Page 27 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 9

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1.8 Lysosome with adjacent lipid droplet in a liver cell
(x10,000).
1.9 Lysosomes in the tubular epithelium of the kidney
of a dog (x22,000).
Lysosomes (lysosoma)
Lysosomes are membrane-bound organelles responsible
for intracellular digestion and enzymatic degradation of
soluble and solid substances.
Lysosomes contain a large range of structurally diverse
enzymes (acid hydrolases). These are synthesised in the
rER and sorted in the Golgi apparatus. Depending on the
function of the cell, lysosomal enzymes consist primarily
of acid phosphatases, glycosidases, peptidases (proteases),
esterases, sulfatases, deoxyribonucleases and ribonucleases
(active at pH 5.0). The enzymes are separated from the
surrounding cytosol by a phosphatide-glycolipid-protein
membrane with a built-in proton-ATPase (proton pump).
Lysosomal enzymes are responsible for the digestion
of substances originating from inside and outside the
cell. The end products (nucleotides, amino acids or sugars)
pass into the cell matrix where they are incorporated into
cellular metabolic processes.
Material that cannot be digested enzymatically is
retained within the lysosome as residual bodies. These
may be ejected from the cell through the plasmalemma. 1.10 Lysosomes in a peritoneal macrophage (x4500).
Those that remain within the cell can lead to formation of
endogenous pigments (e.g. lipofuscin).
Lysosomes are also involved in physiological processes Abnormalities of lysosomal enzyme function can give rise
associated with embryogenesis and play a role in pro- to lysosomal storage diseases. These are characterised by
grammed cell death (apoptosis). the genetically determined absence of functional enzymes,
Disruption of the integrity of the lysosomal mem- resulting in accumulation of undigested substrates.
brane, by endogenous or exogenous factors, results in the
release of lysosomal enzymes into the surrounding cyto- LYSOSOME STRUCTURE
sol, leading to degradation of cellular components within Lysosomes are ovoid to spherical organelles (Figures 1.6
the cytoplasm. Causative factors include oxygen deficiency, to 1.10). Their limiting membrane is reinforced by an
X- and UV radiation and various cytotoxins. This process, abundance of glycolipids. The membrane permits egress
referred to as autolysis, usually results in death of the cell. of metabolic end products into the cytosol and facilitates

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