Lactation in humans.

Postpartum milk secretion is a hormonally regulated process that reflects the highly coordinated actions of endocrine, neuroendocrine, and behavioral mechanisms. Structural development of the mammary gland during pregnancy is stimulated primarily by estrogen, progesterone, and prolactin (PRL), the secretion of which is controlled directly or indirectly by the fetoplacental unit. Following parturition, PRL also plays an important role in initiating and sustaining milk secretion and suppressing the resumption of cyclic hypothalamic-pituitary-ovarian function. Dynamic postpartum PRL secretion is in response to the neurogenic stimulus of suckling, which both acutely releases PRL and maintains the responsiveness of PRL regulatory pathways to subsequent nursing. The central control of PRL secretion at this time involves the integrated activities of inhibitory and stimulatory hypothalamic mechanisms. A major feature of lactation is the degree to which the entire process is controlled by the developing organism itself.