General morphology of the cell

GENERAL MORPHOLOGY OF THE
CELL
Department Of General Histology

CELL DIFFERENTIATION
 The human organism presents about 200 different cell types, all
derived from the zygote, the single cell formed by fertilization of
an oocyte with a spermatozoon. The first cellular divisions of the
zygote produce cells called blastomeres and as part of the inner
cell mass blastomeres give rise to all tissue types of the adult.
Explanted to tissue culture such cells have been termed
embryonic stem cells. During their specialization process,
called cell differentiation, the cells synthesize specific proteins,
change their shape, and become very efficient in specialized
functions. For example, muscle cell precursors elongate into
fiber-like cells that synthesize and accumulate large arrays of
actin and myosin. The resulting cell is specialized to efficiently
convert chemical energy into contractile force.
 The main cellular functions performed by specialized cells in the
body are listed in Table 2–1. It is important to understand that the
functions listed there can be performed by most cells of the body;
specialized cells have greatly expanded their capacity for one or
more functions during differentiation.

CELLULAR FUNCTIONS IN SOME SPECIALIZED
CELLS

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MEMBRANE STRUCTURE. (a): A TEM of a sectioned cell surface
shows the trilaminar unit membrane
with two dark (electron-dense) lines
enclosing a clear (electron-lucent)
band. These three layers of the unit
membrane correspond to reduced
osmium deposited on the hydrophilic
phosphate groups present on each
side of the internal bilayer of fatty acids
where osmium is not deposited. The
"fuzzy" material on the outer surface of
the membrane represents the
glycocalyx of oligosaccharides
attached to phospholipids and proteins.
Components of the glycocalyx are
important for cell-cell recognition in
many biological processes and for
adsorption and uptake of many
molecules by cells. X100,000.
(b): Schematic drawing depicts the trilaminar
ultrastructure (left) and molecular organization (right) of
the lipid bilayer in a cell membrane.

 Membrane phospholipids, such as phosphatidylcholine
(lecithin), consist of two non-polar (hydrophobic or water-
repelling) long-chain fatty acids linked to a charged polar
(hydrophilic or water-attracting) head group. Cholesterol is also
present, often at nearly a 1:1 ratio with the phospholipids in
plasma membranes. Membrane phospholipids are most stable
when organized into a double layer (bilayer) with their
hydrophobic fatty acid chains directed toward the middle away
from water and their hydrophilic polar heads directed outward
to contact water on both sides.
 Cholesterol molecules insert among the close packed the
phospholipid fatty acids, restricting their movement, and thus
modulate the fluidity and movement of all membrane
components. The lipid composition of each half of the bilayer is
different. For example, in red blood cells phosphatidylcholine
and sphingomyelin are more abundant in the outer half of the
membrane, whereas phosphatidylserine and
phosphatidylethanolamine are more concentrated in the inner
half. Some of the lipids, known as glycolipids, possess
oligosaccharide chains that extend outward from the surface of
the cell membrane and thus contribute to the lipid asymmetry

THE FLUID MOSAIC MODEL OF MEMBRANE
STRUCTURE

 (a): The fluid mosaic model emphasizes that a membrane consisting of
a phospholipid bilayer also contains proteins inserted in it or bound to
the cytoplasmic surface (peripheral proteins) and that many of these
proteins move within the fluid lipid phase. Integral proteins are firmly
embedded in the lipid layers. Other proteins completely span the bilayer
and are called transmembrane proteins. Hydrophobic amino acids of
the integral membrane protein interact with the hydrophobic fatty acid
portions of the membrane. Both the proteins and lipids may have
externally exposed oligosaccharide chains. When cells are frozen and
fractured (cryofracture), the lipid bilayer of membranes is often
cleaved along the hydrophobic center.
 (b): Membrane splitting occurs along the line of weakness formed by
the fatty acid tails of membrane phospholipids, since only weak
hydrophobic interactions bind the halves of the membrane along this
line. Electron microscopy of cryofracture preparation replicas is a useful
method of studying membranous structures. Most of the protruding
membrane particles seen (1) are proteins or aggregates of proteins that
remain attached to the half of the membrane adjacent to the cytoplasm
(the P or protoplasmic face). Fewer particles are found attached to the
outer half of the membrane (E or extracellular face). For every protein
particle that bulges on one surface, a corresponding depression (2)
appears in the opposite surface.

