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THE CELL
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Library of Congress Cataloging in Publication Data

Fawcett, Don Wayne, 1917-

The cell.
Edition of 1966 published under title: An atlas of
fine structure.
DON W . FAWCETT. M.D. Includes bibliographical references.
Hersey Professor of Anatomy 1. Cytology -Atlases. 2. Ultrastructure (Biology)-
Harvard Medical School Atlases. I. Title. [DNLM: 1. Cells- Ultrastructure-
Atlases. 2. Cells- Physiology - Atlases. QH582 F278c]
QH582.F38 1981 591.8'7 80-50297
ISBN 0-7216-3584-9

Listed here is the latest translated edition of this book together

with the language of the translation and the publisher.

German (1st Edition)- Urban and Schwarzenberg, Munich, Germany

The Cell ISBN 0-7216-3584-9

1981 by W. B. Saunders Company. Copyright 1966 by W. B. Saunders Company. Copyright under

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 CONTRIBUTORS OF iv

Dr. Jeffrey Pudney

CONTRIBUTORS OF PHOTOMICROGRAPHS

Dr. Manfred Schliwa Dr. John Tersakis

ELECTRON MICROGRAPHS Dr. Eli0 Raviola
Dr. Giuseppina Raviola
Dr.
Dr.
Nicholas Severs
Emma Shelton
Dr. Guy de Th6
Dr. Lewis Tilney
Dr. Janardan Reddy Dr. Nicholai Simionescu Dr. Greta Tyson
Dr. Thomas Reese Dr. David Smith Dr. Wayne Vogl
Dr. Jean Revel Dr. Andrew Somlyo Dr. Fred Warner
Dr. Hans Ris Dr. Sergei Sorokin Dr. Melvyn Weinstock
Dr. Joel Rosenbaum Dr. Robert Specian Dr. Richard Wood
Dr. Evans Roth Dr. Andrew Staehelin Dr. Raymond Wuerker
Dr. Thomas Roth Dr. Fumi Suzuki Dr. Eichi Yamada
Dr. John Albright Dr. Marilyn Farquhar Dr. Shuichi Karasaki Dr. Kogaku Saito Dr. Hewson Swift
Dr. David Albertini Dr. Don Fawcett Dr. Morris Karnovsky Dr. Peter Satir Dr. George Szabo
Dr. Nancy Alexander Dr. Richard Folliot Dr. Richard Kessel
Dr. Winston Anderson Dr. Michael Forbes Dr. Toichiro Kuwabara
Dr. Jacques Auber Dr. Werner Franke Dr. Ulrich Laemmli
Dr. Baccio Baccetti Dr. Daniel Friend Dr. Nancy Lane
Dr. Michael Barrett Dr. Keigi Fujiwara Dr. Elias Lazarides
Dr. Dorothy Bainton Dr. Penelope Gaddum-Rosse Dr. Gordon Leedale
Dr. David Begg Dr. Joseph Gall Dr. Arthur Like
Dr. Olaf Behnke Dr. Lawrence Gerace Dr. Richard Linck
Dr. Michael Berns Dr. Ian Gibbon Dr. John Long
Dr. Lester Binder Dr. Norton Gilula Dr. Linda Malick
Dr. K. Blinzinger Dr. Jean Gouranton Dr. William Massover
Dr. Gunter Blobel Dr. Kiyoshi Hama Dr. A. Gideon Matoltsy
Dr. Robert Bolender Dr. Joseph Harb Dr. Scott McNutt
Dr. Aiden Breathnach Dr. Etienne de Harven Dr. Oscar Miller
Dr. Susan Brown Dr. Elizabeth Hay Dr. Mark Mooseker
Dr. Ruth Bulger Dr. Paul Heidger Dr. Enrico Mugnaini
Dr. Breck Byers Dr. Arthur Hertig Dr. Toichiro Nagano
Dr. Hektor Chemes Dr. Marian Hicks Dr. Marian Neutra
Dr. Kent Christensen Dr. Dixon Hingson Dr. Eldon Newcomb
Dr. Eugene Copeland Dr. Anita Hoffer Dr. Ada Olins
Dr. Romano Dallai Dr. Bessie Huang Dr. Gary Olson
Dr. Jacob Davidowitz Dr. Barbara Hull Dr. Jan Orenstein
Dr. Walter Davis Dr. Richard Hynes Dr. George Palade
Dr. Igor Dawid Dr. Atsuchi Ichikawa Dr. Sanford Palay
Dr. Martin Dym Dr. Susumu It0 Dr. James Paulson
Dr. Edward Eddy Dr. Roy Jones Dr. Lee Peachey
Dr. Peter Elias Dr. Arvi Kahri Dr. David Phillips
Dr. A. C. Faberge Dr. Vitauts Kalnins Dr. Dorothy Pitelka
Dr. Dariush Fahimi Dr. Marvin Kalt Dr. Thomas Pollard
Dr. Wolf Fahrenbach Dr. Taku Kanaseki Dr. Keith Porter
.111
..
 PREFACE

