The Fine Structure of the Nervous System

The Fine Structure of the Nervous System

Neurons and Their Supporting Cells

Third Edition

ALAN PETERS

Waterhouse Professor of Anatomy, Department of Anatomy and Neurobiology Boston University School of Medicine, Boston, Massachusetts

SANFORD L. PALAY

Bullard Professor of Neuroanatomy, Emeritus Harvard Medical School, Boston, Massachusetts

HENRY DBF. WEBSTER

Chief, Laboratory of Experimental Neuropathology National Institute of Neurological Diseases and Stroke, Bethesda, Maryland

New York Oxford OXFORD UNIVERSITY PRESS 1991

Oxford University Press Oxford New York Toronto Delhi Bombay Calcutta Madras Karachi Petaling Jaya Singapore Hong Kong Tokyo Nairobi Dar es Salaam Cape Town Melbourne Auckland and associated companies in Berlin Ibadan

Copyright © 1970, 1976, 1991 by Alan Peters

Copyrighted under the International Copyright Union.

Published by Oxford University Press, Inc., 200 Madison Avenue, New York, New York 10016 Oxford is a registered trademark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior permission of Oxford University Press.

Library of Congress Cataloging-in-Publication Data Peters, Alan, 1929- The fine structure of the nervous system : neurons and their supporting cells Alan Peters, Sanford L. Palay, Henry deF. Webster. —3rd ed. p. cm. Includes bibliographical references. Includes index. ISBN 0-19-506571-9 1. Nervous system—Ultrastructure. I. Palay, Sanford L. II. Webster, Henry deF. III. Title. [DNLM: 1. Nervous System—ultrastructure. WL 101 P481f] QM575.P45 1990 611'.8—dc20 DNLM/DLC for Library of Congress 90-14201

Various aspects of nerve cells in light microscopic preparations. A. A small pyramidal cell from the visual cortex, Golgi method.

The axon (a) descends from the cell body. B. A small neuron in the dentate nucleus of the cerebellum, Golgi

method. The axon (a) is represented only by its initial segment. c. Protoplasmic (velate) astrocyte in the gray matter, Golgi method. D. Oligodendrocyte in the white matter, Golgi method. E. A motor neuron in the spinal cord showing the Golgi apparatus,

osmium tetroxide impregnation. F. A motor neuron in the abducens nucleus showing the distribution

of mitochondria, Altmann-Kull method. G. A motor neuron in the spinal cord showing the distribution

of neurofibrils, Cajal's silver stain. H. A motor neuron in the abducens nucleus showing the disposition

of the Nissl bodies, thionin. . i. A dorsal root ganglion cell, showing the axon coiled about the*

perikaryon and dividing into central and peripheral fibers, j. A myelinated peripheral nerve fiber, showing a node of Ranvier,

Schmidt-Lanterman clefts, Schwann cell nucleus, and neurofibrils.

9 8 7 6 5 4 3 2 Printed in the United States of America on acid-free paper

Preface

We wish to thank our many friends and colleagues who encouraged us to un-dertake a third edition of this book on the fine structure of the nervous system. This revision, like the previous editions (1970 and 1976), aims "to present in words and pictures an account of the salient features of mammalian neurons and neuroglial cells." We have thoroughly revised the text in order to bring it up to date, and we have exchanged many of the original micrographs for ones that we believe better show the characteristics of various structures. Through the generosity of our colleagues, we have been able to add new freeze-fractured material and some deep-etched preparations, as well as examples of various labeling techniques. Consequently, the number of figures has increased from 118 to 137, and 51 of them are new illustrations.

