The lobes of the mammary gland consist of lobules, which contain alveoli and ducts. The epithelium of these structures is made up of two layers of cells: luminal epithelial cells and myoepithelial cells. Adult mammary stem cells are believed to be present within this epithelium. The luminal epithelial cells in the alveoli produce milk, while the luminal cells in the ducts convey and modify the milk. Myoepithelial cells, resembling stellate smooth muscle cells, contract in response to stimuli and expel milk from the alveoli and ducts. Connective tissue provides support for the lobes and lobules, and adipose tissue is another major component of the breast. The lactiferous ducts empty at the nipple, which is a hairless protrusion of the breast designed for suckling. The nipple is surrounded by a pigmented hairless areola lubricated by sebaceous glands. The nipple can protrude in response to various stimuli, including suckling, mechanical stimulation, erotic stimulation, and cold, due to the contraction of smooth muscle fibers stimulated by the sympathetic nervous system.

Hormonal regulation plays a crucial role in the development of the mammary gland. During puberty, estrogen promotes the growth and branching of the ducts. Progesterone and estrogen, particularly during the luteal phases of the ovary, induce ductal growth and the formation of rudimentary alveoli. Breast development occurs to some extent during nonpregnant cycles but then regresses. Estrogen also contributes to the deposition of adipose tissue, which influences breast size and shape. Adipose tissue in the breast expresses CYP19/aromatase, leading to increased local production of estrogens from circulating androgens.

Pregnancy further facilitates breast development, characterized by extensive ductal growth, branching, and lobuloalveolar development. As the parenchyma of the breast grows, the stroma is degraded to accommodate the enlarging lobuloalveolar structures. Several placental hormones, such as estrogen, progesterone, placental lactogen, and a growth hormone variant (GH-V), stimulate breast development. Estrogen acts both directly on the breast and indirectly by increasing pituitary prolactin secretion in the mother. Estrogen stimulates lactotrope hypertrophy and proliferation, resulting in a twofold increase in pituitary volume during human pregnancy. Although epithelial cells express genes related to milk protein and production enzymes, progesterone inhibits the onset of milk production and secretion.

The neuroendocrine reflex triggered by suckling at the nipple leads to the secretion of oxytocin and prolactin. These hormones, in turn, induce continued milk production (galactopoiesis) and milk let-down. Prolactin also causes lactational amenorrhea. The hypothalamus, specifically magnocellular and parvicellular neurons, plays a crucial role in this reflex. The pituitary gland, specifically lactotropes and gonadotropes, is involved in regulating the production of prolactin and other hormones. Oxytocin is responsible for the milk let-down reflex, which is stimulated by sucking at the nipple. The concentration of myoepithelial cells also influences milk let-down.
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).