They occur primarily in the body of the glands. They produce pepsinogen, which is a precursor of the proteolytic enzyme pepsin. The pH optimum of of pepsin is about 2. This enzyme is able to break down collagen. They are situated deeper, between and below chief cells, in lower parts of the gland.
Parietal cells secrete the hydrochloric acid of the gastric juice.
Aside from activating the pepsinogen the hydrochloric acid also effectively sterilizes the contents of the stomach. Note that so far only one type of bacteria has found which can live happily in the stomach - Helicobacter pylori . Unfortunately these bacteria are involved in the pathogenesis of gastritis and gastric ulcers. Parietal cell also secrete intrinsic factor , which is necessary for the resorption of vitamin B12. Vitamin B12 is a cofactor of enzymes which synthesise tetrahydrofolic acid, which, in turn, is needed for the synthesis of DNA components.