tangentially migrating Purkinje cells (beneath EGL) Germinal trigone (upper rhombic lip)

Sojourning Purkinje cells

C1/2


C2 Spreading EGL Tangentially migrating Purkinje cells (beneath EGL) Germinal trigone (upper rhombic lip)

Anterior cerebellum

Future intermediate and lateral hemisphere


PONS


Pontine NEP


C2/3 rhombencephalic superventricle

479

480

A. GW9 Coronal, M841

Future site of vermal fusion

Future EGL trajectory

ABBREVIATIONS:

NEP - Neuroepithelium C1 - Medial cerebellar NEP C2 - Intermediate cerebellar NEP C3 - Lateral cerebellar NEP EGL - External germinal layer G/EP - Glioepithelium/ependyma

Medially spreading EGL


Inferior cerebellar peduncle

fusing isthmal canal

Ascending Purkinje cells approaching EGL


C2+C3 Germinal trigone (upper rhombic lip)

Sojourning and ascending Purkinje cells

B. GW11 Coronal, Y1-59 LATERAL HEMISPHERE

l je ce rkin n u f P tio s o o si as ial p m ic rf

in

ge n Late r

INTERMEDIATE HEMISPHERE

ls

er a te su d pe

EGL

Descended deep nucl ei Interpositus nucleus

Dentate nucleus

VERMIS

Fastigial nucleus

Earlier generated dispersing Purkinje cells

Fusion site of vermal NEP (C1)

Cerebellar G/EP

Germinal trigone (upper rhombic lip)



Figure 39. A. The ascent of Purkinje cells toward the surface in a GW9 embryo as, concurrently, the EGL begins to spread medially. B. The Purkinje cells are settling superficially beneath the canopy of EGL cells in this GW11 fetus, and the fastigial, interpositus, and dentate nuclei are assuming their final position in the depth of the vermis, and the intermediate and lateral hemispheres. Coronal sections.

481 Purkinje cells through the field of deep neurons, and the descent of granule cells through the Purkinje cell layer is that, by leaving behind their trailing axons, the stereotypic input-output circuitry of the cerebellum is established (Altman and Bayer, 1997). The pattern of neurogenesis and migration of hippocampal macroneurons and microneurons is quite different than that of the cerebellum. As it was demonstrated in the rat, hippocampal macroneurons (the pyramidal cells of Ammon’s horn) are produced early during fetal development (Bayer, 1980a, b). The dentate NEP produces an early set of perinatal microneurons (granule cells), while another set of progenitor cells forms the secondary germinal matrix of the subgranular zone (SGZ), which persists through adulthood (e.g., Altman and Bayer, 1975; Kempermann, 2006). It is now known, that the production of hippocampal granule cells through adulthood is not limited to lower mammals but is also a significant phenomenon in primates (e.g., Gould et al., 1999). In the human telencephalon, the hippocampal dentate NEP is present by GW9; but the dentate gyrus is not recognizable as a distinct entity until the second trimester (Volume 3 of this Atlas: Bayer and Altman, 2005). The time course of secondary matrix neurogenesis is again different in the cerebral cortex, a site where, according to experimental evidence in rodents, early- and late-generated neurons settle in an “inside-out” pattern in the gray matter (Angevine and Sidman, 1961). The lateral ventricles that are lined by NEP cells shrink considerably during the third trimester but, where present, an appreciable NEP/SVZ matrix is still present at the time of birth (Volume 2 of this Atlas: Bayer and Altman, 2004a). The SVZ that persists anteriorly through adulthood in animals has been shown to be a source of neurons that, migrating by way of the rostral migratory stream, supply neurons to the olfactory bulb not only in the rat (Altman, 1969) but also in monkeys (e.g., Pencea et al., 2001) and humans (Bedard and Parent, 2004). According to our interpretation of the experimental data obtained in rats, the early-generated layer VI and layer V cortical output neurons are progeny of the cortical NEP, and the later-generated granule cells that settle in the granular layer (IV) and other locally arborizing neurons that settle in the supragranular layers (III-II) are progeny of both the NEP and SVZ (Bayer and Altman, 1991a). Others have argued that the SVZ of the basal ganglia is the source of cortical interneurons (e.g., Anderson et al., 2002). It has been reported that the transcription factor Tlx is necessary for the full formation of the supragranular layers (Land and Monaghan, 2003). In the material available to us, we could not accurately date the emergence of the cortical SVZ because of the difficulty of distinguishing it from the NEP. However, as Kershman (1938) illustrated some time ago, the SVZ is prominent in the GW11 cerebral cortex (Figure 40). The basal ganglionic SVZ may begin to form as early as GW7.5-GW8 (Figure 16H, I), and both the cortical and the striatal SVZ persist as prominent ger-

