Fig. 44.37 ). The lobes in turn are composed of several lobules that contain secretory structures called alveoli and the terminal portions of the ducts. The epithelium of the alveoli and ducts is composed of two cell layers: apical luminal epithelial cells and basal myoepithelial cells. There is strong evidence for the presence of adult mammary stem cells within this epithelium. The luminal epithelial cells of the alveoli are the producers of milk, and the luminal cells of the ducts convey and modify the secreted milk. Myoepithelial cells are stellate smooth muscle–like cells, and contraction of these cells in response to a stimulus (i.e., milk let-down) expels milk from the lumens of the alveoli and ducts. Lobes and lobules are supported within a connective tissue matrix. The other major tissue component of the breast is adipose tissue. The lactiferous ducts empty at the nipple, a highly innervated hairless protrusion of the breast designed for suckling by an infant. The nipple is surrounded by a pigmented hairless areola that is lubricated by sebaceous glands. Protrusion of the nipple, called erection, is mediated by sympathetic stimulation of smooth muscle fibers in response to suckling and other mechanical stimulation, erotic stimulation, and cold.
Hormonal Regulation of Mammary  Gland Development
At puberty, estrogen increases ductal growth and branching. With onset of the luteal phases of the ovary, progesterone and estrogen induce ductal growth and formation of rudimentary alveoli. During nonpregnant cycles the breasts develop somewhat and then regress. Estrogen also increases deposition of adipose tissue, which makes a major contribution to breast size and overall form. Adipose tissue expresses CYP19/aromatase, so accumulation of this tissue in the breast increases local production of estrogens from circulating androgens.
Breast development is facilitated by pregnancy, during which extensive ductal growth and branching and lobuloalveolar development occur. The parenchymal growth of the breast during development occurs at the expense of stroma, which is degraded to make room for enlarging lobuloalveolar structures. Several placental hormones stimulate breast development, including estrogen, progesterone, placental lactogen, and a growth hormone variant (GH-V). Estrogen acts on the breast both directly and indirectly through increasing maternal pituitary PRL. Estrogen increases PRL secretion from pituitary lactotropes. Estrogen also stimulates lactotrope hypertrophy and proliferation, which accounts for the twofold increase in pituitary volume during pregnancy in humans. Although epithelial cells express genes encoding milk protein and enzymes involved in milk production, progesterone inhibits the onset of milk production and secretion 826 SECTION8Berne & Levy Physiology •Fig. 44.38 Neuroendocrinereflexcausedbysucklingatthenippleandleadingtosecretionofoxytocinandprolactin.Inturnthesehormonesinducecontinuedmilkproduction(galactopoiesis)andmilklet-down.Prolactinalsoinduceslactationalamenorrhea.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.)Magnocellular neurons Parvicellular neurons Lactotropes Gonadotropes Lactational amenorrhea Parvicellular neurons Hypothalamus Breast Ovary Pars distalis Pars nervosa ˜ GnRH˜ Dopamine ˜ LH and FSH ° PRF?? ° Prolactin GalactopoiesisConcentration of myoepithelial cells Milk let-down Sucking at nipple ° Oxytocin (lactogenesis).