Since the last edition was published there has been not only an information explosion in neuroscience, but also a notable improvement in microtomes and electron microscopes, so that the production of good electron micrographs poses less of a challenge than it did even a decade ago. At the same time, however, some of the "art" of electron microscopy has been lost. In the 1960s and early 1970s, when the technical demands of electron microscopy were greater, inves-tigators devoted themselves wholeheartedly to acquiring the skills necessary for producing electron micrographs that were both informative and esthetically at-tractive. Sharp and clean images of well-fixed material were the aims of every cytologist. Considerable effort was expended in the pursuit of the most complete rendition of protoplasmic structure possible. Such images permitted neurocytol-ogists to distinguish and describe all the components of the complex tissue that the brain of any animal contains. Today, it is taken for granted that any study that requires them can be illustrated with electron micrographs. But with the increasing facility with the elementary techniques has come a decline in the ex-acting criteria both for acceptable electron micrography and for credible inter-pretation of the profiles displayed within them. Good examples of these changes in standards can be found in the identification of synaptic junctions in tissues taken from tracing experiments or from immunocytochemical studies. While this

vii

decline in standards is regrettable, it reflects the fact that electron microscopy of the nervous system has passed the classic stage of exploration. Electron micro-scopy is now being used to examine specific issues, such as the interconnections among neurons and the locations of specific proteins or neuroactive substances. Fifteen years ago we were using degeneration techniques and tracers, such as horseradish peroxidase or radioactively labeled amino acids, in order to under-stand how the nervous system is constructed. Although important information was obtained through the use of these methods and they are still useful, their place at the forefront has largely been ceded to intracellular filling techniques and combined Golgi-electron microscopy. But the modern explosion in the neu-romorphological sciences has been brought about by the use of antibodies to identify the chemical signatures of neural pathways and individual neurons and synaptic terminals. For all of these new approaches the appreciation of fine structure is more pertinent than ever.

We have rewritten this book in the light of the information obtained through the use of these newer methods because they have led to a much better under-standing of the relationships between neuronal circuits and their functions. Con-sequently we have extensively revised all of the chapters and added many new references to the bibliography. We have, however, retained those references that reflect the foundations upon which our new information is based. A reader fa-miliar with the previous edition of this book will certainly recognize paragraphs and descriptions that have not changed appreciably because no significant new knowledge has come to our notice in that area. Other chapters, such as the chapters on axons, synapses, sheaths, and the neuropil, have been almost en-tirely rewritten. In the chapter on the neuropil we have tried to show the pos-sibilities and limitations of the various techniques, so that this chapter has be-come a vehicle for giving an account of the methods available. To a large extent this strategy has allowed us to eliminate details of techniques from the other chapters.

We hope that in this version of the book we have succeeded in correlating structure and function and in providing a reference source of electron micro-graphs and literature, in which both experienced neuroscientists and students interested in the fine structure of the nervous system can find information be-yond the scope of their immediate interests.

Although most of the illustrations come from our own collections, we have relied on the generosity of many colleagues for illustrations of structures and techniques that we have not explored ourselves. We gratefully acknowledge the contributions of figures from J. Anders, D. J. Allen, Dennis Bray, Milton Bright-man, Mary B. Bunge, Victoria Chan-Palay, M. W. Cloyd, Edward V. Famig-lietti, Martin L. Feldman, James E. Hamos, C. K. Henrikson, John E. Heuser, J. Hirokawa, James Kerns, Frank N. Low, Douglas L. Meinecke, Enrico Mug-naini, Elio Raviola, Thomas S. Reese, Bruce Schnapp, Constantino Sotelo, Deb-orah W. Vaughan, James E. Vaughn, Bruce W. Warr, and Raymond B. Wuer-ker.

We are also grateful to Janet Harry, Mary Alba, Lilian Galloway and Joyce Resil for typing the several versions of the manuscript and references, and to Katherine Harriman, Karen Josephson and Claire Sethares for their expert tech-nical assistance. In addition we wish to thank Dr. R. Hammer and Dr. V. J.

viii PREFACE

DeFeo for providing facilities for one of us (A.P.) to enjoy a session of quiet writing at the University of Hawaii, and Dr. P. Hashimoto of Osaka University for providing facilities for another of us (S.L.P.) during an extended visit.