EXPERIMENT DEMONSTRATING THE FLUIDITY OF
MEMBRANE PROTEINS.
(a, b): Two types of cells were grown in tissue
cultures, one with fluorescently labeled
transmembrane proteins in the plasmalemma (right)
and one without. Cells of each type were fused
together into hybrid cells through the action of
Sendai virus.
(c): Minutes after the fusion of the cell membranes,
the fluorescent proteins of the labeled cell spreads
to the entire surface of the hybrid cells. Such
experiments provide important data in support of
the fluid mosaic model. However, in many cells most
transmembrane proteins show very restricted lateral
movements along the cell membrane and are
anchored in place by other proteins linking them to
the cytoskeleton.

FORMATION AND MATURATION OF CELL
MEMBRANE PROTEINS

 Membrane proteins of the plasmalemma are
synthesized in the rough endoplasmic reticulum and
then move in transport vesicles to a Golgi apparatus,
another cytoplasmic structure with several flattened
membrane saccules or cisternae. While in the Golgi
apparatus, the oligosaccharide chains are added
(glycosylation) to many membrane proteins by enzymes
in the Golgi saccules. When glycosylation and other
posttranslational modifications are complete, the mature
membrane proteins are isolated within vesicles that
leave the Golgi apparatus. These vesicles move to the
cell membrane and fuse with it, thus incorporating the
new membrane proteins (along with the lipid bilayer of
the vesicle) into the cell membrane.

EXPERIMENT DEMONSTRATING THE FLUIDITY
OF MEMBRANE PROTEINS
(a, b): Two types of cells were grown in tissue cultures,
one with fluorescently labeled transmembrane proteins in
the plasmalemma (right) and one without. Cells of each
type were fused together into hybrid cells through the
action of Sendai virus.
(c): Minutes after the fusion of the cell membranes, the
fluorescent proteins of the labeled cell spreads to the
entire surface of the hybrid cells. Such experiments
provide important data in support of the fluid mosaic
model. However, in many cells most transmembrane
proteins show very restricted lateral movements along
the cell membrane and are anchored in place by other
proteins linking them to the cytoskeleton.

THREE MAJOR FORMS OF ENDOCYTOSIS
Endocytosis is a process in which a cell takes in material from the
extracellular fluid using dynamic movements and fusion of the cell
membrane to form cytoplasmic, membrane-enclosed structures containing
the material. Such cytoplasmic structures formed during endocytosis fall
into the general category of vesicles or vacuoles. (a): Phagocytosis
involves the extension from the cell of large folds called pseudopodia which
engulf particles, for example bacteria, and then internalize this material into
a cytoplasmic vacuole or phagosome. (b): In pinocytosis the cell membrane
invaginates (dimples inward) to form a pit containing a drop of extracellular
fluid. The pit pinches off inside the cell when the cell membrane fuses and
forms a pinocytotic vesicle containing the fluid. (c): Receptor-mediated
endocytosis includes membrane proteins called receptors which bind
specific molecules (ligands). When many such receptors are bound by their
ligands, they aggregate in one membrane region which then invaginates and
pinches off to create vesicle or endosome containing both the receptors
and the bound ligands.

ENDOCYTOSIS AND MEMBRANE TRAFFICKING.
Ligands, such as hormones and growth factors, are
internalized by receptor-mediated endocytosis, which
is mediated by the cytoplasmic peripheral membrane
protein clathrin or other proteins which promote
invagination and temporarily coat the newly formed
vesicles. Such coated vesicles can be identified by
TEM. After detachment of the coating molecules, the
vesicle fuses with one or more vesicles of the
endosomal compartment, where the ligands detach
from their receptors and are sorted into other
vesicles. Vesicles of membrane with empty receptors
return to the cell surface and after fusion the
receptors are ready for reuse. Vesicles containing the
free ligands typically fuse with lysosomes, as
discussed below. The cytoskeleton with associated
motor proteins is responsible for all such directional
movements of vesicles.