Layer I

Cortical Plate

Stratified transitional field

Cortical SVZ

Cortical NEP

Figure 40. Kershman’s (1938) illustration of the developing cerebral cortex in a GW11 human fetus, showing the cytological difference between the neuroepithelium (NEP) adjacent to the ventricle and the subventricular zone (SVZ). The spindle-shaped (pseudostratified) NEP cells are oriented at a right angle to the ventricular lining and they undergo mitotic division near the lumen. The variably shaped SVZ cells undergo mitosis within this secondary germinal matrix. Labeling modified.

minal zones in newborns (Volume 2: Bayer and Altman, 2004a). The formation of the internal capsule as early as GW8 (Figure 31B) may initially contain only thalamocortical fibers. The outflow of corticofugal efferents and the formation of the cerebral peduncle between GW10 and GW13.5 (Figure 33A, B) suggests that the cortical output

482 neurons are probably generated before the cortical SVZ is forming or becomes prominent. Indeed, the cortical plate that forms between GW8.0 and GW8.5 (Figure 32C, D) may contain the earliest layer VI corticofugal neurons generated by the primary NEP and the generation of layer V neurons may follow soon thereafter. Gliogenesis and Fate-Restricted Glioepithelia. Within a given brain region, neurogenesis and gliogenesis are generally sequential processes, and the generation of certain types of neuroglia, like astrocytes, antedates the production of oligodendrocytes that myelinate axons. As we have documented in the rat with 3H-thymidine autoradiography, there are many CNS sites where germinal matrices with proliferating cells persist for some time after the cessation of neurogenesis (e.g., Bayer and Altman, 1991). We called these transformed germinal matrices glioepithelia (GEPs). These proliferating cells leave the GEP, become dispersed through the parenchyma, and generate fate-restricted neuroglial precursors that multiply through adulthood (Altman, 1966). We have also found that at sites where the enduring ventricle will be lined with specialized ependymal cells, the administration of 3H-thymidine at a late phase of fetal or early prenatal development tags ependymal cells that remain labeled in adults; we call these germinal sites glioepithelial/ependymal matrices (G/EPs). Finally, cytological observations in the human CNS, in particular in myelin-stained sections of the spinal cord, have established that myelination gliosis (the high rate of glial cell proliferation preceding the myelination of particular fiber tracts) is a late developmental phenomenon (Altman and Bayer, 2001; Volume 1 of this Atlas: Bayer and Altman, 2002). Lineage studies have confirmed that NEP cells can generate both neurons and glia, although it remains unclear whether it is the same line of proliferative cells that first gives rise to neurons and then to neuroglia or that there are two separate lines that are sequentially activated by external signals (Luskin et al., 1988, 1993; Walsh and Cepko, 1992; Williams and Price, 1995). The report that cultured cortical progenitor cells generate neurons when plated with cortical tissue from young embryos but produce neuroglia when plated with cortical tissue from adults (Morrow et al., 2001) supports the idea that extracellular influences can modify NEP cell fate. The gene, Brg1, has been implicated in the switch from neurogenic to gliogenic cell production, and proteins, such as Sox1 and Pax1, implicated in the maintenance of neurogenic potential are drastically reduced in Brg1 mutant mice (Matsumoto et al., 2006). In addition to the NEP-derived, late-generated glial progenitors, there are specialized germinal matrices that from the outset of CNS development are destined to produce only neuroglia. Prominent among these are perifascicular GEP matrices associated with early forming large fiber tracts, such as the fornical GEP. Moreover, we can also distinguish at the latter site a germinal matrix that generates the specialized cells of the choroid plexus (Altman and

Bayer, 1990a). Finally, we present suggestive evidence that, as in the rat brain (Van Hartesveldt et al., 1986), so also in the human CNS, there are specialized types of glia present at certain sites, we call them morphocytes, that play a formative role in the structural transformations of the developing CNS. For instance, in the midline of the spinal cord, morphocytes of the roof plate and the floor plate are responsible for the development of the H-shape configuration of the gray matter (Altman and Bayer, 2001), and in the hindbrain morphocytes seem to act as guy wires to produce and maintain the structural stability of the medullary, pontine, mesencephalic, and diencephalic flexures (Volume 4 of the Atlas and the present volume). Nestin expression has been associated with the midline raphe glial structure in the early human fetus (Takano and Becker, 1997).