By no means of least importance, we wish to pay tribute to our wives. With-out their patience, understanding and support, we could not have completed this revision.

Boston, Massachusetts Alan Peters Concord, Massachusetts Sanford L. Palay Bethesda, Maryland Henry deF. Webster

February 1990

PREFACE ix

Contents

List of Illustrations, xv

1 General Morphology of the Neuron, 3

2 The Neuronal Cell Body, 14

THE PERIKARYON, 14 The Nissl Substance, 14 The Agranular Reticulum, 22 The Golgi Apparatus, 26 Multivesicular Bodies, 33 Lysosomes, 33 Peroxisomes, 34 Lipofuscin Granules, 34 Mitochondria, 38 Microtubules and Neurofilaments, 40 Cilia and Centrioles, 41 Cytoplasmic Inclusions, 42

THE NUCLEUS, 48 General Morphology, 48 The Nuclear Envelope, 52 The Karyoplasm, 58 The Nucleolus, 59 Nuclear Inclusions, 60

THE PLASMA MEMBRANE, 64

3 Dendrites, 70 GENERAL MORPHOLOGY, 70 THE CYTOPLASM OF DENDRITES, 76 THE DENDRITIC SPINES, 82 MYELINATED DENDRITES, 96 GROWING TIPS OF DENDRITES, 98

xi

4 The Axon, 101 AXON HILLOCK AND INITIAL AXON SEGMENT, 101 THE AXON BEYOND THE INITIAL SEGMENT, 108

Neurofilaments and Microtubules, 110 Membranous Components, 119 Cytoskeleton, 122 The Axonal Membrane, 124

AXOPLASMIC FLOW, 126 THE AXON GROWTH CONE, 132 THE IDENTIFICATION OF SMALL AXONS AND DENDRITES, 137

5 Synapses, 138 THE NEUROMUSCULAR SYNAPSE, 138 INTERNEURONAL CHEMICAL SYNAPSES, 147

The Synaptic Junction, 150 The Presynaptic Grid, 154 The Synaptic Cleft, 159 Potsysnaptic Densities, 160 Freeze-Cleavage, 166

Nonsynaptic Junctions Between Neurons, 168 Synaptic Vesicles With Clear Centers, 169

Shapes and Sizes of Vesicles, 169 Correlation Between Vesicle Shape and Function of Chemical Synapses, 176

Granular Vesicles, 178 Neurosecretory Vesicles, 184 Other Presynaptic Organelles, 186 Other Postsynaptic Organelles, 188 Synaptic Relations, 190

Axo-Dendritic Synapses, 190 Axo-Somatic Synapses, 191 Axo-Axonal Synapses, 192 Dendro-Dendritic Synapses, 195 Somato-Dendritic, Dendro-Somatic and Somato-Somatic Synapses, 196 Somato-Axonic Synapses, 198 Dendro-Axonic Synapses, 198 Synaptic Glomeruli, 199

ELECTROTONIC SYNAPSES, 203 MIXED SYNAPSES, 207 "SYNAPSES" INVOLVING NEUROGLIAL CELLS, 210

6 The Cellular Sheaths of Neurons, 212 THE SHEATHS OF UNMYELINATED GANGLION CELLS, 213 THE SHEATHS OF UNMYELINATED NERVE FIBERS, 218 THE SHEATHS OF MYELINATED FIBERS, 222

Internodal Peripheral Myelin, 224 The Formation of the Peripheral Myelin Sheath, 226 Internodal Central Myelin, 232 The Formation of the Central Myelin Sheath, 234 Identification of the Myelin-forming Cell of the Central Nervous System, 242 The Mechanism of Myelin Formation, 246 The Node of Ranvier, 250 The Schmidt-Lanterman Incisures, 261

xii CONTENTS

The Differences Between Peripheral and Central Myelin Sheaths, 262 The Proximity of Adjacent Sheaths, 262 The Thickness of Myelin Lamellae, 263 The Radial Component of the Central Sheath, 264

