Mammary glands exhibit a number of changes during pregnancy in preparation for lactation. Lymphocytes and plasma cells infiltrate the loose connective tissue as the glandular tissue develops. As the cells of the glandular portion proliferate by mitotic division, the ducts branch and alveoli begin to develop at their growing ends. Alveolar development becomes most prominent in the later stages of pregnancy, and accumulation of secretory product takes place in the alveoli. At the same time, lymphocytes and plasma cells become prominent in the loose connective tissue of the developing lobules. Myoepithelial cells proliferate between the base of the epithelial cells and the basal lamina in both the alveolar and the ductal portion of the glands. They are most prominent in the larger ducts.
Both merocrine and apocrine secretion are involved in the production of milk. The protein component is synthesized, concentrated, and secreted by exocytosis in a manner typical for protein secretion. The lipid component begins as droplets in the cytoplasm that coalesce into large droplets in the apical cytoplasm of the alveolar cells and cause the apical plasma membrane to bulge into the alveolar lumen. The droplets are surrounded by a thin layer of cytoplasm and are enveloped in plasma membrane as they are released.
The initial secretion in the first days after birth is called colostrum. This premilk is an alkaline secretion with a higher protein, vitamin A, sodium, and chloride content than milk and a lower lipid, carbohydrate, and potassium content. Considerable amounts of antibodies are contained in colostrum, and these provide the newborn with passive immunity to many antigens. The antibodies are produced by the plasma cells in the stroma of the breast and are carried across the glandular cells in a manner similar to that for secretory IgA in the salivary glands and intestine. A few days after parturition, the secretion of colostrum stops and lipid-rich milk is produced.
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