H. Centro-Central Signaling and the Morphogenetic Maturation of the CNS Centro-Central Induction and Signaling. From the perspective of its input-output organization, the vertebrate CNS has two principal components: (i) first-order structures that are in direct contact with peripheral sense organs, skeletal muscles, smooth muscles, and glands; and (ii) higher-order structures that are directly (synaptically) connected with one another but only indirectly (by way of the first-order structures) with the periphery. Examples of first-order sensory CNS structures are the dorsal horn in the spinal cord, which receives input from the trunk, limb, and neck sensors by way of spinal afferents; the trigeminal nuclei that receive sensory input from the face and mouth by way of trigeminal afferents; and the olfactory bulb that receives direct peripheral input from the olfactory epithelium. Examples of first-order CNS effector structures are the motor nuclei of the ventral horn and the cranial motor nuclei that directly innervate skeletal muscles, and the secretory neurons of the hypothalamus and preoptic area that produce releasing factors that, conveyed to the pituitary gland, control the visceral system. In contrast, the bulk of second- and higher-order CNS structures that relay and process sensory information and issue motor commands have no direct access to the periphery. Examples of intercalated somatosensory structures are the dorsal column nuclei in the medulla, the relay nuclei of the thalamus, the postcentral projection area in the cerebral cortex, and several cortical association areas involved in higher-order somatosensory information processing. From the perspective of morphogenetic regulation, the coordinated development of first-order CNS structures requires, as described earlier, reciprocal periphero-central induction and signaling, whereas the coordinated development of higher-order CNS structures requires centro-central signaling between components of a system that become interconnected during development. It is reasonable to assume that the induction and signaling mechanisms responsible for synchronizing the development of PNS and CNS structures, and those

483 responsible for interconnecting CNS structures with one another are different. The higher-order components of the CNS are interconnected with one another by way of long-distance nerve trunks, regional neural networks, and local circuits. The establishment of modality-specific and topographically organized projection systems—such as the selective and reciprocal interconnections between subdivisions of a particular sensory relay nucleus of the thalamus with a particular area of the neocortex—is dependent on the guided pathfinding of the growing axons of projection neurons that have to navigate over long distances to find their exact target. This requires centro-central signaling between source and target structures, as well as guideposts or beacons along a tortuous path. The regional ramification of the axons of interneurons, the spread and geometric configuration of their dendritic arbors, etc. must likewise be coordinated by reciprocal centro-central signaling among neighboring structures. It may be expected that the signaling mechanisms responsible for long-distance central interconnections, which establish the gross circuitry of the CNS, and the signaling mechanisms responsible for short-distance interconnections, which produce the fine circuitry of a local brain region, are also different. Centrocentral signaling between interconnected brain structures may involve parallel (synchronous) development, serial (hierarchical) development, or a combination of both. For example, the early development of lower-level components of the system (input-output structures) may trigger the later development of its higher level components (processing structures). In the human CNS, at least some of the long-range sensory connections are established by the late first trimester, and to some extent concurrently, at different levels within a system. An example is the early maturation of the medial lemniscal system in the somatosensory pathway and the relatively early maturation of the ventral thalamic nuclear complex (Volume 4 of this Atlas: Bayer and Altman, 2006). However, the establishment of the fine circuitry of CNS structures, through the interdigitation of macroneurons and microneurons, is a lengthy process that extends beyond the second and third trimesters and continues through the prenatal period of brain development. Relatively little is known about the mechanisms of centrocentral signaling except for axonal pathfinding. The Guidance of Axonal Pathfinding. The growing axons of the medial lemniscus exemplify pathfinding to a distant target. Originating in the dorsal column nuclei, these axons cross to the opposite side in the medulla (the arcuate decussation) and, bypassing various intermediate structures in the pons, isthmus, and the midbrain tegmentum, terminate in the somatosensory thalamus. Another example is the corticospinal tract whose axons, originating in the cerebral cortex, penetrate the corona radiata, move through the striatum, join the internal capsule, pass through the cerebral peduncle of the midbrain, penetrate and move

