acini

Acini, the plural of acinus, are small, rounded sac-like structures that serve as the secretory endpieces of many exocrine glands and, in pulmonary anatomy, as the distal gas-exchanging units of the lung. They consist of secretory cells arranged around a central lumen and are typically connected to a ductal system that transports their product to larger ducts or, in the lung, to airspaces.

In exocrine glands, acini are the primary sites of secretion. The secretory cells, called acinar cells, can be serous, mucous, or mixed. Serous cells produce a watery, enzyme-rich fluid; mucous cells secrete mucus. Mixed acini contain both cell types, sometimes with serous demilunes covering mucous cells. Myoepithelial cells may surround acini to facilitate secretion, and intercalated ducts receive the secretions from acini before they join larger ducts.

Pancreas serves as a key example of acinar function. Here the exocrine acini produce digestive enzymes such as amylase, lipase, and proteases, stored in zymogen granules and released into a network of ducts that lead to the duodenum. In salivary glands, acini similarly form the primary secretory units, with serous acini dominating in some glands (e.g., parotid) and mucous acini in others (e.g., sublingual), contributing to enzymatic and mucous components of saliva.

In the lung, the acinus comprises the respiratory bronchiole, the accompanying ductules, and the alveolar sacs and alveoli. This unit is the site of gas exchange and is central to studies of pulmonary physiology and pathology, including emphysema and other forms of parenchymal disease.