Milk Secretion

Lactation involves the production of milk by epithelial cells that line the expanded terminal ends, or alveoli, of an intricate system of ducts. Milk is formed as a mixture of two primary phases: an aqueous phase (including water, electrolytes, proteins, and sugars) released from small vesicles that migrate to the cell surface, and a lipid phase that forms by the coalescence of lipid droplets and is released from the cells as membrane-bound fat globules.[3'4] Most constituents are synthesized within the epithelial cells themselves, but some are transported from blood across the mammary epithelial cells or pass into milk by extracellular routes. Differences in the rates of secretion of the different phases, as well as in the rates of synthesis and/or transport of particular constituents, result in wide variation in milk composition.

Mammary glands do not attain structural or physiological maturity until the onset of reproduction, and thereafter undergo cyclical proliferation and regression. Milk secretion begins shortly before parturition (or egg-laying in monotremes), and is substantially upregulated in the postnatal period. Milk production usually rises to a peak in well-fed mammals and then declines as the offspring switch to solid foods. In some mammals, such as many species of true seals, milk production appears to remain at high levels until the young are abruptly weaned.

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