Acinus

An acinus (plural acini) is a small sac-like structure that serves as a functional unit in several biological systems. In anatomy, the term is used most often to describe the secretory unit of exocrine glands, where clusters of secretory cells surround a central lumen and release their product into a duct system. Acini may be serous, producing a watery enzyme-rich secretion, or mucous, producing mucus; many glands contain a mixture of both. Secretory cells are typically organized around the lumen and may be accompanied by myoepithelial cells that help expel contents.

In the pancreas, acinar cells form the exocrine component and synthesize digestive enzymes such as amylase, lipase, and proteases, stored in zymogen granules. These enzymes are released into small ducts that converge into the main pancreatic duct, contributing to digestion in the small intestine. In salivary glands, acini mirror this arrangement with serous acini in the parotid gland and mucous acini in the sublingual gland, supplying saliva that begins carbohydrate digestion and lubricates the mouth.

In the respiratory system, the acinus refers to the terminal respiratory unit, comprising respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. This unit is the primary site of gas exchange, with oxygen and carbon dioxide diffusing across the alveolar walls and into pulmonary capillaries.

Pathology can involve acini through conditions such as acinar cell carcinoma, a rare pancreatic cancer arising from acinar cells. See also alveolus and secretory unit.