General Cytology
amounts of glycoproteins, pancreatic cells secrete enzymes such
as zymogen, and breast cells produce milk droplets.
Mitochondria produce ATP, the universal fuel of living organ-
isms, by oxidative processing of nutrients. They are located in
the cytoplasm and separated from it by a double membrane
(Fig. 1.4C). On average, an eukaryotic cell contains about 2000
mitochondria. Depending on age and cell type, mitochondrial
size can vary between 0.5 and 10 pm. The highest mitochon-
drial counts can be found in cells with high energy demand,
such as muscle cells, nerve cells, or metabolically active cells in
the liver. Mitochondria are inherited in non-mendelian fashion
via the cytosol of oocytes. During cell division, mitochondria
are divided between the two daughter cells. They are genetically
semi-autonomous since they possess their own circular genome,
but are dependent on a number of proteins encoded by the
nuclear DNA.
The Pap stain does not color mitochondria, but iron hema-
toxylin or acid fuchsin does. A more specific stain for mito-
chondria is rhodamine 123. Stained mitochondria appear as
single spheres or long, branching structures, up to 7 x 0.5 pm
in size. Mitochondria can be found in large numbers in
hepatocellular carcinoma, resulting in a granular appearance
of the cytoplasm. There are many other causes of granular
cytoplasm; the underlying cellular components can only be
visualized by ultrastructural methods. Since mitochondria
represent the energy system of living cells, they are very impor-
tant in the malignant development. Multiple functional and
structural alterations during carcinogenic processes have been
Lysosomes are small vesicles derived from the Golgi apparatus;
they contain up to 40 acidic enzymes (hydrolases) at a pH 5.
The membrane prevents the aggressive enzymes from destroying
cellular structures. Although the contents can vary substantially,
there are basically no morphological differences between func-
tionally different lysosomes. The main function of lysosomes
is the digestion of internal (non-functional cell organelles) and
external (nutrients, bacteria, leukocytes, debris) material. The
processed material is either released to the cytoplasm, secreted,
or stored in lysosomes.
Several storage diseases (e.g. Hunter-Hurler-Syndrome) are
characterized by a deficiency of lysosomal enzymes. These disor-
ders lead to accumulation of incompletely digested mucopoly-
saccharides in the lysosomes.
Cytoskeleton, Centrosome
The cytoskeleton is a complex lattice of various filaments
building the cellular structure and shape; it is responsible for
dynamic activities such as movement in growth and differen-
tiation (Fig. 1.5). Although is has been thought for a long time
that the cytoskeleton is a special feature of eukaryotic cells only,
it is becoming more and more clear that also prokaryotes have
cytoskeleton-like structures.5
Basal membrane
Fig. 1.5
Display of an epithelial cell with
cytoskeleton and cell-cell
The filaments are required for cell movement (cytokinesis),
transport of material across the cell surface, muscle contraction,
intracellular transport, and sorting and dividing of replicated
chromosomes by the mitotic spindle.
Three main classes of cytoskeletal filaments are distin-
guished: actin filaments, intermediate filaments, and micro-
tubules (Fig. 1.5).
Actin filaments have a diameter of 7 nm and are built from
six different actin types; in muscle cells, actin is functionally
linked to myosin. They can be found in all cells, with especially
high numbers in fibroblasts and the highest concentrations
in muscle cells, since actin is part of the contractile structures.
Lamellipodia (bulges of the cell surface for cell motility) and
filopodia (enhanced cell surface for absorption) are built from
bundled actin. Several glandular tissues such as breast and pros-
tate have contractile myoepithelial cells that can forcibly express
the glandular contents.
Intermediate filaments have a diameter of 10 nm, con-
sist of one or more of 19 different cytokeratin molecules, and
are the strongest fibers among the cellular filaments. They are
mainly responsible for the structural framework of a cell and
determine the cell's tensile strength. They build rope-like poly-
mers. Keratins belong to the group of intermediate filaments.
In keratinizing epithelial cells, keratin filaments accumulate
and are cross-linked by other proteins and disulfide bonds. The
keratinizing process starts at the periphery and progresses to the
nuclear area. In fully differentiated cells, the nucleus becomes
more and more pyknotic and finally dissolves.
Other examples for intermediate filaments are desmin in
skeletal muscle, glial filaments in astrocytes, and neurofilaments
in axons.
Microtubules are long, hollow tubes with 25 nm diameter
assembled from microtubule oligomers originating in a mem-
braneless body, the centrosome. The centrosome is the main
microtubule organizing center (MTOC) of a cell and functions as
an important regulator of the cell cycle. The centrosome consists
of two orthogonally arranged centrioles surrounded by peri-
centriolar material and is situated between the nucleus and the
Golgi apparatus. Upon cell division, each daughter cell receives
one centriole. Although in most model organisms, a proper cell
division can be achieved without a functional centrosome, an
organism requires functional centrosomes to survive in the long
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