PREFACE ably used in combination with biochemical, biophysical, and immunocytochemical

techniques. Its use has become routine and one begins to detect a decline in the number
and quality of published micrographs as other analytical methods increasingly capture
the interest of investigators. Although purely descriptive electron microscopic studies
now yield diminishing returns, a detailed knowledge of the structural organization of
The history of morphological science is in large measure a chronicle of the dis- cells continues to be an indispensable foundation for research on cell biology. In under-
covery of new preparative techniques and the development of more powerful optical taking this second edition I have been motivated by a desire to assemble and make
instruments. In the middle of the 19th century, improvements in the correction of easily accessible to students and teachers some of the best of the many informative
lenses for the light microscope and the introduction of aniline dyes for selective stain- and aesthetically pleasing transmission and scanning electron micrographs that form
ing of tissue components ushered in a period of rapid discovery that laid the founda- the basis of our present understanding of cell structure.
tions of modern histology and histopathology. The decade around the turn of this The historical approach employed in the text may not be welcomed by all. In the
century was a golden period in the history of microscopic anatomy, with the leading competitive arena of biological research today investigators tend to be interested only
laboratories using a great variety of fixatives and combinations of dyes to produce in the current state of knowledge and care little about the steps by which we have
histological preparations of exceptional quality. The literature of that period abounds arrived at our present position. But to those of us who for the past 25 years have been
in classical descriptions of tissue structure illustrated by exquisite lithographs. In the privileged to participate in one of the most exciting and fruitful periods in the long
decades that followed, the tempo of discovery with the light microscope slackened; history of morphology, the young seem to be entering the theater in the middle of an
interest in innovation in microtechnique declined, and specimen preparation narrowed absorbing motion picture without knowing what has gone before. Therefore, in the
to a monotonous routine of paraffin sections stained with hematoxylin and eosin. introduction to each organelle, I have tried to identify, in temporal sequence, a few of
In the middle of the 20th century, the introduction of the electron microscope the major contributors to our present understanding of its structure and function. In
suddenly provided access to a vast area of biological structure that had previously venturing to do this I am cognizant of the hazards inherent in making judgments of
been beyond the reach of the compound microscope. Entirely new methods of speci- priority and significance while many of the dramatis personae are still living. My
men preparation were required to exploit the resolving power of this new instrument. apologies to any who may feel that their work has not received appropriate recognition.
Once again improvement of fixation, staining, and microtomy commanded the atten- It is my hope that for students and young investigators entering the field, this book
tion of the leading laboratories. Study of the substructure of cells was eagerly pursued will provide a useful introduction to the architecture of cells and for teachers of cell
with the same excitement and anticipation that attend the geographical exploration of biology a guide to the literature and a convenient source of illustrative material. The
a new continent. Every organ examined yielded a rich reward of new structural infor- sectional bibliographies include references to many reviews and research papers that
mation. Unfamiliar cell organelles and inclusions and new macromolecular components are not cited in the text. It is believed that these will prove useful to those readers who
of protoplasm were rapidly described and their function almost as quickly established. wish to go into the subject more deeply.
