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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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