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Digestive system (apparatus digestorius) 219

epithelial cells by pathogens. The mucus occupies receptors Secretin, produced by S cells, stimulates production
VetBooks.ir used by microorganisms or binds toxins, thus preventing and secretion of pancreatic juice, NaHCO and bile.
3
Enterochromaffin cells (EC cells) stain with silver and
their adhesion to the epithelial surface. Through its lyso-
zyme content, mucus also has bactericidal properties. In chromium salts. Their granules contain serotonin, which
addition, mucus binds to microorganisms and transports increases gut motility (refer to Chapter 9, ‘Endocrine sys-
them aborally, assisted by peristaltic movements of the tem’ and biochemistry texts.)
intestine.
Paneth cells
endocRine cells Paneth cells, or acidophilic granular cells, are located at the
Endocrine cells are present in the lining of the intesti- base of the intestinal glands. They are characterised by the
nal glands of the proximal small intestine. These cells presence of numerous acidophilic granules that accumu-
synthesise peptide hormones and serotonin (5-hydroxy- late towards the apex of the cell. Paneth cells produce a
tryptamine), which regulate secretion of digestive serous, glycoprotein-rich exocrine secretion that also con-
enzymes and peristaltic activity. Together with similar cells tains lysozyme. This enzyme exerts bactericidal effects by
in the stomach and pancreas, the endocrine cells of the dissolving the bacterial cell wall and thus influences the
intestine (enteroendocrine cells) form the gastroentero- microbial flora of the intestine. Paneth cells are present in
pancreatic (GEP) endocrine system. the horse. They have not been demonstrated definitively in
The intra-epithelial endocrine cells deliver their secre- ruminants and pigs and are absent in carnivores.
tory granules to the immediately adjacent subepithelial
capillary network. Based on immunohistochemical tech- LAMINA PROPRIA MUCOSAE
niques and, in some cases, on affinity for chromium and The lamina propria mucosae consists of loose connective
silver salts, the contents of the cytoplasmic secretory gran- tissue. It forms the core of the intestinal villi (Figures 10.49
ules have been identified as particular peptide hormones and 10.50). Much of the lamina propria is occupied by the
and biogenic amines (e.g. serotonin) or their precursors. intestinal glands. Between the glands, the loose con-
A feature common to these cells is the presence of nective tissue is condensed and houses blood and lymph
usually basally located granules containing amino acid vessels, nerve fibres, myofibroblasts and smooth muscle
decarboxylases. These enzymes are involved in the syn- cells. This layer also contains numerous immune cells, in
thesis of biogenic amines. Endocrine cells that contain aggregates of varying density, and reticular fibres.
amino acid decarboxylases are referred to as APUD cells The microvasculature serves primarily to support
(amine precursor uptake and decarboxylation). These hor- the secretory activity of epithelial cells and to transport
mone-producing cells are characterised by the presence of products of digestion, water, electrolytes and vitamins.
surface receptors and the capacity to secrete polypeptides Accordingly, each villus contains a dense and substantial
in response to an appropriate stimulus. capillary network. An arteriole passes centrally to the
Several specific hormone-producing cell types have tip of each villus where it arborises into capillaries. The
been identified, particularly in the deeper portions of the capillaries anastomose to form a subepithelial network
intestinal glands of the duodenum. In addition to basal that extends along the sides of the villus to its base, where
granules (150–450 nm), features common to these cells blood drains via postcapillary venules into one or more
include a usually spherical nucleus and close contact with venules. These combine to form veins of the submucosal
the basal lamina. Endocrine cells vary between species in vascular plexus. Arteriovenous anastomoses regulate the
their distribution in the gastric and intestinal glands, their flow of blood.
size and shape, their electron density and in the contents Also passing through the centre of each villus is a lym-
of their granules. phatic capillary or lacteal. The lacteal begins at the tip of
D cells synthesise somatostatin, which inhibits the syn- the villus as a blind-ended vessel and opens at the base of
thesis of gastric secretions (e.g. gastrin). D cells also occur the villus into a plexus that joins collecting lymphatic ves-
in the pancreas and gastric mucosa. sels in the tela submucosa (Figure 10.50). The lacteal is
G cells secrete gastrin; in addition to the pars pylorica lined with simple squamous epithelium that is specialised
of the stomach, these are found in the duodenum and jeju- for the uptake of chylomicrons.
num. Gastrin stimulates the secretory activity of parietal The loose tissue of the lamina propria is pervaded
cells and mucous cells of the gastric mucosa, as well as cells by networks of fine autonomic nerve fibres that have
of the intestinal mucosa and pancreas. their perikarya in the plexus nervorum submucosus
I cells produce cholecystokinin (CCK), which pro- (Meissner’s plexus) (Figure 10.50). These nerve fibres
motes production of bile by the liver, contraction of the supply exocrine and endocrine gland cells and regulate
gall bladder and secretion of digestive enzymes by the the function of smooth muscle, including the muscular
pancreas. component of blood vessel walls.

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