This bountiful harvest of new structural information brought about an unprecedented The omission of magnifications for each of the micrographs will no doubt draw
convergence of the interests of morphologists, physiologists, and biochemists; this some criticism. Their inclusion was impractical since the original negatives often
convergence has culminated in the unified new field of science called cell biology. remained in the hands of the contributing microscopists and micrographs submitted
The first edition of this book (1966) appeared in a period of generous support of were cropped or copies enlarged to achieve pleasing composition and to focus the
science, when scores of laboratories were acquiring electron microscopes and hundreds reader's attention upon the particular organelle under discussion. Absence was con-
of investigators were eagerly turning to this instrument to extend their research to the sidered preferable to inaccuracy in stated magnification. The majority of readers, I
subcellular level. A t that time, an extensive text in this rapidly advancing field would believe, will be interested in form rather than measurement and will not miss this datum.
have been premature, but there did seem to be a need for an atlas of the ultrastructure Assembling these micrographs illustrating the remarkable order and functional
of cells to establish acceptable technical standards of electron microscopy and to design in the structure of cells has been a satisfying experience. I am indebted to more
define and illustrate the cell organelles in a manner that would help novices in the field than a hundred cell biologists in this country and abroad who have generously re-
to interpret their own micrographs. There is reason to believe that the first edition of sponded to my requests for exceptional micrographs. It is a source of pride that nearly
The Cell: An Atlas of Fine Structure fulfilled this limited objective. half of the contributors were students, fellows or colleagues in the Department of
In the 14 years since its publication, dramatic progress has been made in both the Anatomy at Harvard Medical School at some time in the past 20 years. I am grateful
morphological and functional aspects of cell biology. The scanning electron microscope for their stimulation and for their generosity in sharing prints and negatives. It is a
and the freeze-fracturing technique have been added to the armamentarium of the pleasure to express my appreciation for the forbearance of my wife who has had to
miscroscopist, and it seems timely to update the book to incorporate examples of the communicate with me through the door of the darkroom for much of the year while I
application of these newer methods, and to correct earlier interpretations that have not printed the several hundred micrographs; and for the patience of Helen Deacon who
withstood the test of time. The text has been completely rewritten and considerably has typed and retyped the manuscript; for the skill of Peter Ley, who has made many
expanded. Drawings and diagrams have been added as text figures. A few of the copy negatives to gain contrast with minimal loss of detail; and for the artistry of
original transmission electron micrographs to which I have a sentimental attachment Sylvia Collard Keene whose drawings embellish the text. Special thanks go to Elio
have been retained, but the great majority of the micrographs in this edition are new. and Giuseppina Raviola who read the manuscript and offered many constructive
These changes have inevitably added considerably to the length of the book and there- suggestions; and to Albert Meier and the editorial and production staff of the W. B.
fore to its price, but I hope these will be offset to some extent by its greater informa- Saunders Company, the publishers.
tional content. And finally I express my gratitude to the Simon Guggenheim Foundation whose
Twenty years ago, the electron microscope was a solo instrument played by a few commendable policy of encouraging the creativity of the young was relaxed to support
virtuosos. Now it is but one among many valuable research tools, and it is most profit- my efforts during the later stages of preparation of this work.
v
D ON W. FAWCETT
Boston, Massachusetts
 CONTENTS CONTENTS

