Page 26 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

8 Veterinary Histology of Domestic Mammals and Birds

Cell matrix (cytosol)
· constitutive and
VetBooks.ir · regulated secretory. The cytosol is the site of most of the reactions that, in
biochemical terms, constitute intermediate (intermedi-
ary) metabolism. This involves the liberation of energy
In the constitutive pathway, substances intended for export (ATP) through the breakdown of small molecules and the
from the cell are delivered continuously to the plasma synthesis of new macromolecules that contribute to the
membrane. This is exemplified by the secretion of certain structure of the cell or maintenance of cellular metabolic
proteins immediately after their synthesis in the Golgi processes.
apparatus (e.g. antibodies in plasma cells or collagen pre- The cytosol comprises over half of the volume of
cursors in fibroblasts). the cell. It contains enzymes involved in glycolysis and
Regulated secretory exocytosis is performed by special- gluconeogenesis, and serves as the site of assembly of
ised cells (e.g. endocrine or exocrine gland cells, or nerve sugars, fatty acids, nucleotides and amino acids into com-
cells), in which hormones and other secretory products plex organic molecules. In particular, a large portion of
are stored in the form of secretory granules. The release the protein biosynthetic pathway takes place within the
2+
of these granules is Ca -dependent and is controlled by cytosol. Structural proteins that contribute to the stabil-
hormonal and neural mechanisms (e.g. the liberation of ity, plasticity and motility of the cell are formed here.
zymogen granules by chief cells of the gastric mucosa or The cell matrix has a high binding affinity for water and
acinar cells of the pancreas). electrolytes.
In addition to enzymes, the cytosol contains dissolved
ENDOSOMAL SYSTEM proteins that confer upon the matrix the quality of a gel.
Within the cell, endocytosed vesicles become associated The gel constantly changes its physical state by temporar-
with a system of membrane-bound compartments known ily entering a liquid phase (solgel state).
as endosomes. Early endosomes are usually located While molecular components of cellular metabolism
peripherally. Those located deeper in the cytoplasm (late are typically not morphologically observable, individual
endosomes) generally mature into lysosomes. structures associated with the storage of products of
Two models exist to explain the endosomal system. intermediate metabolism can be visualised with micros-
According to one, endosomes are a stable component of copy. In the synthesis of fat (triglycerides), fatty acids are
the cytoplasm, while in the other they are formed from stored in lipid droplets. Glycogen, a storage form of poly-
endocytotic vesicles. Early endosomes manifest as a merised glucose, can be appreciated morphologically as
tubulovesicular structure, subdivided into cisternae by glycogen granules. Structural proteins and lysosomes can
invagination of their membranes. Proteins are transported also be identified.
from early endosomes to late endosomes by endosomal
vesicles referred to as multivesicular bodies. These con-
tain acid hydrolases, and are found in abundance in liver
cells, epithelial cells of the epididymis and in Type II alveo-
lar epithelial cells in the lung. After undergoing additional
biochemical transformation, late endosomes (also referred
to as prelysosomes) mature into lysosomes.
The primary function of endosomes is the sorting
and recovery of proteins within the endocytotic vesicular
transport system. Ligands are separated from receptors
and the resulting components are directed into processing
pathways for recycling or degradation.

Intracellular metabolism
Intracellular metabolic processes take place either in the
cell matrix or in association with the cellular organelles,
particularly ribosomes and the membrane-bound com-
partments of the endoplasmic reticulum. The nucleus
plays a central role in coordinating the activity of these
(and other) organelles.
Mitochondria and the Golgi apparatus also participate
in intracellular metabolism, as do lysosomes and peroxi- 1.7 Components of lysosome biogenesis in a mac-
somes (see below). rophage (x10,000).

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