The stomach is famous for its secretion of acid, but acid is only one of four major secretory products of the gastric epithelium, all of which are important either to the digestive process or to control of gastric function:
- Mucus: The most abundant epithelial cells are mucous cells, which cover the entire lumenal surface and extend down into the glands as "mucous neck cells". These cells secrete a bicarbonate-rich mucus that coats and lubricates the gastric surface, and serves an important role in protecting the epithelium from acid and other chemical insults.
- Acid: Hydrochloric acid is secreted from parietal cells into the lumen where it establishes an extremely acidic environment. This acid is important for activation of pepsinogen and inactivation of ingested microorganisms such as bacteria.
- Proteases: Pepsinogen, an inactive zymogen, is secreted into gastric juice from both mucous cells and chief cells. Once secreted, pepsinogen is activated by stomach acid into the active protease pepsin, which is largely responsible for the stomach's ability to initiate digestion of proteins. In young animals, chief cells also secrete chymosin (rennin), a protease that coagulates milk protein allowing it to be retained more than briefly in the stomach.
- Hormones: The principal hormone secreted from the gastric epithelium is gastrin, a peptide that is important in control of acid secretion and gastric motility.
A number of other enzymes are secreted by gastric epithelial cells, including a lipase and gelatinase. One secretory product of considerable importance in man is intrinsic factor, a glycoprotein secreted by parietal cells that is necessary for intestinal absorption of vitamin B12.
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