MITOCHONDRIA ................................................................................. 410

Structure of Mitochondria .......................................................................... 414
Matrix Granules ...................................................................................... 420
Mitochondria1 DNA and RNA ................................................................... 424
Division of Mitochondria ........................................................................... 430
Fusion of Mitochondria ............................................................................. 438
Variations in Internal Structure .................................................................. 442
CELL SURFACE................................................................................... 1 Mitochondria1 Inclusions ........................................................................... 464
Numbers and Distribution ......................................................................... 468
Cell Membrane ........................................................................................ 1
Glycocalyx or Surface Coat ....................................................................... 35 LYSOSOMES ......................................................................................... 487
Basal Lamina .......................................................................................... 45
Multivesicular Bodies ............................................................................... 510
SPECIALIZATIONS O F T H E FREE SURFACE .................................... 65
PEROXISOMES ..................................................................................... 515
Specializations for Surface Amplification...................................................... 68
Relatively Stable Surface Specializations ...................................................... 80
LIPOCHROME PIGMENT .................................................................... 529
Specializations Involved in Endocytosis ....................................................... 92
MELANIN PIGMENT ........................................................................... 537
JUNCTIONAL SPECIALIZATIONS ...................................................... 124
Tight Junction (Zonula Occludens).............................................................. 128 CENTRIOLES ....................................................................................... 551
Adhering Junction (Zonula Adherens).......................................................... 129
Sertoli Cell Junctions ................................................................................ 136 Centriolar Adjunct ................................................................................... 568
Zonula Continua and Septate Junctions of Invertebrates ................................. 148
Desmosomes ........................................................................................... 156 CILIA AND FLAGELLA ...................................................................... 575
Gap Junctions (Nexuses)........................................................................... 169
Intercalated Discs and Gap Junctions of Cardiac Muscle ................................ 187 Matrix Components of Cilia ....................................................................... 588
Aberrant Solitary Cilia .............................................................................. 594
Modified Cilia.......................................................................................... 596
NUCLEUS ............................................................................................ 195 Stereocilia ............................................................................................... 598
Nuclear Size and Shape ............................................................................ 197
Chromatin............................................................................................... 204 SPERM FLAGELLUM .......................................................................... 604
Mitotic Chromosomes ............................................................................... 226
Mammalian Sperm Flagellum ..................................................................... 604
Nucleolus ............................................................................................... 243
Urodele Sperm Flagellum .......................................................................... 619
Nucleolar Envelope .................................................................................. 266
Insect Sperm Flagellum............................................................................. 624
Annulate Lamellae ................................................................................... 292

ENDOPLASMIC RETICULUM ............................................................. 303

CYTOPLASMIC INCLUSIONS ............................................................. 641
Glycogen ................................................................................................ 641
Rough Endoplasmic Reticulum ................................................................... 303
Lipid ...................................................................................................... 655
Smooth Endoplasmic Reticulum ................................................................. 330
Crystalline Inclusions ............................................................................... 668
Sarcoplasmic Reticulum ............................................................................ 353
Secretory Products ................................................................................... 691
Synapses ................................................................................................ 722
GOLGI APPARATUS ............................................................................ 369
Role in Secretion ..................................................................................... 372 CYTOPLASMIC MATRIX AND CYTOSKELETON .............................. 743
Role in Carbohydrate and Glycoprotein Synthesis ......................................... 376
Microtubules ........................................................................................... 743
Contributions to the Cell Membrane............................................................ 406
vii
Cytoplasmic Filaments .............................................................................. 784
 LYSOSOMES

