Initiation of Lactation

Pregnancy transforms the mammary gland from a simple ductal tree into a highly efficient exocrine organ with expansive lobuloalveolar structures. This transformation is hormonally regulated and involves changes in the cellular composition of the mammary gland and alterations in the structural, cellular, and biochemical properties of alveolar cells that are critical to development of efficient solute transport and secretory functions. Alveolar epithelial cells begin to differentiate into secretory cells at midpregnancy in most species. The differentiation process occurs het-erogeneously and has been divided into initiation and activation phases based on differences in the composition of mammary secretions, gene expression, and structural and functional properties of alveolar cells. Alveolar cells become capable of limited secretion of some milk components during the initiation phase, which in humans is detected by measurement of increased concentrations of lactose and a-lactalbumin in the plasma. Copious milk secretion, however, is induced during the secretory activation phase (sometimes called lactogenesis II) that occurs in response to the decrease in serum progesterone levels. In rodents and ruminants, this decrease is closely associated with parturition; in humans it occurs after parturition.

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