through the pontine gray and, upon reaching the medulla, most of them cross to the opposite side and descend in the spinal cord. We have described earlier the developmental timetable of the latter process in the human CNS (Altman and Bayer, 2001). Modern research indicates that the directed growth of pioneer axons toward distant targets depends on molecular guidance agents along their route, cues that are detected by receptor elements within the axon’s growth cones (Tessier-Lavigne and Goodman, 1996). These cues, acting as chemorepellants or chemoattractants, may adhere to or emanate from specific brain structures along the axon’s path, and in that way, the growing axon can use a trial-and-error method to locate its target. Specifically, the growth cone tips that encounter repellants withdraw or collapse, and those that encounter attractants expand. Among recognized repellant cues are the semaphorins that, interacting with the growth cone receptors (plexins and neuropilins) of advancing axons, cause the depolymerization of microtubules and F-actin in the growth cones that leads to their collapse (Fan et al., 1993; He et al., 1997; Fujisawa and Kitsukawa, 1998; Pasterkamp and Kolodkin, 2003). Netrin-1 (Métin et al., 1997; Richards et al., 1997; Finger et al., 2002) and semaphorins (Polleux et al., 1998) are among the putative chemoattractants implicated in the directed growth of corticofugal axons. Netrin-1 may also be involved in the guided growth of thalamocortical fibers (Braistead et al., 2000) and in the normal development of the circuitry of the hippocampus (Barralobre et al., 2000). In some brain regions, netrin-1 signaling is required for commissural fibers to cross to the opposite side (Serafini et al., 1996); in the medulla, netrin-1 is secreted by the floor plate and its absence in homozygous mutant mice results in the failure of medial lemniscal fibers to cross to the opposite side and turn to ascend rostrally (Kubota et al., 2004). Still another example of directed, long-distance axonal pathfinding is the guidance of the pioneering fibers of the ascending thalamocortical afferents. The internal capsule that contains the bulk of the thalamocortical fibers, is still absent in the developing human brain on GW7.5 (Figure 31F), and the cortical plate has yet to form. By GW8 thalamocortical fibers enter the expanding internal capsule (Figure 31G), which establishes a bridge between the diencephalon and the cortex, and a thin cortical plate begins to form dorsolaterally. By GW10, there is a massive outflow of afferents from the thalamus through the expanded internal capsule and, in addition to the thickening cortical plate, the different layers of the STF are beginning to form (Figure 33A). By GW11, we can track the different trajectories of thalamocortical fibers to some areas of the cerebral cortex; in particular, the visual radiation from the lateral geniculate nucleus, which makes a 180° turn (Meyer’s loop) around the caudate nucleus, proceeds caudally, and terminates in the occipital lobe (Volume 4 of this Atlas: Bayer and Altman, 2006). According to one hypothesis there are intrinsic regional differences in the proliferative

484 matrix of the cerebral cortex that will become the target of different afferent fibers and these regional differences are the foundation of areal specification (Rakic, 1988). We have demonstrated anterior-to-posterior (longitudinal) and lateral-to-dorsomedial (transverse) neurogenetic gradients in the rat cerebral cortex (Bayer and Altman, 1991), and suggested that these NEP maturation gradients may control not only the modality-specific selectivity of thalamocortical innervation but also the lateral-to-medial order in the ingrowth of thalamic afferents, as demonstrated in the mouse (Caviness and Frost, 1980). One set of studies has implicated Tbr1, Gbx2, and Pax6 in the initial outgrowth of thalamic fibers to the cortex and in the guidance of their long-range trajectory (Hevner et al., 2002; Molnár et al., 2003). Another series of investigations implicates Emx1, Emx2, Pax6, Gsh2, COUP-TFI, and Fgf8, which are expressed in dissimilar dorsal-to-ventral and anterior-toposterior gradients in the developing cerebral cortex (Gulisano et al., 1996; Liu et al., 2000; Stoykova et al., 2000; Yun et al., 2001; Torreson et al., 2000; Muzio et al., 2002; Bishop et al., 2003; Garel et al., 2003). These studies have not yet taken into consideration our evidence that rather than proceeding directly to the cortical plate, the sojourning cells of the STF are an intermediary target of thalamocortical afferents, and that there are pronounced regional differences in STF lamination patterns (Altman and Bayer, 2002; Volume 3 of this Atlas: Bayer and Altman, 2005). Once thalamocortical modality-specific projection has been established, the formation of topographic maps and the termination of arborizing axons in layer IV are probably controlled by different signaling agents than those guiding the initial growth of thalamic afferents. Among these signaling molecules are reelin secreted by the early generated Cajal-Retzius cells of the primordial plexiform layer, already discussed in the context of cell migration, and ephrins and their receptors (Castellani et al., 1998; Vanderhaeghen et al., 2000; Dufour et al., 2003; Bolz et al., 2004). One experiment compared the growth of thalamic axons toward cultured membranes from cortical layer IV to that of membranes from cortical layer V, a stratum that the thalamocortical fibers bypass on their way to layer IV. The thalamic axons exhibited arrested growth and increased branching density on their appropriate target tissues but interference with ephrin expression abolished this preferential termination pattern (Mann et al., 2002). Still other studies implicated N-cadherin (Huntley and Benson, 1999; Poskanzer et al., 2003) and Slit2 (Ozdinler and Erzurumlu, 2002) in the laminar termination of thalamocortical fibers in the cortex. N-cadherin (Riehl et al, 1997) and ephrins also appear to serve as guidance cues in the growth of retinal fibers (Birgbauer et al., 2001). According to a recent study (Lambot et al., 2005), there are differences in the gradients of ephrin expression in the developing visual system of animals with lateral eyes, in which most (or all) retinal axons cross to the opposite side in the optic chiasma, with ephrin expression in the developing visual

system of humans with medial eyes, where only the nasal half of the retina projects contralaterally but the temporal half projects ipsilaterally.

