The act of suckling during breast-feeding initiates sensory impulses from receptors in the nipple to the hypothalamus. The impulses inhibit the release of prolactin-inhibiting factor, and prolactin is then released from the adenohypophysis. The sensory impulses also cause the release of oxytocin in the neurohypophysis. Oxytocin stimulates the myoepithelial cells that surround the base of the alveolar secretory cells and the base of the cells in the larger ducts, causing them to contract and eject the milk from the alveoli and the ducts. In the absence of suckling, secretion of milk ceases, and the mammary glands begin to regress and atrophy. The glandular tissue then returns to an inactive condition.
Involution of the Mammary Gland
The mammary gland atrophies or its specialized stroma involutes after menopause. In the absence of ovarian hormone stimulation, the secretory cells of the TDLUs degenerate and disappear, leaving only ducts to create a histologic pattern that resembles that of male breast. The connective tissue also demonstrates degenerative changes, marked by a decrease in the number of fibroblasts and collagen fibers, and loss of elastic fibers.
FIGURE 23.35 • 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 because their epithelium also secretes. Outside the lobules is a large excretory duct. 60. 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. 700.
 FOLDER 23.6
Clinical Correlation: Cervix and Human Papillomavirus (HPV) Infections