Figure 2234

Photomicrograph of an active mammary gland during late pregnancy. a. This low-magnification H&E-stained specimen shows the marked proliferation of the duct system, giving rise to the secretory alveoli that constitute the major portion of the lobules. The intralobular ducts are difficult to identify, as their epithelium also secretes.

Outside the lobules is a large excretory duct. x60. b. A higher magnification of an area in a. The secretory alveolar cells are mostly cuboidal here. A myoepithelial cell (mEp) as well as a number of plasma cells (arrows) can be identified in the adjacent loose connective tissue. x700.

dense lysosomes (Fig. 22.35). Depending on the secretory state, large lipid droplets and secretory vesicles may be present in the apical cytoplasm. The secretory cells produce two distinct products that are released by different mechanisms:

• Merocrine secretion. The protein component of the milk is synthesized in the rER, packaged into membrane-limited secretory vesicles for transport in the Golgi apparatus, and released from the cell by fusion of the vesicle's limiting membrane with the plasma membrane.

• Apocrine secretion. The fatty or lipid component of the milk arises as lipid droplets free in the cytoplasm. The lipid coalesces to form large droplets that pass to the apical region of the cell and project into the lumen of the acinus. The droplets are invested with an envelope of plasma membrane as they are released. A thin layer of cytoplasm is trapped between the plasma membrane and lipid droplet and is released with the lipid, but the cytoplasmic loss in this process is minimal.

The secretion released in the first few days after childbirth is known as colostrum. This premilk is an alkaline, yellowish secretion with a higher protein, vitamin A, sodium, and chloride content and a lower lipid, carbohydrate, and potassium content than milk. It contains considerable amounts of antibodies that provide the newborn with some degree of passive immunity. The antibodies in the colostrum are believed to be produced by the lymphocytes and plasma cells that infiltrate the loose connective tissue of the breast during its proliferation and development and are secreted across the glandular cells as in salivary glands and intestine. As these wandering cells decrease in number after parturition, the production of colostrum stops, and lipid-rich milk is produced.

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