INTERNALIZATION OF LOW-DENSITY
LIPOPROTEINS
Endocytosis of low-density lipoproteins (LDL) is an important mechanism that keeps the concentration of LDL in extracellular
body fluids low and is a well-studied example of endocytosis and membrane trafficking. LDL, which is often rich in cholesterol,
binds with high affinity to its specific receptors in the cell membranes. This binding activates the formation of clathrin-coated
endocytotic pits that form coated vesicles. The vesicles soon lose their coat proteins, which return to the inner surface of the
plasmalemma. The uncoated vesicles fuse with endosomes and the free LDL and the receptors are sorted into separate
vesicles. Receptors are returned to the cell surface and the LDL is transferred to lysosomes for digestion and separation of
their components to be utilized by the cell.

EXOCYTOSIS
 In exocytosis a membrane-limited cytoplasmic
vesicle fuses with the plasma membrane, resulting
in the release of its contents into the extracellular
space without compromising the integrity of the
plasma membrane (Figure 2–6). Often exocytosis
of stored products from epithelial cells occurs
specifically at the apical domains of cells, such as
in the exocrine pancreas and the salivary glands
(see Chapter 4). The fusion of membranes during
exocytosis is a highly regulated process involving
interactions between several specific membrane
proteins. Exocytosis is triggered in many cells by
transient increase in cytosolic Ca2+.

SIGNAL RECEPTION AND TRANSDUCTION
 In endocrine signaling, the signal molecules (called
hormones) are carried in the blood to target cells throughout
the body.
 In paracrine signaling, the chemical mediators are rapidly
metabolized so that they act only on local cells very close to
the source.
 In synaptic signaling, a special kind of paracrine interaction,
neurotransmitters act only on adjacent cells through special
contact areas called synapses.
 In autocrine signaling, signals bind receptors on the same
cell type that produced the messenger molecule.
 In juxtacrine signaling, important in early embryonic tissue
interactions, signaling molecules remain part of a cell's
surface and bind surface receptors of the target cell when the
two cells make direct physical contact.

RECEPTORS AND THEIR LIGANDS
Cells respond to certain external
chemical signals that act as ligands,
such as hormones and lipoproteins,
according to the library of receptors
they have. Such receptors are always
proteins, typically transmembrane
proteins. In this schematic
representation, three cells appear with
different receptors. The extracellular
environment is shown to contain
several ligands, which can interact only
with the appropriate specific receptors.
Considering that the extracellular
environment contains a multitude of
molecules, it is important that ligands
and the respective receptors exhibit
complementary morphology and great
binding affinity.

 Hydrophilic signaling molecules, including most hormones,
local chemical mediators (paracrine signals), and
neurotransmitters activate receptor proteins on the surface of
target cells. These receptors, often transmembrane proteins,
relay information to a series of intracellular intermediaries that
ultimately pass the signal (first messenger) to its final
destination in either the cytoplasm or the nucleus in a process
called signal transduction. One of the best-studied classes of
such intermediary proteins, the G proteins, binds guanine
nucleotides and acts on other membrane-bound intermediaries
called effectors which propagate the signal further into the cell.
 Effector proteins are usually ion channels or enzymes that
generate large quantities of small second messenger
molecules, such as 1,2-diacyglycerol (DAG), cyclic adenosine
monophosphate (cAMP), and inositol 1,4,5-triphosphate (IP3).
The ions or second messengers diffuse through the cytoplasm,
amplifying the first signal and triggering a cascade of molecular
reactions that lead to changes in gene expression or cell
behavior.

G PROTEINS AND INITIATION OF SIGNAL
TRANSDUCTION

 When a hormone or other signal binds to a membrane
receptor, the hormone can begin to cause changes in the
cell's activities after a signal transduction process initiated by
the bound receptor. The first step in receptor signaling often
involves G proteins which bind guanosine diphosphate
(GDP) when inactive and are activated when GDP is
exchanged for GTP. A simplified version of G protein activity is
shown here. Conformational changes occur in the receptor
when it binds its ligand and the changed receptor activates
the G protein–GDP complex. A GDP-GTP exchange releases
the subunit of the G protein, which then moves laterally to bind
with a transmembrane effector protein, activating it to
propagate the signal further by various mechanisms. The
subunit GTP is rapidly converted back to GDP, allowing the
polypeptide to reassociate with the rest of the G protein
complex, ready to be activated again when the receptor is
again bound by hormone.