I. Summary: The Epochs, Phases, and Mechanisms of CNS Development The reviewed descriptive and experimental evidence suggests that the prenatal development of the CNS consists of successive epochs, each with multiple phases, that are guided by different morphogenetic mechanisms. The first epoch (Figure 41) consists of several phases. The initial phase, which is not dealt with in this Atlas, is the formation of the neural plate, containing pluripotent progenitor cells that give rise to the neural elements of the CNS and PNS as well as some non-neural tissue and organs. The next phase is the fusion of the central component of the neural plate, which results in the formation of the neural tube (the future spinal cord) along the trunk caudally, and the cephalic vesicles (the future brain) in the head region rostrally (Figure 22). This is followed by the proliferation of stockbuilding NEP cells in association with the earliest specification of their future diversity through reciprocal periphero-central transactions with the primordial components of the developing body, including the neural crest, somites, and notochord caudally, and the preplacodes rostrally. That early diversification becomes manifest in the future spinal cord as the tubular NEP is partitioned into sensory and motor compartments. This occurs under the peripheral inductive and signaling influence of the neural crest and somites, on the one hand, and the notochord, on the other. The diversification of the stockbuilding NEP cells in the different divisions of the future brain is under a different set of peripheral influences. Rostrally, the stockbuilding prosencephalic NEP expands greatly in association with the ballooning of the prosencephalic superventricle, and it is under the reciprocal influence of the cephalic (olfactory, optic, and pituitary) preplacodes. Caudally, the diversifying rhombomere NEPs, which grow in association with the expansion of the rhombencephalic superventricle, interact with the branchial (orofacial, octaval, and visceral) preplacodes, and arches I, II, III, and IV with which they are associated. The second epoch of CNS development, likewise, consists of several phases, and these phases differ caudally in the developing spinal cord and rostrally in the developing brain (Figure 42). In the developing spinal cord, several periphero-central morphogenetic events take place concurrently. (i) The skin senses of the trunk, limbs, and neck, and (ii) the bipolar neurons of the spinal dorsal root ganglia differentiate peripherally. The latter (iii) interconnect the sense organs with the sensory-relay neurons in the dorsal horn and dorsal column nuclei centrally. At about the same time, (iv) the various peripheral muscles of the trunk, limb and neck conjointly differentiate with (v) the motor neurons that innervate them, which form distinct columns in

485 the ventral horn centrally. In the developing brain, the different components of the ventral telencephalon and diencephalon (forebrain) interact with the diversifying cephalic placodes, and different components of the rhombencephalon (hindbrain) do the same with the diversifying branchial placodes. In the developing forebrain, aided by reciprocal periphero-central induction and signaling, (i) the neurons in the olfactory epithelium, the progeny of the olfactory placode, differentiate conjointly with the neurons in the olfactory bulb. (ii) The cells in the crystalline lens of the eye, derived from the lens placode, differentiate conjointly with the neurons in the retina and cells in the pigment epithelium. (iii) The secretory cells in the adenohypophysis, derived from the pituitary placode, differentiate conjointly with the neurosecretory cells in the hypothalamus and pituicytes in the neurohypophysis. In the developing hindbrain, (iv) the peripheral neurons in the orofacial (trigeminal and facial) ganglia, derived from branchial placodes, conjointly differentiate with central neurons derived from R2 and R3. (v) The peripheral neurons in the vestibulocochlear ganglion, derived from the otic vesicle, conjointly differentiate with the central neurons derived from R4 and R5. (vi) The glossopharyngeal and vagal ganglia, derived from branchial placodes, conjointly differentiate with the central glossopharyngeal and vagal neurons derived from R6 and R7. In sharp contrast to these forebrain and hindbrain regions that have direct peripheral connections, many brain regions are devoid of surrounding placodes such as the cerebral cortex, basal ganglia, thalamus, tectum, and cerebellum. In the absence of direct contact with the developing peripheral sense organs, muscles, and other effectors, the coordinated development of these brain structures is dependent on endogenous mechanisms and centro-central induction and signaling. One facet of this process, as we described earlier, is the directed growth of axons that interconnect different brain structures and produce the brain’s gross and fine circuitry. Another is the migratory movement of young neurons that contact one another in passing and leave behind trailing axons, much like a spider building its web. This phenomenon is exemplified by the choreographed movements of sequentially generated cerebellar neurons, as deduced from experimental studies in the rat. The first phase of cerebellar development is the formation and expansion of the stockbuilding cerebellar NEP (Figure 43A). The second phase is the radial migration of the earliest set of differentiating neurons, the future deep nuclei, which form the first parenchymal cell layer abutting the NEP (Figure 43B). The third phase is the exodus of a new set of differentiating neurons from the cerebellar NEP, the Purkinje cells, which displace the layer of deep neurons outward (Figure 43C). The fourth phase is the tangential migration of deep neurons over the cerebellar surface (Figure 43D), a process coupled with the sprouting of axons that cross to the opposite side in the vermis. During the fifth phase, deep neurons and Purkinje cells exchange

