Page 40 - Textbook of Pathology, 6th Edition

24 c) Vimentin (found in cells of mesenchymal origin). ribosomes on it. RER is especially well-developed in cells
d) Glial fibrillary acidic protein (present in astrocytes and active in protein synthesis e.g. Russell bodies of plasma cells,
ependymal cells). Nissl granules of nerve cells.
e) Neurofilaments (seen in neurons of central and peripheral ii) Smooth endoplasmic reticulum (SER) is devoid of ribosomes
nervous system). on its surface. SER and RER are generally continuous with
Their main function is to mechanically integrate the cell each other. SER contains many enzymes which metabolise
organelles within the cytoplasm. drugs, steroids, cholesterol, and carbohydrates and partake
iii) Microtubules are long hollow tubular structures about in muscle contraction.
25 nm in diameter. They are composed of protein, tubulin. 5. GOLGI APPARATUS. The Golgi apparatus or Golgi
SECTION I
Cilia and flagella which project from the surface of cell are complex is generally located close to the nucleus. Morpho-
composed of microtubules enclosed by plasma membrane logically, it appears as vesicles, sacs or lamellae composed
and are active in locomotion of the cells. Basal bodies present of unit membrane and is continuous with the endoplasmic
at the base of each cilium or flagellum and centriole located reticulum. The Golgi apparatus is particularly well-
at the mitotic spindle of cells are the two other morpho- developed in exocrine glandular cells.
logically similar structures composed of microtubules. Its main functions are synthesis of carbohydrates and
2. MITOCHONDRIA. Mitochondria are oval structures complex proteins and packaging of proteins synthesised in
and are more numerous in metabolically active cells. They the RER into vesicles. Some of these vesicles may contain
are enveloped by two layers of membrane—the outer smooth lysosomal enzymes and specific granules such as in
and the inner folded into incomplete septa or sheaf-like ridges neutrophils and in beta cells of the pancreatic islets.
called cristae. Chemically and structurally, membranes of 6. LYSOSOMES. Lysosomes are rounded to oval
mitochondria are similar to cell membrane. The inner membrane-bound organelles containing powerful lysosomal
membrane, in addition, contains lollipop-shaped globular digestive (hydrolytic) enzymes. There are 3 forms of
structures projecting into the matrix present between the lysosomes:
layers of membrane. The matrix of the mitochondria contains i) Primary lysosomes or storage vacuoles are formed from the
enzymes required in the Krebs’ cycle by which the products various hydrolytic enzymes synthesised by the RER and
of carbohydrate, fat and protein metabolism are oxidised to packaged in the Golgi apparatus.
produce energy which is stored in the form of ATP in the ii) Secondary lysosomes or autophagic vacuoles are formed by
lollipop-like globular structures. Mitochondria are not static fusion of primary lysosomes with the parts of damaged or
structures but undergo changes in their configuration during worn-out cell components.
General Pathology and Basic Techniques
energised state by alteration in the matrix and intercristal iii) Residual bodies are indigestible materials in the lysosomes,
space; the outer membrane is, however, less elastic. e.g. lipofuscin.
Mitochondria perform the important metabolic function
of oxidative phosphorylation, and in the process generate 7. CENTRIOLE OR CENTROSOME. Each cell contains a
free radicals injurious to membranes. They also have role in pair of centrioles in the cytoplasm close to nucleus in the
apoptosis. Mitochondria contain 37 genes out of which area called centrosome. Centrioles are cylindrical structure
13 encode for synthesising proteins. In addition, composed of electron-dense evenly-shaped microtubules.
mitochondria also have some DNA and ribosomes. They perform the function of formation of cilia and
flagellae and constitute the mitotic spindle of fibrillary
3. RIBOSOMES. Ribosomes are spherical particles which protein during mitosis.
contain 80-85% of the cell’s RNA. They may be present in
the cytosol as ‘free’ unattached form, or in ‘bound’ form when INTERCELLULAR COMMUNICATION
they are attached to membrane of endoplasmic reticulum.
They may lie as ‘monomeric units’ or as ‘polyribosomes’ All cells in the body constantly exchange information with
when many monomeric ribosomes are attached to a linear each other to perform their functions properly. This process
molecule of messenger RNA. is accomplished in the cells by direct cell-to-cell contact
Ribosomes synthesise proteins by translation of (intercellular junctions), and by chemical agents, also called
messenger RNA into peptide sequences followed by as molecular agents or factors (molecular interactions between
packaging of proteins for the endoplasmic reticulum. cells) as under.

4. ENDOPLASMIC RETICULUM. Endoplasmic reticulum Intercellular Junctions
is composed of vesicles and intercommunicating canals. It is
composed of unit membrane which is continuous with both Plasma membranes of epithelial and endothelial cells, though
nuclear membrane and the Golgi apparatus, and possibly closely apposed physically, are separated from each other
with the cell membrane. The main function of endoplasmic by 20 nm wide space. These cells communicate across this
reticulum is the manufacture of protein. Morphologically, space through intercellular junctions or junctional complexes
there are 2 forms of endoplasmic reticulum: rough (or visible under electron microscope and are of 4 types
granular) and smooth (or agranular). (Fig. 3.4):
i) Rough endoplasmic reticulum (RER) is so-called because 1. Occluding junctions (Zonula occludens). These are tight
its outer surface is rough or granular due to attached junctions situated just below the luminal margin of adjacent