mammary gland

The mammary gland is the milk-making organ inside the breast, a branching network of glands and tubes shaped a bit like a tree: leaf-like clusters that make milk, connected by ducts that carry it to the nipple. It is a remarkable example of an organ that is mostly built and switched on by hormones rather than at birth.

It is a modified exocrine gland that develops in stages under hormonal direction. At puberty, estrogen promotes duct growth and progesterone supports the gland-forming clusters. During pregnancy, a surge of estrogen, progesterone, prolactin and placental hormones drives the breast to mature its milk-producing alveoli. After birth, prolactin sustains milk synthesis and oxytocin drives its ejection.

Functionally, each milk-secreting unit is a cluster called an alveolus, lined by secretory cells and wrapped in contractile myoepithelial cells. The secretory cells make milk; the myoepithelial cells squeeze it out during let-down.

A caveat: because the mammary gland is so hormone-responsive, it is also a key site of hormone-related disease. The same estrogen and progesterone pathways that build it can drive abnormal growth, which is why breast tissue is central to discussions of hormone exposure and cancer risk.