THE SHEATHS OF MYELINATED GANGLION CELLS, 265 MYELIN SHEATHS OF DENDRITES IN THE CENTRAL NERVOUS SYSTEM, 266 FUNCTIONS OF SATELLITE AND SCHWANN CELLS, 266

Early Development, 266 Axon Ensheathment and Myelin Formation, 267 Biochemical Relationships, 269 Breakdown of Myelin, 271 Other Functions, 272

7 The Neuroglial Cells, 273 THE DEVELOPMENT OF NEUROGLIA, 274 ASTROCYTES, 276

Fibrous Astrocytes, 277 Protoplasmic Astrocytes, 281 Functions of Astrocytes, 284 Structural Support, 284 Guidance for Neuroblast Migration and Axon Growth, 286 Graft Survival and Function, 288 Isolation of Receptive and Nodal Surfaces of Neurons, 288 Interactions with Oligodendroglia: Role in Myelination, 290 Blood-Brain Barrier, 290 Interactions with the Immune System, 293 Repair, 294

OLIGODENDROCYTES, 295 General Morphology, 295 Functions of Oligodendrocytes, 298

NEUROGLIAL CELLS INTERMEDIATE BETWEEN ASTROCYTES AND OLIGODENDROCYTES, 302 MICROGLIA, 304

General Morphology, 306 Functions, 308 Discussion, 308

8 The Ependyma, 312 THE MORPHOLOGY OF EPENDYMAL CELLS, 313 THE MORPHOLOGY OF TANYCYTES, 318 INTRAVENTRICULAR NERVE ENDINGS, 322 THE SUBEPENDYMA, 324 FUNCTIONS OF CELLS IN THE EPENDYMA, 325

Movements of Cerebrospinal Fluid, 325 Capture of Materials Present in the Cerebrospinal Fluid, 325 Proliferation, 325 Support, 326 Sensory Function, 326 Secretion, 326 Transport of Substances, 327

CONTENTS xiii

9 Choroid Plexus, 328 THE CHOROIDAL EPITHELIUM, 330 THE VASCULARIZED CONNECTIVE TISSUE CORE, 336 FUNCTIONS OF THE CHOROID PLEXUS, 338

10 Blood Vessels, 344 CAPILLARIES, 344 ARTERIES AND ARTERIOLES, 350 VEINS, 354

11 The Neuropil, 356 THE IDENTIFICATION OF PROFILES IN THE NEUROPIL, 356 THE ORGANIZATION OF THE NEUROPIL AND SYNAPTIC CONNECTIONS, 364

Golgi—Electron Microscope Technique, 366 Intracellularly Injected Markers, 368 Reconstruction of Neurons and Their Processes, 370 Experimental Degeneration, 372 Intracellular Transport of Radioisotopes, 375 Antibodies to Neurotransmitters, 375 Antibodies to Neuropeptides, 380 Techniques Using Two Antibodies, 381 Combined Techniques, 382

12 Connective Tissue Sheaths of Peripheral Nerves, 384 EPINEURIUM, 384 PERINEURIUM, 385 ENDONEURIUM, 388 FUNCTIONS OF CONNECTIVE TISSUE SHEATHS, 392

13 The Meninges, 395 DURA MATER, 396 ARACHNOID MATER, 398 PIA MATER, 400 ENTRY OF PERIPHERAL NERVES INTO THE CENTRAL NERVOUS SYSTEM, 402 ARACHNOID VILLI, 404

