The two main types of exocrine secretory cells of the stomach are parietal cells and chief cells. Parietal cells secrete hydrochloric acid and chief cells secrete digestive enzymes such as pepsin. These cells secrete their products when activated by signals from the body such as hormones and neurotransmitters.
Parietal cells are the exocrine cells of the stomach that secrete hydrochloric acid (HCl). The acid which is secreted by the cells are able to help in the digestion of food by causing them to unfold. Parietal cells secrete HCl at a concentration of 160 mM, which is a pH of 0.8. The secretion of the parietal cells contains 3 million times more hydrogen ions than there are hydrogen ions in the bloodstream.
Parietal cells secrete hydrochloric acid when stimulated by hormones such as gastrin, molecules such as histamine (which causes allergies) and neurotransmitters from nerve cells such as acetylcholine.
Chief cells are the other type of exocrine secretory cell in the stomach. They secrete digestive enzymes that cleave the proteins in food into smaller pieces. Pepsin is secreted as an inactive enzyme called pepsinogen which is secreted by the chief cell. Pepsinogen becomes active when it encounters an acidic environment and is cut apart. Chief cells start secreting digestive enzymes when they are activated by hormones and neurotransmitters. Activating hormones include secretin, vasoactive intestinal peptide and gastrin.