Chapter 46 - Lactation and its Hormonal Control

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The mammary gland undergoes its final development in the adult animal, filling the mammary fat pad with a ductal tree and rudimentary alveoli during puberty, and expanding and fully differentiating during pregnancy. The hormones of pregnancy, prolactin (PRL), placental lactogen, growth hormone, and progesterone (P4) cooperate to produce a gland that is fully developed but nonfunctional. Secretory activity commences around parturition, with the withdrawal of progesterone and maintained levels of PRL and glucocorticoid. During lactation PRL provides a comprehensive signal that fosters synthesis and secretion of milk components and the survival of the alveolar cell. The lactating gland produces milk of a composition defined for the species using several specialized pathways including: (1) exocytosis for the secretion of milk proteins, lactose and divalent ions; (2) a unique lipid secretion pathway that produces membrane-bound milk fat globules; (3) transport systems for monovalent ions, glucose, and amino acids; and (d) transcytosis for the secretion of immunoglobulins and other milk components. Tight junctions form a gasket around the apical surface of the epithelial cells that is open to traffic of large and small molecules in pregnancy but tightly closed in lactation. The volume of milk produced is determined by milk removal from the gland, a function dependent on oxytocin secretion by the posterior pituitary and contraction of myoepithelial cells to force milk out of the alveoli. With the termination of milk removal an orderly involution process involving interactions with immune cells returns the gland to its resting state. In short, we provide a summary of our present understanding of the cellular and molecular biology of mammary development and milk secretion in the context of the whole body mechanisms that ensure adequate flux of nutrients to the gland to provide sufficient milk to meet the needs of the neonate.