Alveoli in Breast Tissue
Alveoli are the milk-producing glandular structures in the breast that develop during pregnancy and serve as the primary sites of milk secretion during lactation. 1
Structural Definition
Alveoli (also called acini) are saccular outpouchings from the ductal system that appear as small, grape-like clusters throughout the breast tissue. 2, 3 These structures represent the functional endpoint of mammary gland development, where actual milk production occurs. 4
Development and Formation
During pregnancy, thousands of alveoli sprout from the ductal system in mid to late gestation, transforming the breast from a simple branched duct network into a milk-producing organ. 2, 3
Alveoli are lined by milk secretory cells (lactocytes) that produce and release milk components into the alveolar lumen. 1
Myoepithelial cells wrap around each alveolar unit like a basket, contracting to forcibly expel milk from the alveolar lumen into the ductal system during milk ejection. 1, 5
Functional Role in Lactation
Alveoli become distended with milk after delivery when prolactin stimulates secretory changes and the lobular acini fill with milk. 1
The alveolar structures secrete milk that is then propelled through the branched ductal system to the nipple by coordinated myoepithelial cell contraction. 4
Mammary gland stem cells can differentiate into alveolar, ductal, and myoepithelial cells during lactogenesis, supporting the formation of new mammary tissue. 1
Clinical Relevance
During pregnancy and lactation, the massive proliferation of alveolar structures causes increased breast volume, firmness, and nodularity, making clinical breast examination more challenging. 1, 6
After weaning, alveoli undergo involution with massive epithelial cell death and tissue remodeling over approximately 12-18 months postpartum. 6
The distinction between terminal ductal structures varies by species: terminal ductal lobular units in humans, lobules and alveolar buds in rats, and terminal ducts in mice. 1