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Review: Mammary gland development in swine: embryo to early lactation.

Milk production by the sow is a major factor limiting the growth and survival of her litter. Understanding the process of morphogenesis of the sow's mammary gland and the factors that regulate mammary development are important for designing successful management tools that may enhance milk production. Primordia of the mammary glands are first observable in the porcine embryo at approximately 23 days of gestation. The glands then progress through a series of morphologically distinct developmental stages such that, at birth, each mammary gland is composed of the teat, an organized fat pad and two separate lactiferous ducts each with a few ducts branching into the fat pad. The glands continue to grow slowly until about 90 days of age when the rate of growth increases significantly. The increased rate of mammary gland growth coincides with the appearance of large ovarian follicles and an increase in circulating estrogen. After puberty, the continued growth of the gland and elongation and branching of the duct system into the fat pad takes place in response to the elevated levels of estrogen occurring as part of the estrous cycles. After conception, parenchymal mass of each gland increases slowly during early pregnancy and then grows increasingly rapidly during the final trimester. This growth is in response to estrogen, progesterone, prolactin and relaxin. Lobuloalveolar development occurs primarily during late pregnancy. By parturition, the fat pad of the mammary gland has been replaced by colostrum-secreting epithelial cells that line the lumen of the alveoli, lobules and small ducts. All mammary glands develop during pregnancy, however, the extent of development is dependent on the location of the mammary gland on the sow's underline. The mammary glands undergo significant functional differentiation immediately before and after farrowing with the formation of colostrum and the transition through the stages of lactogenesis. Further growth of the glands during lactation is stimulated by milk removal. Individual glands may grow or transiently regress in response to the intensity of suckling during the initial days postpartum. Attempts to enhance milk production by manipulation of mammary development at stages before lactation generally have met with limited success. A more in depth understanding of the processes regulating porcine mammary gland morphogenesis at all stages of development is needed to make further progress.