The history of most cell organelles has been early description by microscopists
followed many years later by isolation and biochemical characterization. The lysosome
is an exception in that it originated as a biochemical concept and morphological
identification followed. Among investigators isolating mitochondria and microsomes by
differential centrifugation of cell homogenates, there was some disagreement as to
which fraction contained the enzyme acid phosphatase. Some found it in the mitochon-
drial fraction (Palade, 1951; Schneider and Hogeboom, 1952), while others, using
somewhat lower centrifugal force, found it in their microsome fraction (Novikoff et al.,
1953). This problem was resolved by DeDuve and his associates, who were able to
separate the classical mitochondrial fraction into two subfractions. The lighter (L)
fraction consisted of particles rich in acid phosphatase but lacking the mitochondrial
enzyme cytochrome oxidase (DeDuve et al., 1953). These particles, which also
contained cathepsin, ribonuclease, deoxyribonuclease, and /3-glucuronidase, were
recognized as a new particulate component of cells distinct from mitochondria and were
given the name lysosomes to draw attention to their richness in hydrolytic enzymes
(DeDuve et al., 1955).
Another defining characteristic of lysosomes was the fact that they were imperme-
able to their substrates and were enzymatically active in vitro only after disruption or
treatment with a surface active agent. It was thus inferred that they must be enclosed by
a membrane-like barrier. When centrifugal pellets of fractions enriched in lysosomes
were examined in thin sections with the electron microscope, a high proportion of the
granules were obviously different from mitochondria. They had a dense heterogeneous
content and, as postulated, they were enclosed by a membrane (Novikoff, Beaufay and
DeDuve, 1956).
Fortunately a dependable cytochemical method was available for the lysosomal
enzyme acid phosphatase (Gomori, 1952). Cytochemical observations at the light and
electron microscope level confirmed that the acid phosphatase of liver was not localized
in mitochondria but in membrane-bounded dense bodies in the vicinity of the bile
canaliculi (Novikoff, 1959; Holt, 1959; Barka, 1960). Unlike mitochondria and other cell
organelles which have a consistent, clearly defined and easily recognizable structure,
the granules exhibiting acid phosphatase activity varied in size and were highly
heterogeneous in their internal structure. Some were spherical with a uniform content
of moderate density; others were irregular in outline and contained aggregations of very
dense granules in a less dense matrix. Others contained myelin figures or crystalline
inclusions. It was this extraordinary pleomorphism that had prevented cytologists from
recognizing lysosomes as a distinct entity. This was the first instance of an organelle
that could not be identified with confidence solely by morphological criteria. Bio-
chemical or cytochemical demonstration of acid hydrolase activity was required. By
1962, the number of hydrolytic enzymes identified had increased to 10. It was apparent
that while most of these were present in all lysosomes, their proportions probably
varied considerably from tissue to tissue.
The assembly of multiple acid hydrolases in the same organelle led naturally to the
speculation that lysosomes functioned as an intracellular digestive system and that the
substances degraded were either exogenous - brought into the cell by phagocytosis -
or endogenous, involving the controlled autolytic elimination of other organelles and
inclusions in the course of normal renewal or in response to an altered state of
physiological activity. An essential feature of this concept was the belief that in their
487
488 LYSOSOMES LYSOSOMES 489

lysosomes cells contained the seeds of their own destruction. In normal function, it was their lysosomal enzymes. The secretion of collagenase and other proteases by
assumed that the digestive processes had to be sequestered within membranes to osteoclasts is involved in the resorption of matrix in the normal remodeling of bone.
protect the surrounding cytoplasm, but in pathological conditions autolysis of cells was The binding of complement or antigen-antibody complexes to leucocytes or mac-
a consequence of breakdown of this protective barrier and release of digestive enzymes rophages stimulates their secretion of hydrolases that may destroy basal laminae,
into the cytoplasm (DeDuve, 1963). cartilage matrix, collagen or elastin and result in damage to kidneys, joints, or lungs
Evidence for the physiological degradation of cell components, called autophagy, (Davies and Allison, 1976; Weissmann et al., 1971).
rested upon numerous electron microscopic observations of mitochondria, profiles of
endoplasmic reticulum or aggregations of glycogen particles enclosed in membrane-
limited uutophugic vacuoles (Ashford and Porter, 1962; Novikoff and Essner, 1962). It
is believed that the organelles to be degraded are first surrounded by a membrane of Endoplasmic reticulum Golgi complex
uncertain provenance and that lysosomes then coalesce with the autophagic vacuole so
Primary lysosome (storage
Primary granule)
ly%Sbme
formed, adding their lytic enzymes to its contents. \
The relationship of lysosomes to exogenous material incorporated in heterophugic
vacuoles was demonstrated experimentally in many cell types. In the kidney, intrave-
nously injected peroxidase is taken up by endocytosis in the proximal convoiuted
tubules in phagosomes which have the morphological and biochemical properties of
lysosomes (Strauss, 1959; DeDuve, 1961). The phagocytic Kupffer cells of hepatic
sinusoids are rich in iysosomes and after administration of peroxidase or particulate
markers, these substances are found sequestered in acid phosphatase-positive vacuoles Autophagic vacuoles
or secondary lysosomes (Novikoff et al., 1960). Observations of phagocytosis by living (cytolysosome)

