Solutes enter milk through five general pathways (Figure 2). Endogenously generated substances, including the major milk proteins, oligosaccharides, and nutrients such as lactose, citrate, phosphate, and calcium, are secreted through an exocytotic pathway (pathway I). Lipids and lipid-associated proteins are secreted by a process that is unique to mammary epithelial cells (pathway II). The transcytosis pathway (pathway III) transports a wide range of macro-molecular substances derived from serum or stromal cells, including serum proteins such as immunoglo-bulins, albumin, and transferrin; endocrine hormones such as insulin, prolactin, and insulin-like growth factor-1; and stromal-derived agents such as IgA, cytokines, and lipoprotein lipase. In addition, various membrane transport pathways (pathway IV) exist for the transfer of ions and small molecules, such as glucose, amino acids, and water, across basal and apical plasma membranes. Finally, there is a paracellular pathway (pathway V) that provides a direct route for entry of serum and interstitial substances into milk. This pathway, however, closes during the first few days of lactation in the human. Transport through these pathways is affected by the functional state of the mammary gland and regulated by direct and indirect actions of hormones and growth factors. The general cellular and physiological properties of these pathways are summarized next.
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