Exocrina

Exocrina, in biology, refers to glands and secretions that are discharged through ducts to an epithelial surface, such as the skin or the mucous membranes of the digestive and respiratory tracts. This contrasts with endocrine secretions, which are released directly into the bloodstream to act on distant targets. Exocrine glands secrete substances intended for local effects or digestion, rather than systemic hormonal signaling.

The hallmark of exocrine glands is their ductal system, which carries secretions from the secretory units to a designated surface. Secretory units are commonly organized as acini (clusters of secretory cells) or tubules. Secretions can be serous (watery and enzyme-rich), mucous (rich in glycoproteins), or mixed. Secretory activity is typically regulated by neural inputs and hormonal signals, as well as local stimuli such as the presence of food in the digestive tract.

Common secretion mechanisms include merocrine secretion, where products are released by exocytosis without cell loss; apocrine secretion, where a portion of the cell’s cytoplasm is shed with the product; and holocrine secretion, in which entire cells disintegrate to release their contents.

Examples of exocrine glands include the salivary glands, gastric glands, and the exocrine portion of the pancreas, which releases digestive enzymes into the small intestine. Other exocrine glands include the sweat glands, lacrimal glands, mammary glands, and sebaceous glands. Dysfunction of exocrine secretions can lead to clinical conditions such as pancreatic insufficiency, cystic fibrosis, or obstructive biliary disorders.

Overall, exocrina describes a functional class of glands optimized for local, surface-directed secretion via ducts.