leucocytes provided convincing support for the postulated mechanism involved in

heterophagocytosis. The granules of these cells contain lysosomal hydrolases and is
s)
antibacterial agents. When observed with the light microscope, the granules can be seen
to coalesce with the phagocytic vacuoles, resulting in a progressive degranulation of the
cells (Cohn and Hirsch, 1960; Hirsch, 1962).
Residual body
The morphological heterogeneity of lysosomes is attributable in part to the fact that
they represent various stages in a dynamic process of intracellular digestion. Small Phagocytic vacuole
granules with relatively homogeneous content are called primary lysosomes, an
inactive storage form or ready reserve of hydrolytic enzymes not yet involved in
digestive events. Somewhat larger bodies with a heterogeneous content that may include
recognizable residues of ingested material are heterophagic vacuoles (phagosomes, or Heterophagic vacuole
secondary lysosomes) formed by fusion of primary lysosomes with endocyto- Lipofuscin
pigment granule
\Residual body
(digestive vacuole)

sis vacuoles. As digestion of the contents of the heterophagic vacuoles progresses, they
are reduced in size and become residual bodies filled with granules of varying size and
density in an amorphous matrix. Schematic representation of the role of lysosomes in heterophagy (lower right) and in autophagy
The fate of undigestable residues is not entirely clear. In amoebae they appear to (upper left). Bacteria or other foreign matter may be taken into the cell by phagocytosis. Primary
be eliminated by exocytosis, but unambiguous evidence for such a process in cells of lysosomes fuse with the heterophagic vacuole and their enzymes degrade the foreign matter. Effete
higher forms is lacking. Instead, residual bodies appear to coalesce and become or supernumerary organelles may be enclosed by membranes to form autophagic vacuoles. Subsequent
consolidated into the large, irregularly shaped aggregations of dense material tradition- fusion of primary lysosomes with these vacuoles results in degradation of their contents. End products
of either autophagy or heterophagy can be recognized in cells as residual bodies and lipofuscin
ally designated lipofuscin, or wear-and-tear pigment. pigment. (From G . Bloom and D. W. Fawcett, Textbook of Histology, 10th Ed., W. B. Saunders Co.,
Primary lysosomes are generally believed to be formed in much the same manner 1975.)
as secretory granules, with synthesis of the enzymes in the rough endoplasmic reticulum
and segregation and packaging in the Golgi apparatus. Some investigators, however,
believe they originate in the GERL, a differentiated region of the endoplasmic reticulum
associated with the secretory face of the Golgi.
Some 50 lysosomal enzymes have now been identified. These include several
proteolytic enzymes, giycosidases, nucleases, phosphatases, phospholipases, and
sulfatases. The pH optima of nearly all of these are in the acid range. This battery of
enzymes can break down most constituents of cells into small molecules that can pass
through the lysosome membrane into the cytoplasm. An inherited defect in the
synthesis of one or more of these is the basis for 20 or more rare storage diseases of
children (Neufeld et al., 1975). When a lysosomal enzyme is deficient, its normal
substrate accumulates in large membrane-bounded structures in the cytoplasm which
are, in effect, autophagic vacuoles that are unable to digest their contents.
While the main site of lysosomal function is intracellular, evidence has accumulat-
ed in recent years indicating that under certain conditions phagocytic cells can secrete
I LYSOSOMES

The aggregation of primary lysosomes in the accompanying micrograph shows some

of the variability that makes their microscopic identification difficult. They may be
round, elliptical, or highly irregular in shape. They are invariably limited by a membrane
but their content may be of homogeneous density or highly heterogeneous. Other vari-
ations in their appearance are recorded in the micrographs that follow.

Figure 262. Lysosomes in the supranuclear region of an epithelial cell from the epididymis of the Figure 262
bandicoot, Parameles nasuta.