THE CHEMIOSMOTIC PROCESS OF ENERGY
TRANSDUCTION.

 The movement of electrons along the units of the inner mitochondrial
membrane electron transport system (middle) is accompanied by the
directed movement of protons (H+) from the matrix into the
intermembranous space. The inner membrane is highly impermeable to
protons and the result is an electrochemical gradient across the
membrane. The other membrane-associated proteins (left) make up the
ATP synthase systems, each of which forms a 10 nm, multisubunit
globular complex on a stalk-like structure projecting from the matrix side
of the inner membrane.
 A channel runs through this enzyme complex and specifically allows
protons to flow through it, down the electrochemical gradient and across
the membrane back into the matrix. Passage of protons through this
narrow path causes rapid spinning of specific polypeptides in the globular
ATP synthase complex. In this manner the energy of proton flow is
converted into the mechanical energy of protein movement.
 Other subunit proteins of the complex store this energy in the new
phosphate bond of ATP which leaves the mitochondrion for use
throughout the cell. It is estimated that each ATP synthase complex
produces more than 100 molecules of ATP per second. In some
mitochondria, particularly those in cells of multilocular adipose tissue,
another inner membrane protein called thermogenin forms a shunt for
the return of protons into the matrix (right).
 This reflux of protons does not produce ATP, but instead dissipates
energy as heat which warms blood flowing through the tissue

MITOCHONDRIA
The two mitochondrial membranes and
central matrix can be seen here in the
diagram and the TEM. The outer
membrane is smooth and the inner
membrane, shown at left, has many sharp
folds called cristae which increase its
surface area greatly. Cristae are most
numerous in mitochondria of highly active
cells. The matrix is a gel containing
numerous enzymes. The inner membrane
surface in contact with the matrix is
studded with many multimeric protein
complexes resembling globular units on
short stalks. These contain the ATP
synthase complexes that generate most
of the cell's ATP.

MITOCHONDRIA IN THE LIGHT MICROSCOPE
In sectioned cells stained with H&E,
such as certain cells of the stomach
inner lining, mitochondria typically
appear as numerous eosinophilic
structures throughout the cytoplasm.
The mitochondria usually appear round
or slightly elongated and are more
numerous in cytoplasmic regions with
higher energy demands, such as near
the cell membrane in cells undergoing
much active transport. The central
nuclei are also clearly seen in these
cells.

Entire mitochondria can be shown in cultured
cells, such as the endothelial cells shown here
and often appear as the elongated structures
(shown in yellow or orange here), usually
arrayed in parallel along microtubules. These
preparations along with TEM studies indicate
that the elongated shape is typical of
mitochondria and that their shape can be quite
plastic and variable. Specific mitochondrial
staining such as that shown here involves
incubating living cells with specific fluorescent
compounds that are specifically sequestered
into these organelles, followed by fixation and
immunocytochemical staining of the
microtubules. In this preparation, microtubules
are stained green and mitochondria appear
yellow or orange, depending on their
association with the green microtubules. The
cell nucleus was stained with DAPI. (Figure 2–
11b, with permission, from Invitrogen.)

 (a): Many ribosomes attach to the same mRNA and
move along it during translation, with each
ribosome producing and at the end of the mRNA
releasing one copy of the protein encoded by that
message.
 (b): Proteins that are to be incorporated into
membranes, or eventually to be extruded from the
cytoplasm (secreted proteins) or sequestered into
lysosomes, are made on polysomes that adhere to
the membranes of endoplasmic reticulum. The
proteins produced by these ribosomes are
segregated during translation into the interior of the
endoplasmic reticulum's membrane cisternae.

ENDOPLASMIC RETICULUM
(a): Electron microscopy
shows that some regions of
endoplasmic reticulum, called
smooth ER (foreground), are
devoid of ribosomes, the small
granules that are present in
the rough ER (background).
Both types of ER are
continuous with one another.
The interconnected
membranous cisternae of the
smooth ER are frequently
tubular, whereas those in the
rough ER are flattened sacs.

