Physiology of lactation

Abstract

SummaryIn adult life the human breast, unlike that of other species, is sufficiently developed to allow milk secretion to occur after only a brief period of hormonal stimulation. During pregnancy the ducts and secretory alveoli develop under the influence of both oestrogen and progesterone. Growth hormone and placental lactogen are not required; the role of prolactin in mammogenesis remains questionable. Lactogenesis, the onset of milk secretion, is inhibited during pregnancy despite high levels of prolactin, by a direct inhibitory action of steroids on the breast. The clearance of placental steroids after delivery removes this inhibition and milk secretion is initiated.Inhibition of prolactin secretion prevents milk secretion, suggesting that prolactin is the essential hormone for lactation in man. Prolactin release occurs in response to suckling and the amount released depends on the strength and duration of the suckling process. No release of prolactin occurs in response to stimuli other than stimulation of the nipple.Removal of milk from the mammary gland is effected by the milk-ejection reflex (MER) involving the release of oxytocin in response to suckling. Unlike prolactin, oxytocin may be released in response to stimuli associated with breast-feeding, e.g. the cry of the infant. The MER may be inhibited by psychological and physical stress, either by inhibiting oxytocin release or by preventing its action upon the breast contractile elements. The susceptibility of the MER to disturbance requires consideration when encouraging the establishment of breast-feeding.Breast-feeding is geared directly to the needs of the infant, since not only does the suckling infant obtain its present meal by inducing oxytocin release, but also by stimulating prolactin release it orders the next (Tindal, 1972).