The female breast is structured to produce and deliver milk to an infant. Inside the breast tissue, branching networks of lactiferous ducts converge toward the nipple, ending in clusters of secretory cells known as alveoli. These alveoli are the functional units where milk is synthesized and stored. Surrounding each alveolus are myoepithelial cells, specialized muscle cells that contract to push milk into the ductal system.
During pregnancy, rising hormone levels drive mammogenesis, preparing the glandular tissue for milk production. Estrogen stimulates the growth and branching of the ductal system, while progesterone promotes the development of the alveolar secretory structures. High levels of circulating estrogen and progesterone during gestation inhibit actual milk secretion, keeping the gland in a preparatory state until delivery occurs.
Lactogenesis is the process through which the mammary gland becomes competent to secrete milk. It is divided into distinct phases occurring before and after parturition.
Two key pituitary hormones govern the production and release of breast milk, working through a delicate balance of supply and demand.
Prolactin, secreted by the anterior pituitary, drives milk synthesis in the alveolar cells. Every time an infant suckles, sensory impulses travel from the nipple to the hypothalamus, causing a sharp surge in prolactin levels that signals the alveoli to manufacture milk for the next feeding.
Oxytocin, released by the posterior pituitary, is responsible for milk ejection. The baby's suckling triggers oxytocin release, which travels through the bloodstream to contract the myoepithelial cells surrounding the alveoli, squeezing milk down into the ducts.
The milk ejection reflex, commonly known as the let-down reflex, is a neuroendocrine response. Sensory nerve endings in the nipple are stimulated by the infant's suckling. These signals travel via the spinal cord to the hypothalamus, prompting the posterior pituitary gland to release oxytocin into the bloodstream.
This reflex can be conditioned and is easily influenced by maternal emotions. Stress, anxiety, or pain can trigger the release of epinephrine, which constricts mammary blood vessels and blocks oxytocin from reaching the breast. Conversely, the sight, sound, or thought of the baby can trigger spontaneous milk let-down.
Galactopoiesis is the maintenance of established milk production. Once lactation is successfully initiated, continued milk secretion relies primarily on autocrine control—the supply-and-demand mechanism.
A protein called Feedback Inhibitor of Lactation (FIL) plays a central role here. When milk accumulates and is not removed from the alveoli, FIL builds up and suppresses further milk synthesis by the secretory cells. Regular removal of milk through nursing or expressing clears FIL and prompts the alveoli to produce more milk. If milk removal stops, increased pressure and FIL accumulation lead to involution of the mammary tissue.
Human breast milk changes dynamically over time to meet the growing infant's nutritional and immunological requirements.
Various complications can disrupt the normal physiology of lactation, causing distress for both mother and infant.