ENDOPLASMIC RETICULUM
In a very thin cultured endothelial cell, both ER (green) and mitochondria
(orange) can be visualized with vital fluorescent dyes that are sequestered
specifically into those organelles. This staining method with intact cells clearly
reveals the continuous, lacelike ER present in all regions of the cytoplasm.
(Figure 2-15b, with permission, from Invitrogen.)

ROUGH AND SMOOTH ER
As seen with the TEM the cisternae of
rough ER are flattened, with polyribosomes
on their outer surfaces and concentrated
material in their lumens. Such cisternae
appear separated in sections made for
electron microscopy, but they actually form
a continuous channel or compartment in
the cytoplasm. Smooth ER is continuous
with rough ER but is involved with a much
more diverse range of functions. Three
major activities associated with smooth ER
are (1) lipid biosynthesis, (2) detoxification
of potentially harmful compounds, and (3)
sequestration of Ca++ ions. Specific cell
types with well-developed smooth ER are
usually specialized for one of these
functions.

MOVEMENT OF POLYPEPTIDES INTO THE RER.

PROTEIN SYNTHESIS AND CELL
MORPHOLOGY.
(a): Cells that make few or no
proteins for secretion have very
little rough ER, with essentially all
polyribosomes free in the
cytoplasm.

(b): Cells that synthesize,
segregate, and store various
proteins in specific secretory
granules or vesicles always have
rough ER, a Golgi apparatus, and
a supply of granules containing the
proteins ready to be secreted.

(c): Cells with extensive RER
and a well-developed Golgi
apparatus show few secretory
granules because the proteins
undergo exocytosis immediately
after Golgi processing is
complete. Many cells, especially
those of epithelia, are polarized,
meaning that the distribution of
RER and secretory vesicles is
different in various regions or
poles of the cell.

(d): Epithelial cells specialized for
secretion have distinct polarity, with
RER abundant at their basal ends and
mature secretory granules at the
apical poles undergoing exocytosis into
an enclosed extracellular compartment,
the lumen of a gland.

PHOSPHOLIPID TRANSPORT
Phospholipids or more complex
lipids such as cholesterol are
generally synthesized by enzymes
located in smooth ER. The products
are inserted into the lipid bilayers
of that organelle and are distributed
in membranes throughout the cell
by movement through the ER, Golgi
apparatus, secretory vesicles, and
other organelles. As shown here
however, individual phospholipids
can also be transported from
smooth ER directly to membranes
elsewhere in the cell but only after
binding a water-soluble transporting
protein. There is a specific transfer
protein (also called exchange
proteins) for each specific type of
phospholipid and each can be
reused many times. Phospholipid
transfer proteins are an important
mechanism for redistributing lipids
between different membrane-
enclosed compartments or
organelles, such as the ER and
mitochondria.

GOLGI APPARATUS
Morphological aspects of the Golgi apparatus are revealed by the SEM, which shows a three-dimensional
snapshot of the region between RER and the Golgi membrane compartments. Cells may have multiple
Golgi apparatuses, each with stacks of cisternae and dynamic cis and trans faces, and these typically are
situated near the cell nucleus. This has been shown in careful TEM studies but is also clearly seen in
intact cultured cells.

(c): The fibroblast was processed by immunocytochemistry using an antibody
against golgin-97 to show many complexes of Golgi vesicles (green), all near the
nucleus, against a background of microfilaments organized as stress fibers and
stained with fluorescent phalloidin (violet). Because of the abundance of lipids in
its many membranes, the Golgi apparatus is difficult to visualize by light
microscopy in typical paraffin-embedded, H&E stained sections. In cells with
very active Golgi complexes however, such as developing white blood cells, the
organelle can sometimes be seen as a faint unstained juxtanuclear region
(sometimes called a "Golgi ghost") surrounded by basophilic cytoplasm.

