Structure of Gastric Mucosa
There are about 20,000 depressions or gastric pits per square centimeter of gastric mucosa. Each pit receives secretions from three to seven gastric glands. The glands comprise several types of cells (see top figure). Chief (peptic) cells are located near the base of the glands and secrete their inactive precursor of pepsin, pepsinogen. Parietal (oxyntic) cells are distributed along the length of the gland and secrete HCl and intrinsic factor, which the small intestine needs to absorb vitamin B12. Intrinsic factor is released by the same substances that stimulate HCl secretion from the parietal cells. The stomach antrum lacks parietal cells, but chyme has been acidified by the time it gets there (see bottom figure). G cells of the lower part of the stomach secrete gastrin. Surface mucosa cells in the pyloric region secrete a thick, alkaline-rich mucus that protects the epithelium of the stomach and duodenum from harsh acid conditions of the lumen. This is known as the gastric mucosal
barrier. These cells are stimulated by mechanical and chemical irritation and parasympathetic inputs. This protective mucus barrier can be damaged by bacterial and viral infection, certain drugs, and aspirin.