References, 407 Index, 487

xiv CONTENTS

List of Illustrations

1-1 The Neuronal Cell Body, 11 2-1 The Cell Body of a Pyramidal Cell, 17 2-2 A Purkinje Cell, 19 2-3 Pyramidal Neuron, 21 2-4 Granule Cells of the Cerebellum, 23 2-5 The Cytoplasm of a Purkinje Cell, 25 2-6 The Cytoplasm of a Dorsal Root Ganglion Cell, 29 2-7 The Cytoplasm of a Dorsal Root Ganglion Cell, 31 2-8 Nissl Bodies in an Anterior Horn Cell, 35 2-9 The Golgi Apparatus and the Nissl Substance of a Purkinje Cell, 37 2-10 Golgi Apparatus of the Purkinje Cell, 39 2-11 Two Views of the Golgi Apparatus in a Freeze-Fractured Preparation, 43 2-12 Golgi Apparatus, Lysosomes, Nematosomes, and Fibrillary Inclusions, 45 2-13 Lipofuscin Granules, Cilia, and Centrioles, 47 2-14 Laminated Inclusion Body, 49 2-15 The Nuclear Envelope, Nissl Bodies, and Golgi Apparatus, 55 2-16 Nuclear Pores, 57 2-17 The Nucleolus, 61 2-18 Intranuclear Inclusions, 63 2-19 Diagram of Freeze Fracturing, 65 2-20 The Edge of a Purkinje Cell, Freeze-Fractured Preparation, 67 3-1 Pyramidal Neuron in Cerebral Cortex, 73 3-2 The Apical Dendrites of Pyramidal Cells, 75 3-3 Dendrite of a Purkinje Cell, 79 3-4 Dendrite of a Purkinje Cell, 81 3-5 Dendrites in Longitudinal and Transverse Section, 83 3-6 Dendrites in the Neuropil of the Anterior Horn: Transverse Section, 85 3-7 Dendrites in the Neuropil of the Cerebral Cortex, 87 3-8 Dendrites in Cerebellar and Cerebral Cortex, 89 3-9 A Spiny Branchlet of a Purkinje Cell Dendrite, 91

xv

3-10 Olfactory Bulb, 93 3-11 Myelinated Dendrite in Olfactory Bulb, 97 3-12 Dendrite Growth Cones, 99 4-1 Axon Hillock and the Initial Axon Segment, 103 4-2 Axon Hillock and the Initial Axon Segment, 105 4-3 The Initial Axon Segment, Longitudinal Section, 107 4-4 The Initial Segment, Transverse Section, 109 4-5 The Initial Axon Segment and the Node of Ranvier Compared, 111 4-6 Axon Hillock and Initial Segment of a Trigeminal Ganglion Cell, 113 4-7 The Initial Segment of a Trigeminal Ganglion Cell, 115 4-8 Microtubules, Neurofilaments, and Neuroglial Filaments, 117 4-9 Axoplasmic Organelles, 121 4-10 Quick-frozen and Deep-etched Axoplasm, 123 4-11 Quick-frozen and Deep-etched Axoplasm, 125 4-12 Growth Cone from a Sympathetic Neuron in Tissue Culture, 129 4-13 Growth Cone from a Sympathetic Neuron in Tissue Culture, 131 4-14 Small Axons in the Molecular Layer of the Cerebellum, 133 4-15 Unmyelinated Axons Entering the Olfactory Bulb, 135 5-1 Motor End Plate, 141 5-2 Motor End Plate, 143 5-3 Freeze-Fractured Motor End Plates to Show Vesicle Release, 145 5-4 Puncta Adhaerentia, 149 5-5 Axon Terminal Emerging from the Myelin Sheath, 151 5-6 Synapses in the Cerebral Cortex, 153 5-7 Asymmetric Synapses, Cerebral Cortex, 155 5-8 Presynaptic Grid, 157 5-9 Synapses in the Cerebellum, 161 5-10 The Synaptic Junction Between an Axon and a Dendritic Thorn, 163 5-11 Asymmetric and Symmetric Synapses, 165 5-12 Synapses in the Anterior Horn of Spinal Cord, 167 5-13 Anterior Horn of Spinal Cord, 171 5-14 The Glomerulus, Cerebellar Cortex, 173 5-15 The Presynaptic Membrane, P face, 175 5-16 The Presynaptic Membrane, E face, 181 5-17 A Variety of Synapses, 183 5-18 Axo-axonic Synapse and Dense-cored Vesicles, 185 5-19 Dendro-dendritic and Somato-dendritic Synapses, 189 5-20 The Glomerulus, Lateral Geniculate Nucleus, 197 5-21 Electrotonic Synapses, 205 5-22 A Mixed Synapse, 209 6-1 The Sheath Surrounding a Dorsal Root Ganglion Cell, 215 6-2 The Sheath Surrounding a Trigeminal Ganglion Cell, 217 6-3 Unmyelinated Axons, Adult Peripheral Nerve, 219 6-4 Unmyelinated Axons, Adult Peripheral Nerve, 221 6-5 Myelinated Axon, Adult Peripheral Nerve, 227