places. Deep neurons migrate downward, while Purkinje cells migrate upward (Figure 43E). That establishes the mature pattern (Figure 43F) where the Purkinje neurons form a superficial cortical layer and the deep nuclear neurons settle in the core of the cerebellum. This important event occurs conjointly with the formation of a new neurogenic proliferative matrix, the EGL, which spreads superficially to form a subpial canopy over the expanding cerebellum (Figure 43E, F). Perhaps due to some dual attractant/ repellant force exerted by the EGL, the sojourning Purkinje cells ascend to the surface and the deep neurons descend. We hypothesized earlier (Altman and Bayer, 1997) that cohorts of deep neurons and Purkinje cells make enduring contacts with each other while they become intermingled for a period and then pass each other. The deep neurons, which already have extracerebellar inputs and are sprouting cerebellofugal efferents, descend together with the trailing axons of Purkinje cells attached. The ascending Purkinje cells carry with them branches of some extracerebellar afferents (climbing fibers) that also contact the deep neurons. As the superficially situated masses of Purkinje cells disperse beneath the canopy of the EGL to form a monolayer (Figure 43G), the subsequent phases of cerebellar development begin to unfold leading to formation of the fine circuitry of the cerebellar cortex. One facet of this is the descending migration of granule cells from the EGL, through the molecular and Purkinje cell layers, and into the granular layer (arrows, Figure 43G). The descending granule cells leave their axons (the parallel fibers) behind in the molecular layer and establish contacts with growing Purkinje cell dendrites. Granule cell dendrites sprout in the granular layer to establish contact with the specialized endings of mossy fiber afferents. The major function of the elaborate choreographed movements of different cerebellar neurons, in conjunction with the directed growth of their axons and dendrites, is to produce the stereotyped complex circuitry of the cerebellar cortex.

J. A Note on the Functional Maturation of the Human CNS The Functional Maturation of the CNS. The function of the CNS is to gather information about prevailing conditions and salient events in the external world in relation to the changing conditions and needs of the body interior, process and integrate that information, and initiate appropriate behavioral and physiological actions and reactions to them. Neither the proliferative progenitors of neurons nor the migrating and sojourning young neurons lacking axons and dendrites can mediate these functions. It is only after the settled neurons have begun to receive afferent input and establish synaptic connections with one another, and the fibers of motor neurons contact muscles, that the CNS can commence to perform its complex regulatory functions. The maturation of these functional CNS networks and circuits is a protracted process that commences during the early fetal period and continues through the late-fetal, neoText continues on page 489

486

RHOM

BENCEPHA

Cerebellar NEP

N

LON

Medullary velum


SOMITIC/NEURAL CREST DOMAINS (Dorsal)

Precerebellar NEP

R h o m b e s R2 R3 R4 o m e r R7 R5 R6

central canal

PR ( O C E P H A L I C tic) lf a c BRANCHIAL (Op tory) PREPLACODES

Arch II

Arch I (Orofacial)

NOTOCHORDAL DOMAINS (Ventral)

Arch IV Arch III (Visceral) OTIC VESICLE (Octaval)

P L A C O D A L

DORSAL NEP VENTRAL NEP

E

PL (H A C O yp oph D E S y s e a l)

O

P

N


pr su ose pe nc rv ep en ha t r l ic ic l e

PR

OSENCEPH

A

L

PRIMORDIAL NEPs

SE

E NC

LO HA

M

E

SPINAL CORD

D O M A I N S

Figure 41. The first epoch of CNS development. Schematic illustration of the reciprocal periphero-central signaling potential between components of the peripheral cephalic preplacodes (red) and the ventral telencephalic NEP (pink); between components of the peripheral branchial placodes (dark green) and the rhombomeric NEP compartments (light green); and between the peripheral somites and notochord (orange) and the dorsal and ventral spinal NEPs (blue).

CENTRO-CENTRAL INDUCTION FIELD

T

d supience erv pha ent lic ric le Hypotha lamus


um


Dorsal root ganglia and dorsal roots of spinal nerves

Medullary velum

rhombencephalic superventricle Pon

s

R2 R3 R4

Nerve I (olfactory) Pigment epithelium

SKIN SENSES

rebel l Ce

s

Nerve II (optic)

