In the stomach wall, gastric glands contain secretory cells. The opening to the glands is the gastric pit. One set of cells (parietal cells, mucous neck cells, and chief cells) secrete chemicals that continue the process of digestion and protect the stomach lining. Parietal cells secrete HCl (gastric acid) and intrinsic factor, which complexes vitamin B12. Gastric acid unfolds (denatures) proteins, exposing their peptide bonds to proteases. Mucous neck cells secrete bicarbonate and mucus, forming a layer that protects the stomach lining. Chief cells secrete pepsinogen, which is converted to active pepsin by acid hydrolysis. Pepsin begins hydrolysis of denatured proteins.
Another set of cells (G cells and ECL cells) secrete hormones that regulate stomach activity. G cells, which are concentrated in the antrum and pylorus at the exit of the stomach, produce gastrin. ECL cells (enterochromaffin-like) secrete histamine that acts as a paracrine to locally stimulate HCl secretion.
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