xvi ILLUSTRATIONS

6-6 Developing Schwann Cell Sheaths, 229 6-7 Developing Schwann Cell Sheaths, Later Stage, 231 6-8 Diagrammatic Representation of the Formation of Peripheral Myelin Sheaths, 233 6-9 Myelinated Nerve Fibers, Central Nervous System, 235 6-10 Myelin Sheaths: Central Nervous System, 237 6-11 Developing Myelin Sheaths, Central Nervous System, 239 6-12 Developing Myelin Sheaths, Central Nervous System, 241 6-13 Diagrammatic Representation of the Formation of Myelin in the Central Nervous System, 243 6-14 The Myelin Forming Cell, Central Nervous System, 245 6-15 The Node of Ranvier, Peripheral Nervous System, 249 6-16 The Node of Ranvier, Central Nervous System, 251 6-17 The Node and the Paranode, Central Nervous System, 253 6-18 Freeze-Fractured Myelin Sheaths, 255 6-19 Freeze-Fractured Myelin Sheaths, 257 6-20 Freeze-Fractured Myelin Sheaths, 259 6-21 Diagram of Membrane Particle Distribution at the Paranode, 260 7-1 Fibrous Astrocytes, 279 7-2 Protoplasmic Astrocytes, 283 7-3 Protoplasmic Astrocytes, 285 7-4 Protoplasmic Astrocyte, 287 7-5 Glial Limiting Membrane; Cerebral Cortex, 289 7-6 Orthogonal Assemblies and Gap Junctions of Astrocytes in Freeze-Fracture Preparations, 291 7-7 Perineuronal Oligodendrocytes, 297 7-8 An Oligodendrocyte, 299 7-9 Interfascicular Oligodendrocyte, 301 7-10 A Perineuronal Microglial Cell, 303 7-11 A Microglial Cell in a Senile Plaque, 307 8-1 The Ependyma, 315 8-2 Ependymal Surface, 317 8-3 The Cilia of Ependymal Cells, 319 8-4 Ependymal Cell Cytoplasm, 321 8-5 Ependymal Cell Junctions, 323 9-1 Scanning Electron Micrograph of the Choroid Plexus, 329 9-2 Epithelium and Stroma of the Choroid Plexus, 331 9-3 The Choroid Plexus, 333 9-4 Choroid Plexus, Intercellular Junctions, 335 9-5 Choroid Plexus, Intercellular Junctions, 337 9-6 Choroid Plexus, Surface Structures, 339 9-7 The Basal Ends of Choroidal Cells, 341 9-8 Kolmer Cells, 343 10-1 Capillary and Pericyte, 347 10-2 Capillaries, 349 10-3 Small Blood Vessel, 351 10-4 Intracerebral Arterioles, 353 10-5 An Arteriole, 355