Though only snapshots of this highly
dynamic organelle, electron micrographs of
the Golgi apparatus provided early
evidence about how this organelle
functions, evidence that has now been
strengthened by biochemical and other
studies. To the right is a cisterna (arrow)
of the rough ER containing granular
material. Close to it are small vesicles
containing apparently similar material.
These are very close to the cis face of the
Golgi apparatus. In the center are the
characteristic flattened, curved, and
stacked medial cisternae of the complex.
Dilatations (upper left arrow) are seen
extending from the ends of the cisternae.
Similar dilatations gradually detach
themselves from the cisternae and fuse at
the trans face, forming the secretory
granules (1, 2, and 3). Near the plasma
membranes of two neighboring cells is
more rough ER and smooth ER. X30,000.
Inset: a small region of a Golgi apparatus in
a 1- m section impregnated with silver,
which demonstrates the abundance of
glycoproteins within some cisternae.
X1200.

SUMMARY OF GOLGI APPARATUS STRUCTURE
AND FUNCTION

SECRETORY GRANULES
TEM of one area of a
pancreatic acinar cell shows
numerous mature, electron-
dense secretory granules (S)
in association with condensing
vacuoles (C) of the Golgi
apparatus (G). Such granules
form as the contents of the
Golgi vacuoles becomes more
condensed. In H&E stained
sections secretory granules are
often shown as intensely
eosinophilic structures, which in
polarized epithelial cells are
concentrated at the apical
region prior to exocytosis.

LYSOSOMES
Cells in a kidney tubule show numerous
purple lysosomes (L) in the cytoplasmic
area between the basally located nuclei (N)
and apical ends of the cells at the center of
the tubule. Using endocytosis, these cells
actively take up small proteins in the lumen
of the tubule, degrade the proteins in
lysosomes, and then release the resulting
amino acids for reuse.

Lysosomes in cultured
vascular endothelial cells can
be specifically stained using
fluorescent dyes sequestered
into these organelles (green),
which are abundant around
the blue Hoechst-stained
nucleus. Mitochondria (red)
are scattered among the
lysosomes.

In the TEM lysosomes (L)
have a characteristic very
electron-dense appearance
and are shown here near
groups of Golgi cisternae (G)
and a centriole (C). Less
electron-dense lysosomes
represent heterolysosomes in
which digestion of the
contents is underway. The
cell is a macrophage with
numerous fine cytoplasmic
extensions (arrows).

LYSOSOMAL FUNCTIONS
Synthesis of the digestive enzymes
occurs in the rough ER, and the
enzymes are packaged in the Golgi
apparatus. Heterophagosomes, in which
bacteria are being destroyed, are formed
by the fusion of the phagosomes and
lysosomes. Autophagosomes, such as
those depicted here with ER and
mitochondria in the process of digestion,
are formed after nonfunctional or
surplus organelles become enclosed with
membrane and the resulting structure
fuses with a lysosome. The products of
digestion can be excreted from the cell
by exocytosis, but may remain in a
membrane-enclosed residual body,
containing remnants of indigestible
molecules. Residual bodies can
accumulate in long-lived cells and be
visualized as lipofuscin granules. In some
cells, such as osteoclasts, the lysosomal
enzymes are secreted to a restricted
extracellular compartment.

AUTOPHAGOSOMES
Autophagy is a process in which the cell uses lysosomes to dispose of obsolete or non-functioning
organelles or membranes. Details of the process are highly regulated but not well-understood. Membrane
of unknown origin encloses the organelles to be destroyed, forming an autophagosome which then fuses
with a lysosome for digestion of the contents. In the TEM autophagosomes can sometimes be recognized
by their contents, as shown here. Upper right: Two autophagosomes containing portions of the RER that
are slightly more electron-dense than neighboring normal RER. Center: An autophagosome containing
what may be mitochondrial membranes (arrow) plus RER. Left: A vesicle that may represent a residual
body with indigestible material.

PEROXISOMES
(a): By TEM peroxisomes generally show a homogenous matrix of moderate electron-
density, but may include darker crystalloid internal structures representing very dense
concentrations of enzymes. The arrows indicate small aggregates of glycogen. (x30,000)
(b): A cultured endothelial cell processed by immunocytochemistry shows many
peroxisomes (green) distributed throughout the cytoplasm among the vitally stained
elongate mitochondria (red) around the DAPI-stained nucleus (blue). Peroxisomes shown
here were specifically stained using an antibody against the membrane protein PMP70.

