Acinar

Acinar is an anatomical term used to describe glandular tissue organized into sac-like secretory units called acini (singular: acinus). An acinus consists of secretory cells surrounding a small duct and is typically connected to a duct system that conveys its secretions to a larger duct.

In the pancreas, acini are the exocrine component of the gland. They are formed by serous acinar cells that produce digestive enzymes stored in zymogen granules. These enzymes are released into tiny ducts that merge into the pancreatic duct, where secretions are delivered to the duodenum. Enzyme production and secretion are regulated by hormones such as secretin and cholecystokinin.

In the salivary glands, acini are present as serous and mucous acini. Serous acini secrete a watery, enzyme-rich fluid containing amylase, while mucous acini produce mucins. Many salivary glands contain mixed acini, with both serous and mucous components. Secretions pass from acini into intercalated and intralobular ducts before reaching larger salivary ducts.

In the lungs, the term is used in the concept of the pulmonary acinus, the functional gas-exchange unit. A respiratory acinus includes the respiratory bronchioles, the alveolar ducts, and the alveolar sacs with alveoli. Gas exchange occurs across the alveolar-capillary membrane, and the acinus is supplied by a network of pulmonary vessels.

In pathology, acinar-related terms describe certain neoplasms and conditions. Acinar cell carcinoma is a rare pancreatic tumor arising from acinar cells and characterized by acinar differentiation with zymogen granules. Acinar atrophy can occur in the pancreas, reducing exocrine function. Overall, the term acinar denotes a grape-like cluster of secretory cells across multiple organ systems.