m su es pe en rv ce en ph t a me ri li Teg nt cle c

mu Isth

Olfactory bulb

SPINAL CORD

m

a

T e c t

um

al gang li Bas

a

C e r e b r

us am us l ha lam ha

u

T

r t e x

l

c

o

l l a M e d u

R5

R6

central canal

R7

DORSAL HORN VENTRAL HORN

Lens Retina

Eye

Adenohypophysis Neurohypophysis

Pituitary gland

Trigeminal ganglion and nerve V

Facial ganglion and nerve VII

CEPHALIC

PREPLACODAL DERIVATIVES

Ventral roots of spinal nerves

OTIC VESICLE

Vestibulocochlear ganglion and nerve VIII

BRANCHIAL

MUSCLES Vagal Glossopharyngeal ganglia and ganglia and nerve X nerve IX

NEURAL CREST/ SOMITIC DERIVATIVES

PERIPHERO-CENTRAL INDUCTION FIELD Figure 42. Second epoch of CNS development. Schematic illustration of the reciprocal periphero-central signaling potential between the olfactory epithelium, the lens of the eye, and the adenohypophysis, derived from the cephalic placodes (red), with the olfactory bulb, the retina, and the neurohypophysis, respectively, derived from the ventral telencephalic NEP (pink); between the cranial ganglia and otic vesicle derived from the branchial placodes (dark green) and the differentiating neurons of the rhombomeric NEPs (light green); and the differentiating spinal ganglia and muscles (orange) and the differentiating neurons of the dorsal horn and ventral horn derived from the spinal NEP (blue).

487

488

A

Connection to isthmus and inferior colliculus

Trochlear nucleus Primary cerebellar NEP

Purkinje cell layer

G

Upper rhombic lip

B

Neurogenic EGL produces descending granule cells

Granule cell layer

Deep neurons Radially migrating deep neurons

C

Tangentially migrating deep neurons Sojourning Purkinje cells

Germinal trigone remains large

Radially migrating Purkinje cells Radially migrating (short arrows) deep neurons (long arrows)

D


F

Stockbuilding EGL covers the entire cortical surface

Choroid plexus continues to expand

Ascending Purkinje cells disperse and sort into discrete clumps

Dispersing EGL


E

Deep neurons accumulate in specific nuclei

Ascending Purkinje cells

Germinal trigone

Descending deep neurons Choroid plexus

Figure 43. Third epoch of CNS development (centro-central induction) as illustrated by the several stages of cerebellar development. Light green, primary cerebellar neuroepithelium (NEP); yellow, deep neurons; red, Purkinje cells; dark green, external germinal layer (EGL); blue, granule cell layer; brown, choroid plexus Arrows indicate cell migrations. See text on page 485 for more details.

489 natal, and juvenile periods, and may not end until late adulthood. Importantly, there are pronounced differences in the maturation of different components of the CNS in relation to the specific functions they mediate and in terms of their position in the serial and hierarchic organization of those functions. For instance, in the maturating human spinal cord, motor fibers begin to exit the ventral horn as early as GW5.0. Sensory fibers of the spinal ganglia begin to penetrate the dorsal horn by GW5.5 (Altman and Bayer, 2001; Volume 1 of this Atlas: Bayer and Altman, 2002). In the maturing brain, somatosensory neurons of the trigeminal ganglion line up outside the trigeminal NEP (R2) as early as GW5.5 (Figure 27B), and its fibers penetrate it by GW6.5 (Figure 29A). This, of course, does not mean that these afferents and efferents are functional in the sense that they convey sensory messages and trigger motor commands. There is considerable scientific and public interest in the prenatal development of the human brain in relation to the mental status of the embryo and fetus. Studies in the first half of the 20th century with aborted fetuses have indicated that embryos of about 20 to 21 mm CR length (corresponding to the GW7.5 specimens illustrated in Volume 4 of this Atlas: Bayer and Altman, 2006) begin to reliably respond to tactile stimulation with holokinetic (“total pattern”) body movements (Fitzgerald and Windle, 1942; Hooker, 1942). In GW10 fetuses (CR 48.5 mm) ideokinetic or isolated movements were also elicited, such as partial closure of the fingers (though not effective grasping) when the palm of the hand is stimulated (Humphrey, 1964). The more recent introduction of ultrasonic recording techniques has permitted the observation of the emergence of “spontaneous” fetal behavior in normal embryos and fetuses in utero. A pioneering study (de Vries, 1982, 1985) showed that the holokinetic “startle” response emerges as early as GW6, and isolated arm and leg movements emerge by GW7. These embryos would represent some of the oldest specimens presented in this volume. According to a more recent study with improved ultrasonic recording methods (Kurjak et al., 2005), isolated arm and leg movements increase in frequency during the late first trimester but head turning, and the hand contacting the head, do not occur with high frequency until the second trimester. Are the late-embryonic and early-fetal movements reflex reactions mediated by lower-level spinal cord and brain stem mechanisms, or are they budding voluntary activities carried out under higher-level cortical guidance, by mental processes, such as feelings and emotions, perception and volition? We have raised these questions earlier in the context of our study of the development of the sensorimo-

