The mammary gland is the defining characteristic of mammals, responsible for producing milk to nourish offspring. These are exocrine glands, meaning they secrete a substance—in this case, milk—out of the body. While most associated with females, rudimentary glandular tissue exists in both sexes. The gland is not fully developed at birth and undergoes significant changes in size and function throughout different life stages.
Anatomy of the Mammary Gland
The external anatomy of the mammary gland consists of the nipple and the surrounding areola. Internally, the gland is a complex network of tissues designed for milk production and transport. The primary structures are lobes, and each breast contains approximately 10 to 20 of these. These lobes are subdivided into smaller structures called lobules, which house the milk-producing units.
Within the lobules are the alveoli, which are tiny, hollow sacs lined with milk-secreting cells called cuboidal cells. The gland’s structure is often compared to a bunch of grapes; the alveoli represent the grapes, and the ducts that carry the milk represent the stems. This network of ducts merges into larger channels, eventually leading to the nipple.
Surrounding the alveoli are myoepithelial cells, which are muscle-like cells that contract to squeeze milk out of the alveoli and into the ductal system. The entire glandular structure is embedded within a matrix of fat and connective tissue, which determines the size and shape of the breast. A portion of this tissue, known as the tail of Spence, can extend into the axillary region, or armpit.
Hormonal Control and Lactation
The function of the mammary gland is regulated by the endocrine system. During pregnancy, elevated levels of estrogen and progesterone stimulate the growth and branching of the ductal system and the development of the alveoli. Progesterone also has an inhibitory effect on prolactin, preventing milk production before the baby is born.
Following childbirth, a sharp drop in progesterone levels allows the hormone prolactin to initiate and sustain milk synthesis. The glands first produce colostrum, a nutrient-dense pre-milk substance rich in antibodies that helps protect the newborn. As nursing continues, the composition transitions to mature milk to meet the infant’s nutritional demands.
The process of milk release is governed by the milk ejection reflex, commonly known as the “let-down.” When an infant suckles at the nipple, nerve signals are sent to the brain, triggering the pituitary gland to release the hormone oxytocin. Oxytocin travels to the breast, where it causes the myoepithelial cells surrounding the alveoli to contract. This contraction pushes the stored milk into the ducts.
Lifecycle of the Mammary Gland
The mammary gland undergoes distinct changes throughout an individual’s life. At birth, the gland consists of only rudimentary ducts and remains immature through childhood, with little structural difference between males and females. At puberty, hormonal shifts cause the female mammary gland to develop more rapidly.
The most significant transformation occurs during pregnancy and lactation, when the gland reaches full maturity. This functional state is temporary; upon the cessation of lactation, a process called involution begins. During involution, the milk-producing alveoli regress, and much of the glandular tissue returns to a resting state.
This cycle of expansion and regression can occur multiple times throughout a woman’s reproductive years. Later in life, as a woman approaches menopause, the glandular tissue undergoes further changes. The ductal system’s tissues become more fibrous and degenerate, leading to a natural shrinkage as the gland is replaced by fat. Following menopause, the gland loses its capacity to produce milk.