An acinus (//; plural, acini; adjective, acinar // or acinous) refers to any cluster of cells that resembles a many-lobed "berry," such as a raspberry (acinus is Latin for "berry"). The berry-shaped termination of an exocrine gland, where the secretion is produced, is acinar in form, as is the alveolar sac containing multiple alveoli in the lungs.
Acinar exocrine glands are found in many organs, including:
The thyroid follicles can also be considered of acinar formation but in this case the follicles, being part of an endocrine gland, act as a hormonal deposit rather than to facilitate secretion.
Mucous acini usually stain pale, while serous acini usually stain dark.
The end of the terminal bronchioles in the lungs mark the beginning of a pulmonary acinus that includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.