tor circuitry of the human spinal cord (Altman and Bayer, 2001). The substrate of the sensorimotor reflex arc in the spinal cord begins to form between GW7 and GW8 because the collateral branches of dorsal root afferents reach the ventral horn motor neurons during this period. Since the earliest corticospinal tract fibers do not reach the spinal cord until about GW19, we proposed that the isolated limb movements displayed by embryos of that age must be reflex reactions rather than voluntary activities. We can expand on this inference on the basis of the morphogenetic evidence presented in Volume 4 of this Atlas (Bayer and Altman, 2006) and in the present volume by stating that it is most unlikely that first-trimester embryos can experience cortically mediated mental processes. Cortically mediated sentient responses to somatosensory stimulation, which could be the source of internally generated sensations or perceptions in utero, presumes the operation of the following hierarchically arranged neural mechanisms: conduction of nerve impulses through the dorsal funiculus and through a chain of relay neurons and afferents in the dorsal column nuclei of the medulla and the medial lemniscus; synaptic maturation of the somatosensory relay nuclei of the thalamus, the site where the thalamocortical fibers terminate and afferents of the somatosensory cortex originate; and finally, the settling of late-generated cortical neurons in the cortical plate and their synaptic maturation in the cortical gray matter. While the relay neurons of the dorsal column nuclei and some of the thalamic neurons are generated during the early first trimester (we have no information when synapses are beginning to form here), the internal capsule that contains the ascending thalamocortical fibers to the cortex does not begin to form until GW8. It is at this age that, passing through the internal capsule, the earliest somatosensory fibers approach the base of the formative cerebral cortex. These fibers begin to penetrate the sojourn zone of the stratified transitional field by GW9 and that process continues through GW11. It is probable (but this needs to be experimentally verified) that the neurons of layer IV of the cortex, the principal target of thalamocortical fibers, are still in the stratified transitional field during that period and that synaptic connections in the formative gray matter, the cortical plate, are yet to develop. If this is correct, there is no functional connection between the thalamus and cells of the cortical plate during the first trimester of embryonic and fetal development and, therefore, the neural substrate for cortically mediated sentient responses to somatosensory stimuli is still missing during this period. It is possible, but this needs to be investigated morphologically and physiologically, that the neural mechanisms of rudimentary sentience begin to mature slowly some time during the second trimester.

490

APPENDIX APPENDIX

Timespans Timespansof ofNeurogenesis Neurogenesis A neuron is born when a proliferating neurogenic precursor cell gives rise to a postmitotic cell that displays cytological features or expresses molecular markers of a young neuron. While many current studies rely on molecular markers as the criterion of neurogenesis, we have used 3 H-thymidine autoradiography to determine the cessation of mitotic division and the birthdays of different neuronal populations. To label proliferating cells in the embryonic nervous system, we injected pregnant rats on two consecutive days with 3H-thymidine. To label proliferating cells in the infant, juvenile, and adult nervous systems, we injected 3 H-thymidine on two to four consecutive days. The embryonic, infant, and juvenile animals survived to 60 postnatal days; by that time neurons are settled in the parenchyma and can be easily identified. The changes in the percentage of labeled and unlabeled neurons over a series of injection groups allowed us to accurately calculate the proportion of neurons generated on a single day in a given population. But how are we to date neurogenesis in the human nervous system? No experimental manipulation can be done. What can be done, however, is to apply the experimentally obtained data in rats to humans by matching their morphological appearance at different stages of development. During this matching procedure, we have found that morphological maturation of the spinal cord and brainstem are very similar in sequence. The exact chronology is different because days in rats translate to several days or a week in human development (Bayer et al., 1993). We have re-examined this chronological relationship in the new material presented in this Atlas. In the spinal cord (Altman and Bayer, 2001; Volume 1 of this Atlas; Bayer and Altman, 2002) the following developmental sequences can be matched between embryonic days (E) in rats and gestational weeks (GW) in humans: (i) expansion of stockbuilding NEP cells without differentiating neurons [E12=GW3.2], (ii) early motor neuron differentiation [E13=GW4.5], (iii) entry of dorsal root fibers into the spinal cord [E14=GW5.5], (iv) emergence of the dorsal root bifurcation zone [E16=GW7.0], and (v) formation of the dorsal funiculus [E18=GW8.5]. We have made similar comparisons in the rhombencephalon, mesencephalon, diencephalon, and telencephalon. For instance, the comparison of whole brain development in sagittal sections of rats and humans, (Figure 17A, B) suggests a chronological equivalence of E18=GW9.0, and the comparison of diencephalic and telencephalic development in coronal

TABLE 1

Developmental equivalance between rat and human CNS Rat

Human

(embryonic day)

(GW range)

11

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The Human Brain During the Late First Trimester (Atlas of Human Central Nervous System Development, Volume 4)

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Atlas of the Human Brain, Third Edition

Atlas of the Human Brain, 3rd Edition

The Human Nervous System, Second Edition

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