ILLUSTRATIONS xvii

11-1 The Neuropil, Anterior Horn, Spinal Cord, 359 11-2 The Neuropil, Cerebellar Cortex, 361 11-3 The Neuropil, Cerebral Cortex, 363 11-4 Lateral Geniculate Body Glomerulus, 365 11-5 Degenerating Boutons, 367 11-6 Filamentous Degeneration and Horseradish Peroxidase-labeled Neurons, 369 11-7 Golgi-Electron Microscope Technique, 371 11-8 Intracellular Horseradish Peroxidase Injection, 373 11-9 Glutamic Acid Decarboxylase Immunoreactive Axon Terminals, 377 11-10 Vasoactive Intestinal Polypeptide in the Cerebral Cortex, 379 12-1 Connective Tissue Sheaths of Nerves, 387 12-2 Epineurial and Perineurial Sheaths, 389 12-3 Perineurium and Endoneurium of Peripheral Nerve, 391 13-1 Meninges by Scanning Electron Microscopy, 397 13-2 Dura Mater, 399 13-3 Arachnoid Mater, 401 13-4 Pia Mater and Glia Limitans, 403

xviii ILLUSTRATIONS

Dedicated to the Memory of

Jan Evangelista Purkinje, 1787-1869

Louis-Antoine Ranvier, 1835-1922

Camillo Golgi, 1843-1926

Santiago Ramon у Cajal, 1852-1934

The Fine Structure of the Nervous System

1

General Morphology of the Neuron

Anyone who has studied the early history of cy-tology cannot fail to be impressed by the slow development of the concept of the nerve cell. Most types of cells do not have a history. Once the idea was grasped, in the theory of Schleiden (1838) and Schwann (1839), that cells are the architec-tonic units of living things, it was fairly quick work to recognize them in the various tissues and to proceed to the study of their contents, their interrelations, and their functions. But the nerve cell was more perplexing. It occasioned so much difficulty for its students that almost a century passed before they could agree upon its shape. At first it was thought to be an independent globular corpuscle suspended among nerve fibers, which looped and coiled about it and which it somehow nourished (Valentine, 1836). Later, when the con-tinuity between the perikaryon and the nerve fi-bers was finally established (Remak, 1838, 1841; Helmholtz, 1842; Hannover, 1844; Kolliker, 1844; Bidder, 1847; Wagner, 1847), then the nerve cell appeared to have no definite boundaries and seemed endless. Except for the fibers attached to organs in the periphery, the processes of all nerve cells seemed to be equivalent and to be confluent with one another. The nerve cells seemed to be only nodal points in an enormously intricate reticulum pervading the nervous system (Gerlach, 1858, 1872). It appeared that the cell theory did not really apply to the nervous system; one had rather to speak of cell territories or spheres of influence surrounding nucleated centers.

It seems clear that one of the major obstacles to the appreciation of the cellular nature of the

nervous system lay in the shape of the nerve cell itself and to some extent in its size. The medusa-like nerve cell, with its corona of seemingly endless processes, was bizarre. Other cells had relatively simple shapes—globular, cylindrical, squamous, fusiform, and so on. Some fitted one into the other like pieces of a jigsaw puzzle to form an epithe-lium; others lay free and definable in the tissue fluids. Many, such as cartilage or certain epithelial cells, were clearly circumscribed by walls. Only pigment cells, astrocytes, myoepithelial cells, and a few others had shapes even roughly approxi-mating those of nerve cells. But aside from the fact that some of these examples were unknown in the early days, such cells could be easily encom-passed in a single field or at least in a single preparation under the microscope. The multipolar nerve cell, however, with its meter-long axon did not fit into a single section and could not be easily plucked from its context or distinguished from its neighbors by the methods used for other cells. New methods had to be developed. And so a true cell theory of the nervous system did not emerge until the discovery and exploitation of special techniques that had the merit of bringing into view entire nerve cells as if dissected or isolated from the central nervous system.

Actually, the first successful method was mi-crodissection of whole nerve cells from hardened specimens of brain and spinal cord. On the basis of experience with such isolated cells, Deiters (1865) was able to distinguish between the numerous branching processes that we now call dendrites and the single process that slips into a myelin

3

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