49 1
 LYSOSOMES

Another example of lysosomes clustered in the Golgi region. On these the limiting
membrane, which is one of their defining characteristics, is clearly visible. It is often
very closely applied to contents of similar density and is therefore difficult to re-
solve.

Figure 263
Figure 263. Region of the cell center from hamster suprarenal cortex.
 LYSOSOMES

The lysosomes that have been most thoroughly studied are those of hepatic cells
which were first centrifugally isolated from the mitochondria1 fraction of liver homoge-
nates. The primary lysosomes tend to have a dense homogeneous content but the
secondary lysosomes may contain a variety of products of hydrolysis of differing
density and texture. The majority of the membrane-bounded dense bodies in the
accompanying figures are secondary 1ysosomes.
In the absence of a universally applicable set of morphological criteria, identifica-
tion of lysosomes rests upon a general resemblance to the dense particles comprising
the "lysosome fraction" of liver homogenates. Since one of their defining characteris-
tics is the presence of hydrolytic enzymes, verification of the lysosomal nature of
cytoplasmic particles requires the cytochemical demonstration of their acid phospha-
tase or beta glucoronidase activity.

Figure 264, upper Figure 265, lower

Figures 264 and 265. Liver of phenobarbital-treated hamster showing a variety of lysosomes.

495
 LYSOSOMES

In the internal remodeling of cells associated with diminished physiological

activity, excess organelles are eliminated by autophagy. The organelle or inclusion to
be destroyed is first surrounded by a membrane to form an autophagic vacuole.
Lysosomes then fuse with it, discharging their hydrolytic enzymes into its interior. The
origin of the membrane surrounding the material to be digested has not been
established, but the fact that there are often two parallel limiting membranes suggests
the possibility that a cistern of the smooth endoplasmic reticulum or GERL is involved.
The accompanying micrographs present typical examples of autophagic vacuoles.
The upper figures are of liver from an animal in the period of recovery after
experimental induction of drug-metabolizing enzymes. The content of the autophagic
vacuoles is mainly smooth endoplasmic reticulum and free ribosomes. In the figure at
the lower left, a mitochondrion and a peroxisome have been sequestered, and the
autophagic vacuole at the lower right contains a mitochondrion in a more advanced
state of dissolution but with cristae still identifiable.

Figures 266, 267, and 269. Autophagic vacuoles from liver several days after withdrawal of pheno- Figure 266, upper left Figure 267, upper right
barbital. (Micrographs courtesy of Robert Bolender.)
Figure 268, lower left Figure 269, lower right
Figure 268. Autophagic vacuole from normal rat liver. (Micrograph courtesy of Daniel Friend.)
 LYSOSOMES

Some epithelia contain membrane-limited spherical granules that appear to be large

primary lysosomes, but their moderately dense homogeneous content makes them
morphologically indistinguishable from the secretory granules of glandular cells. They
occur, however, in cells not known to have a secretory function; they show no
tendency to accumulate at the luminal surface and are never observed in the process of
exocytosis. They are therefore presumed to be either lysosomes or peroxisomes.
Cytochemical demonstration of acid phosphatase or peroxidase activity is required for
their unambiguous identification.

Figure 270. Cytoplasm of a cell from a ductus efferens of the ground squirrel Citellus lateralis. Figure 270
(Micrograph courtesy of Jeffrey Pudney.)
 LYSOSOMES

Certain segments of the epididymal epithelium normally have large numbers of

dense, spherical lysosomes concentrated in the supranuclear region of the columnar
cells, as illustrated in the accompanying micrograph. Other segments are characterized
by numerous multivesicular bodies in the apical cytoplasm. The physiological signifi-
cance of the presence of these two categories of lysosome in the same epithelium is not
known.

Figure 271. Micrograph of epithelium from the proximal portion of the cauda epididymis of rabbit. Figure 271
(Micrograph courtesy of Roy Jones.)
 LYSOSOMES

Among the commonest inclusions in secondary lysosomes of all cell types are
concentric systems of thin dense lamellae that are interpreted as myelin forms of
hydrated phospholipid. The accompanying micrographs include extreme examples
routinely observed in epithelial cells in the distal portion of the cauda epididymis in
rabbits. Similar inclusions are found in the smaller lysosomes on the previous page, but
in this more distal region of the duct they may attain diameters of several microns.

