Page 28 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

10 Veterinary Histology of Domestic Mammals and Birds

The oxidation of substrates by peroxisomes results
VetBooks.ir in reduction of oxygen to hydrogen peroxide (H O ).
2
2
Produced by many intracellular oxidative reactions, hydro-
gen peroxide is harmful to the cell. This is broken down,
and thus detoxified, by peroxisomal catalase.
Peroxisomes also contribute to lipid biosynthesis. They
produce the lipid layer of the myelin membrane of neu-
rons and synthesise the fatty components of sebaceous
glands. Free acetyl groups are used by peroxisomes in the
biosynthesis of cholesterol.
1.11 Ribosomes and polyribosomes in a canine liver Peroxisomes measure 0.15–0.25 μm in diameter. Their
cell (x45,000). generally homogeneous internal structure may be inter-
rupted by crystalline or tubular inclusions. Peroxisomal
the entry of H ions for activation of lysosomal enzymes. membranes are formed by budding from the tubules of
+
Lysosomes vary in size and in the density of their matrix. the smooth endoplasmic reticulum; the contents of the
Most are 0.2–0.6 μm in diameter but may reach 5 μm in peroxisome are synthesised in the cytosol.
cells with high endocytotic activity.
For many years, lysosomes were considered to arise Organelles of anabolism
from the Golgi apparatus as primary lysosomes, which A major proportion of cellular metabolism involves
then merged with late endosomes to become second- organelles that function primarily to synthesise protein
ary lysosomes. Recent findings suggest that lysosome (ribosomes and rough endoplasmic reticulum) and lipid
biogenesis is a more complex process, involving merg- (smooth endoplasmic reticulum).
ing of post-Golgi vesicles with the endocytic pathway. In
this model, early endosomes or phagosomes containing RIBOSOMES (RIBOSOMA)
internalised materials are converted into late endosomes, Ribosomes are cytosolic ribonucleoprotein particles.
which mature into lysosomes. Lysosomes also feature in Numerous enzymes involved in intermediate metabolism
autophagy, the process by which intracellular compo- (multi-enzyme complexes) are synthesised in these organelles.
nents (organelles, membranes, excess cellular products) Ribosomes translate the information encoded in messenger
are degraded. These components are isolated and engulfed ribonucleic acid (mRNA) into amino acid sequences and
in a membrane to form an autophagosome. Fusion of an link the individual amino acids to form polypeptide chains
autophagosome with a lysosome gives rise to an autol- (for further information, refer to biochemistry texts).
ysosome (autophagolysosome). Autophagy plays an
important part in cell regeneration and in the recycling of Ribosome stRuctuRe
organelles and cell membranes. Ribosomes are spheroid to ovoid structures (diameter 15–30
Under certain physiological conditions, lysosomes can nm). They consist of one larger and one smaller subunit.
be transferred out of the cell, becoming active in the extra- According to their sedimentation coefficients, the subunits
cellular compartment, e.g. breakdown of connective tissue are designated 60S and 40S. The larger subunit comprises
and digestion of ground substance in bone (osteoclasts). two ribonucleic acids (RNA) and 45 different proteins. One
Lysosomes also play an important role in lysis of tissue ribonucleic acid and 33 proteins make up the smaller sub-
fragments in inflammatory reactions. unit. The overall structure and function of the ribosome is
In polymorphonuclear neutrophilic granulocytes, determined by interactions between ribosomal RNA (rRNA)
aggregated lysosomes can be observed with the light and mRNA produced in the nucleus. The composition of
microscope as azurophilic granules. They are frequently ribosomes is around 40% rRNA and 60% associated protein.
also identifiable in macrophages as phagocytosed frag- Within the cytoplasm, ribosomes are either present as
ments of bacteria or cell debris. individual structures or are assembled into spiral chains
referred to as polyribosomes (polysomes, polyribosoma).
Peroxisomes (peroxisoma) Polyribosomes consist of several individual ribosomes
Peroxisomes (microbodies) are membrane-bound organ- attached at intervals to a single mRNA molecule. Free ribo-
elles that contain oxidative enzymes (e.g. D-amino acid somes and polyribosomes primarily synthesise proteins
oxidase, urate oxidase and catalase). They are present that are required by the cell itself (e.g. structural proteins,
in almost all cells. Peroxisomes catalyse the breakdown enzymes) (Figures 1.6 and 1.11).
(β-oxidation) of fatty acids producing acetyl-CoA. They The number of ribosomes is related to the function of
also perform an important role in detoxification reac- the cell. Rapidly growing cells or cells actively synthesising
tions, and thus are abundant in liver and kidney cells. protein contain considerably more ribosomes than degen-

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