The mammary glands, or breasts, are a distinguishing feature of mammals. They are structurally dynamic organs, varying with age, menstrual cycle, and reproductive status of the female. During embryologic development, growth and development of breast tissue occur in both sexes. Multiple glands develop along paired epidermal thickenings called mammary ridges (milk lines) that extend from the developing axilla to the developing inguinal region. In humans, normally only one group of cells develops into a breast on each side. An extra breast (polymastia) or nipple (polythelia) may occur as an inheritable condition in about 1% of the female population. These relatively rare conditions may also occur in men.
In females, mammary glands develop under the influence of sex hormones.
Until puberty, both females and males mammary glands develop in similar fashion. At the onset of puberty in males, testosterone acts on the mesenchymal cells to inhibit further growth of the mammary gland. In the same time, the mammary glands in women undergo further development under hormonal influence of estrogen and progesterone. Estrogen stimulates mesenchymal cells further development. The mammary gland increases in size, mainly due to the growth of interlobular adipose tissue. The ducts extend and branch into the expanding connective tissue stroma. Proliferation of epithelial cells is controlled by interactions between the epithelium and the specialized intralobular hormone-sensitive loose connective tissue stroma. By adulthood, the complete ductal architecture of the gland has been established.
The mammary glands remain in inactive state until pregnancy, during which the mammary glands assume their complete morphologic and functional maturation. This occurs in response to estrogens and progesterone initially secreted from the corpus luteum and later from placenta, prolactin from pituitary gland, and gonadocorticoids produced by adrenal cortex. By the end of pregnancy, secretory vessicles are found in the epithelial cells, but milk production is inhibited by high levels of progesterone. The actual initiation of milk secretion occurs immediately after birth and is induced by prolactin (PRL) secreted by the adenohypophysis. The ejection of the milk from the breast is stimulated by oxytocin released from the neurohypophysis. With the change in the hormonal environment at menopause, the glandular component of the breast regresses or involutes and is replaced by fat and connective tissue. In men, some additional development of the mammary glands normally occurs after puberty, and the glands remain rudimentary.
Constant hormonal exposure and genetic predisposition are the major risk factors for the development of breast cancer. It is the most common malignancy in women in the United States. Each year an estimated nearly
FIGURE 23.30 • Schematic drawing of the human breast as seen during lactation. The breast is composed largely of terminal duct lobular units (TDLUs) containing branched tubuloalveolar glands. TDLUs are contained within an extensive connective tissue stroma and variable amounts of adipose tissue. (Modified from Warwick R, Williams PL, eds. Gray’s Anatomy, 35th ed. Edinburgh: Churchill Livingstone, 1973.) 200,000 women (and also 1,700 men) are diagnosed with breast cancer. Most breast cancers are linked to hormonal exposure (which increases with age, early menarche, late menopause, and with older age of a first full-term pregnancy). About 5% of all breast cancers are attributable to mutation in autosomal dominant breast cancer genes (BRCA1 and BRCA2).
Mammary glands are modified tubuloalveolar apocrine sweat glands.