Figures 272 and 273. Lysosomes in the basal cytoplasm of the principal cells in the cauda epididymis of Figure 272, upper Figure 273, lower
the rabbit. (Micrographs courtesy of Roy Jones )

503
 LYSOSOMES

Additional examples of laminated inclusions in lysosomes are illustrated here in

micrographs from Leydig cells which, like other steroid-secreting cells, are rich in
lysosomes. The clear spaces interposed among the lamellae are negative images of thin
tabular crystals of unknown nature that have been dissolved in the course of specimen
preparation.

Figure 274
Figure 274. Lysosomes in the interstitial cells of Leydig in the testis of the domestic boar, Sus serofa
505
 LYSOSOMES

In cells that are actively engaged in heterophagy, the lysosomes are very
heterogeneous, exhibiting a broad spectrum of primary lysosomes, heterophagic
vacuoles, residual bodies, and lipofuscin pigment. The Sertoli cell of the seminiferous
epithelium degrades the residual cytoplasm of spermatids left behind in the release of
spermatozoa. The accompanying micrographs from the basal region of Sertoli cells
show the typical appearance of primary lysosomes and residual bodies.

Figure 275, upper and lower

Figure 275. Sertoli cells from the testis of chinchilla.

507
508 LYSOSOMES

The specific granules of neutrophilic leucocytes were formerly considered to be

lysosomes. Further cytochemical study demonstrated that two types of granules are
formed sequentially in the differentiation of these cells. In promyelocytes and early
myelocytes, the so-called azurophil granules contain peroxidase, acid phosphatase,
/3-glucuronidase, and esterase and therefore are considered to be lysosomal in nature.
Later in differentiation, only specific granules are formed and these come to outnumber
the azurophil granules. The specific granules of neutrophils contain antibacterial
proteins and alkaline phosphatase and therefore are not lysosomes.
The accompanying micrograph of an early myelocyte from human bone marrow
contains numerous azurophil granules which have the cytochemical properties of lyso-
somes.

Figure 276
Figure 276. Myelocyte from human bone marrow.
MULTIVESICULAR BODIES

Membrane-bounded vacuoles containing small vesicles are observed in many cell

types. These are called multivesicular bodies, and because they stain with histochemi-
cal reactions for acid phosphatase, they are regarded as a morphologically distinct
category of lysosomes. Short segments of their limiting membrane may be coated,
suggesting that fusion of coated vesicles may contribute to their enlargement.
Multivesicular bodies are invariably present in considerable numbers in the
epithelium lining the epididymal duct. This epithelium is active in endocytosis; coated
vesicles and multivesicular bodies are a conspicuous feature of their apical cytoplasm
(at arrows). The more common dense lysosomes are also abundant.

Figure 277. Lipochrome pigment in a cell from human adrenal cortex. (Micrograph courtesy of John
Long.) Figure 277

511
 LYSOSOMES

The upper figure on the facing page permits a comparison of the familiar dense
lysosomes with a typical multivesicular body. The contents of the latter organelle may
consist of a few vesicles or many. Occasional pits (at arrows) in the otherwise smooth
contour on the surface suggest that the vesicles in the interior may form by invagination
and budding off from the limiting membrane. The vesicles tend to have a fuzzy-coated
membrane but smooth vesicles also occur.
The origin of multivesicular bodies is unclear, but it is apparent from their broad
range of sizes that they grow by accretion, probably as a result of fusion and
incorporation of vesicles into their limiting membrane. The variation in multivesicular
body size and a possible developmental sequence is illustrated in the examples
numbered 1 to 5 in the lower figure.

Figures 278 and 279. Multivesicular bodies in human epididymal epithelium. (Micrograph courtesy of Figure 278, upper Figure 279, lower
Anita Hoffer.)

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 LYSOSOMES

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