Chapter 3 - Physiology of Lactation

Abstract

Abstract Lactation represents the completion of the reproductive cycle and occurs as one of the major stages of mammary gland development: embryogenesis; mammogenesis; lactogenesis, or secretory differentiation (stage 1 lactogenesis) and secretory activation (stage II lactogenesis); lactation (or stage III lactogenesis), or full milk secretion; and involution. Hormones play a central role in mammary gland development and lactation (estrogen and progesterone, prolactin, insulin, hydrocortisone, human placental lactogen, human growth hormone, oxytocin). Milk ejection is both a neural and endocrinologic process, whereby suckling stimulates sensory nerve endings in the areola and nipple, which activates the afferent neural reflexes leading to secretion and release of prolactin and oxytocin. Lactation changes the mother’s metabolism greatly, redistributing the blood supply and increasing the demand for nutrients. Milk synthesis and secretion in the mammary alveolus include four major transcellular pathways and one paracellular pathway: exocytosis of milk protein and lactose; milk-fat secretion via the milk-fat globule; secretion of ions and water across the apical membrane; pinocytosis/exocytosis of immunoglobulins; and the paracellular pathway for plasma components and leukocytes.