MOLECULAR ORGANIZATION OF A
MICROTUBULE
Microtubules are rigid structures which assemble from heterodimers of and tubulin.
Microtubules have an outer diameter of 24 nm and a hollow lumen 14 nm wide. Tubulin
molecules are arranged to form 13 protofilaments, as seen in the cross section in the upper
part of the drawing. The specific orientation of the tubulin dimers results in structural polarity of
the microtubule. Microtubules elongate or rapidly shorten by the addition or removal of tubulin
at the ends of individual protofilaments. The lengths and locations of cytoplasmic microtubules
vary greatly during different phases of cell activity, with assembly dependent on shifting
balances between polymerized and unpolymerized tubulin and other factors in "dynamic
instability."

Actin micro filaments (MF) and microtubules (MT)
can both be clearly distinguished in this TEM photo
of fibroblast cytoplasm. The image also provides a
good comparison of the relative diameters of these
two cytoskeletal components.

The ultrastructural view can be compared to the appearance of microfilaments and microtubules in a
cultured cell stained by immunocytochemistry. Actin filaments (red) are most concentrated at the cell
periphery, forming prominent circumferential bundles from which finer filaments project into the transient
cellular extensions at the edge of the cell and push against the cell membrane. Such an arrangement of
actin filaments forms a dynamic network important for cell shape changes such as those during cell
division, locomotion, and formation of cellular processes, folds, pseudopodia, lamellipodia, veils, microvilli,
etc. which serve to change a cell's surface area or give direction to a cell's crawling movements.
Microtubules (green/yellow) are present throughout the cytoplasm and are oriented in arrays which
generally extend from the area around the nucleus into the most peripheral extensions.

CILIA
Cilia are motile structures projecting from a cell, typically the apical end of
epithelial cells. Each cilium is covered by the cell membrane and contains
cytoplasm dominated by a specialized assembly of unusually stable microtubules,
the axoneme. Shifting movements between microtubules of an axoneme produce
whip-like motions of the cilia. Most epithelial cells lining the respiratory tract, such
as those shown in the three micrographs here, have numerous cilia which move to
propel mucus along the tract toward the pharynx. Between the ciliated cells are
mucus-producing, non-ciliated goblet cells (G) with basal nuclei and apical
cytoplasm filled with mucus granules. The relative size and spacing of the ciliated
cells and goblet cells is seen in micrographs. (a): Light micrograph. X400.
Pararosaniline-toluidine blue,

(c): TEM shows the axonemes of cilia cut
in different orientations and their basal
bodies in the apical cytoplasm. X9200.

MICROTUBULES
(a): cross-section by TEM after fixation with tannic acid in glutaraldehyde, which leaves the unstained
tubulin subunits delineated by the dense tannic acid. Cross sections of tubules reveal the ring of 13
subunits of dimeric tubulin which are arranged lengthwise as protofilaments. Changes in microtubule
length are caused by the addition or loss of individual tubulin subunits from protofilaments.

CILIA
(b): A diagrammatic cross-section through a cilium
reveals a cytoplasmic core of microtubules called an
axoneme. The axoneme consists of two central
microtubules surrounded by nine peripheral
microtubular doublets associated with several other
proteins. In the doublets, microtubule A is complete,
consisting of 13 protofilaments, whereas microtubule B
shares some of A's protofilament heterodimers. A series
of protein complexes containing ciliary dynein, the inner
and outer dynein arms, are bound to microtubule A
along its length. When activated by ATP, the dynein
arms briefly link microtubule B of the adjacent doublet
and provide for slight sliding of the doublets against
each other, which is then immediately reversed. This
rapid back-and-forth shift between adjacent doublets,
produced by the ciliary dynein motors, causes the
rhythmic changes of axonemal shape that bring about
the flailing motion of the entire cilium.
Each axoneme is continuous with a basal body located
at the base of the cilium. Basal bodies are structurally
very similar to centrioles, which nucleate and organize
the growth of microtubules during formation of the
mitotic spindle.

