Lactation

Lactation is the production and release of milk by the mammary glands, and it is the process by which the mother nourishes the newborn after birth. The mammary glands begin enlarging and undergoing structural differentiation during pregnancy itself, under the combined influence of oestrogen (which promotes growth of the duct system) and progesterone (which promotes development of the milk-secreting alveoli), together with human placental lactogen and, later in pregnancy, prolactin; however, actual copious milk secretion is held in check during pregnancy by the very high circulating levels of oestrogen and progesterone, and begins in earnest only after childbirth, once the delivery of the placenta causes these hormone levels to fall sharply.

Two hormones govern lactation after delivery, each responsible for a different part of the process. Prolactin, secreted by the anterior pituitary, is chiefly responsible for lactogenesis — stimulating the alveolar (secretory) cells of the mammary gland to actually synthesise milk, including its proteins (such as casein and lactalbumin), fats and lactose. Oxytocin, secreted by the posterior pituitary, is chiefly responsible for milk ejection, commonly called the 'let-down' reflex: when the infant suckles at the nipple, sensory nerve endings there send signals to the mother's hypothalamus, which triggers a burst of oxytocin release; the oxytocin acts on specialised myoepithelial cells that surround each milk-secreting alveolus and the small ducts leading from it, causing these cells to contract and actively squeeze stored milk out of the alveoli and along the ducts to the nipple, where the infant can draw it out by suckling. Both hormone releases are stimulated by the same suckling stimulus, so that prolactin (making more milk for the next feed) and oxytocin (releasing the milk already made) are typically triggered together each time the infant feeds, a reflex arrangement that closely couples milk production to the infant's actual demand. …