 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 (mainly secretory IgA) 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.
Hormonal Regulation of the Mammary Gland
The initial growth and development of the mammary gland at puberty occur under the influence of estrogens and progesterone produced by the maturing ovary. Under hormonal influence, the TDLUs develop and differentiate into dynamic functional units. Subsequent to this initial development, slight changes in the morphology of the glandular tissue occur during each ovarian cycle. During the follicular phase of the menstrual cycle, estrogen in the circulation stimulates proliferation of the lactiferous duct components. After ovulation in the luteal phase, progesterone stimulates growth of alveoli; intralobular stroma become edematous. Clinically, during the luteal phase, women perceive tenderness and a progressive increase of breast tissue mass. During pregnancy, the corpus luteum and placenta continuously produce estrogens and progesterone, causing a massive increase in TDLUs. It is now believed that the growth of the mammary glands also depends on the presence of prolactin, which is produced by the adenohypophysis; hCS, which is produced by the placenta; and adrenal glucocorticoids.
FIGURE 23.34 • Photomicrograph of an inactive mammary gland. a. This low-magnification H&E–stained specimen shows several lobules within the dense connective tissue of the breast. The epithelial component consists of a branching duct system that makes up the lobule. The clear areas (arrows) are adipose cells. 60. b. A higher magnification of the area in the rectangle of a. The epithelial cells of the ducts are columnar and exhibit interspersed lymphocytes (arrows) that have entered the epithelium. The surrounding stained material (arrowheads) represents the myoepithelial cells (MEp) and collagen bundles in the adjacent connective tissue. 700.
Lactation is under the neurohormonal control of the adenohypophysis and hypothalamus.
Although estrogen and progesterone are essential for the physical development of the breast during pregnancy, both of these hormones also suppress the effects of prolactin and hCS, the levels of which increase as pregnancy progresses. Immediately after birth, however, the sudden loss of estrogen and progesterone secretion from the placenta and corpus luteum allows prolactin to assume its lactogenic role. Production of milk also requires adequate secretion of growth hormone, adrenal glucocorticoids, and parathyroid hormones.