Scattered in the mucosa of the GI tract and in the pancreas area variety of endocrine cells which secrete peptide and amine hormones such as gastrin, secretin, CCK, and others. The hormones constitue a balanced system of agonists and antagonists which collectively regulate and coordinate most aspects of GI activity in concert with the autonomic nervous system.

Many of these cells can be stained specifically with chromium salts, and thus are called enterochromaffin cells (arrow). This term is currently applied only to a specific cell type which is responsible for the secretion of serotonin (5-hydroxytryptamine).