Page 224 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

206 Veterinary Histology of Domestic Mammals and Birds

PYLORIC GLANDS (GLANDULAE PYLORICAE) gland lumen occurs via H / K -ATPase proton pumps. For
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VetBooks.ir rus, extending to a varying extent into the body of the basal surface of the cell to be taken up by the bloodstream.
The relatively short pyloric glands are located in the pylo-
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each transported H ion, a HCO ion is exchanged at the
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stomach (Figure 10.29). They typically open into elon-
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Cl ions function optimally in the pH range 1.8–3.5; this
gated, dilated gastric pits (Figure 10.34). Pyloric glands are range is achieved through the buffering effect of gastric
branched and extensively coiled. The deep portion of the mucus and saliva.
lumen is usually expanded. The glandular cell (exocri-
nocytus pyloricus) is cuboidal to columnar with an oval Regulation of gastRic juice PRoduction
nucleus (may be flattened due to accumulation of mucus). Control of gastric acid secretion can be divided into
These cells primarily secrete a mucous substance that also cephalic, gastric and intestinal phases.
contains lysozyme. Characteristic of the glandular epi- In the cephalic phase, the senses of vision, olfaction
thelium is the presence of endocrine G cells. Release of and gustation influence the motor and secretory activ-
gastrin from G cells into the bloodstream stimulates the ity of the stomach. Activation of the vagus nerve via the
activity of parietal cells. nucleus dorsalis induces secretion of gastrin by G cells in
The histological features of the gastric glands are sum- the pyloric antrum. Concurrently, H and pepsinogen are
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marised under Table 10.2. secreted by parietal and chief cells.
The gastric phase is the period during which ingesta
GASTRIC MUCUS are present in the stomach. During this phase, gastric
Gastric mucus comprises the secretory product of surface secretion is influenced by vagal, hormonal (gastrin, his-
epithelial cells, mucous neck cells and other gastric gland tamine), mechanical, chemical (amino acids, protein rich
cells (Figure 10.36). It is composed of neutral long-chain nutrients) and immune factors. Excessively acid condi-
glycoproteins with amino sugar, hexose and sialic acid tions in the stomach have an inhibitory effect on gastrin
residues. The highly viscous mucus produced by surface secretion.
cells forms the layer that lies on the epithelium. This is Acetylcholine, gastrin and histamine are key transmit-

overlaid by a less viscous layer produced by the gastric ters in the control of gastric secretion. The activity of
glands. these molecules is controlled in turn by mediators such
The protective function of the gastric mucus as prostaglandins and leukotrienes. In addition to regulat-
includes buffering of free H ions arising from the secre- ing gastrin secretion by G cells, the vagus nerve influences
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tion of carbonic acid by the tunica mucosa. Under normal blood supply to the stomach. Direct contact of dietary
physiological conditions, the gastric mucus provides an peptides with the gastric mucosa has a stimulatory effect.
effective barrier against the denaturing effects of gastric Acting as antigens, these induce local immune responses
acid. Factors favouring acid production can disrupt this that are mediated by products of lymphatic cells within
relationship, resulting in auto-digestion of the gastric the tunica mucosa (e.g. leukotrienes, gamma-interferon,
mucosa. Production of gastric mucus is regulated by lymphokines) and via paracrine signalling by mast cell
neural, hormonal and immune mechanisms. secretions (histamine, prostaglandins and leukotrienes).
Together, these responses give rise to increased acid and
GASTRIC JUICE pepsinogen secretion. Protection of the gastric mucosa
The components of gastric juice include organic and against decreasing pH (cytoprotection) is facilitated by
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inorganic substances. Of greatest significance are H , Cl , concurrent stimulation of gastric mucus production and
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Na , K , Mg , HPO , SO , pepsin, lipase, water and, in increased mucosal perfusion. The combination of these
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young animals, rennin. Gastric juice also mixes with gas- various regulatory mechanisms preserves the functional
tric mucus. The production of H and Cl involves special integrity of the mucosa.
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features at the internal surface of the plasmalemma of In the intestinal phase, the stomach contents induce
parietal cells. secretion of gastrin by the small intestinal mucosa. The
Upon stimulation of parietal cells (e.g. by gastrin enterogastric reflex results in a gradual decrease in gastric
or histamine) the tubulovesicular system (see above) motility. Gastric acid secretion is reduced by inhibitory
transforms into a secretory component of the intracel- substances such as cholecystokinin and gastric inhibitory
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lular canaliculi. Evidence indicates that H and Cl are peptide (GIP).
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actively transported against a concentration gradient into
the lumen of the gland. Carbonic acid, H CO is formed Ruminant forestomach
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within the parietal cell from CO and H O in a reaction The primary function of the ruminant forestomach is the
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catalysed by the enzyme carbonic anhydrase. Carbonic enzymatic cleavage of vegetable matter by microbial flora
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acid dissociates into H and HCO . Transport of H ions to form predominantly short-chain fatty acids. These are
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through the walls of the intracellular canaliculi into the taken up, together with other products of microbial metab-
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