CENTRIOLES
(c): Each centriole consists of nine
relatively short microtubular triplets
linked together in a pinwheel-like
arrangement. In the triplets,
microtubule A is complete and
consists of 13 protofilaments,
whereas microtubules B and C share
protofilaments. Under normal
circumstances, these organelles are
found in pairs and are oriented at
right angles to one another. The pair
of centrioles is called a centrosome.

ACTIN FILAMENT TREADMILLING
Actin filaments or microfilaments
are helical two-stranded polymers
assembled from globular actin
subunits. The filaments are flexible
structures, with diameters in
various cells of 5-9 nm, depending
on associated proteins. Assembly
of actin filaments (F-actin) results
in their polarity, with actin subunits
(G-actin) added to the plus (+) end
and removed at the minus (–) end.
Even actin filaments of a constant
length are highly dynamic
structures, balancing G-actin
assembly and disassembly at the
opposite ends, with a net
movement or flow along the
polymer known as treadmilling.

WITHIN CELLS, ACTIN MICROFILAMENTS (F-ACTIN)
CAN BE ORGANIZED IN SEVERAL FORMS.
 1. In skeletal muscle, they assume a stable array
integrated with thick (16-nm) myosin filaments.
 2. In most cells, microfilaments form a thin sheath or
network just beneath the plasmalemma. These filaments
are involved in all cell shape changes such as those
during endocytosis, exocytosis, and cell locomotion.
 3. Microfilaments are intimately associated with several
cytoplasmic organelles, vesicles, and granules and play
a role in moving or shifting cytoplasmic components
(cytoplasmic streaming).
 4. Microfilaments are associated with myosin and form a
"purse-string" ring of filaments whose constriction results
in the cleavage of mitotic cells.
 5. In crawling cells actin filaments are organized into
parallel contractile bundles called stress fibers

INTERMEDIATE FILAMENTS OF KERATIN
Intermediate filaments display an average
diameter of 10-12 nm, between that of actin
filaments and microtubules, and serve to
provide mechanical strength or stability to
cells. Unlike the other two cytoskeletal
polymers, intermediate filaments are
composed of various protein subunits in
different types of cells. All such subunits
appear to be rodlike rather than globular and
undergo step-wise assembly into a structure
resembling a cable with many strands. A large
and important class of intermediate filaments
is composed of keratin subunits, which are
prominent in epithelial cells. Bundles of keratin
filaments associate with certain classes of
intercellular junctions common in epithelial
cells and are easily seen with the TEM, as
shown here in two extensions in an epidermal
cell bound to a neighboring cell.

CELLULAR INCLUSIONS-1
Lipid droplets are abundant in cells of the adrenal
cortex, and appear with the TEM as small spherical
structures with homogenous matrices (L).
Mitochondria are also seen here. As aggregates of
hydrophobic lipid molecules these inclusions are
enclosed by a single monolayer of phospholipids with
various peripheral proteins, including enzymes for lipid
metabolism. In routine processing of tissue for
paraffin sections fat droplets are generally removed,
leaving empty spaces in the cells. Common fat cells
have cytoplasm essentially filled with one large lipid
droplet. X19,000.

TEM of a liver cell cytoplasm shows
numerous individual or clustered electron-
dense particles representing glycogen
granules, although these granules lack
membrane. Glycogen granules usually form
characteristic aggregates such as those
shown. Glycogen is a ready source of energy
and such granules are often abundant in cells
with high metabolic activity. X30,000.

Pigment deposits (PD) occur in many cell types and
may contain various complex substances, such as
lipofuscin or melanin. Lipofuscin granules represent
an accumulating by-product of lysosomal digestion in
long-lived cells, but melanin granules serve to protect
cell nuclei from damage to DNA caused by light.
Many cells contain pigmented deposits of
hemosiderin granules containing the protein ferritin,
which forms a storage complex for iron. Hemosiderin
granules are very electron-dense, but with the light
microscope appear brownish and resemble lipofuscin.
The liver cells shown have large cytoplasmic regions
filled with pigment deposits which probably represent
iron-containing hemosiderin. X400

General morphology of the cell

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General morphology of the cell