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mammary gland, organ of the female mammal that produces and secretes milk for the nourishment of the young. A mammal may have from 1 to 11 pairs of mammary glands, depending on the species. Generally, those mammals that bear larger litters have more glands. The mammary gland of the cow and of some other mammals is known as the udder.
In humans, there is one pair of mammary glands, also known as mammae, or breasts. They are rudimentary in both sexes until the age of puberty when, in response to ovarian hormones, they begin to develop in the female. During pregnancy, they distend still further in preparation for nursing the infant. Pregnant women are prevented from lactating (producing milk) by the presence in the blood of high levels of estrogen and progesterone, secreted by the placenta until birth occurs.
After birth, response to prolactin, the milk-stimulating hormone, is no longer inhibited by placental hormones, and lactation begins. Mammary tissue contains between 15 and 20 compartments called lobes, each of which is divided into smaller compartments called lobules. The lobes and lobules are connected by a network of tubes whose cells manufacture the liquid and fatty substances that form milk. The tubes of each lobe connect with a duct, and all ducts lead to the nipple, where the milk is secreted when the nipple is sucked by the young. The letdown of milk during the nursing process is aided by oxytocin, a hormone secreted by the pituitary. The physical force of an infant's sucking on the breast is a major stimulus to milk production. Disorders of the mammary gland include mastitis and breast cancer.
The human mammary gland, usually well developed in the adult female but rudimentary in the male. Each adult female breast contains 15–20 separate, branching glands that radiate from the nipple. During lactation their secretions are discharged through separate openings at the base of the nipple.
In the female, hormonal changes in adolescence cause enlargement of breast tissue, but much of this is connective tissue although some glandular buds form. With the advent of full menstruation ovarian estrogenic hormones influence breast development. If pregnancy ensues, the glandular tissue reaches full development and full lactation begins shortly after birth. After cessation of lactation the breasts regress considerably and once again reflect cyclic regulation. See Lactation
Breast disorders may result from congenital or developmental abnormalities, inflammations, hormonal imbalances, and, most important, from tumor formation.
Congenital defects are usually unimportant except for their psychic or cosmetic implications. Supernumerary nipples and breasts or accessory breast tissue are common examples.
Inflammations are not encountered frequently and usually result from a staphylococcal or streptococcal invasion incurred during lactation. A special form of inflammation may result from fat necrosis. Although any age is susceptible, older women show a slightly higher incidence of fat necrosis, the commonest cause of which is injury from trauma. See Staphylococcus, Streptococcus, Syphilis, Tuberculosis
Hormonal imbalances are believed to be responsible for the variants of the commonest nontumorous breast disorder of women, cystic hyperplasia. The changes are thought to result from exaggeration or distortion of the normal cyclic alterations induced during the menstrual interval. Although a wide range of clinical and pathologic variation is commonplace, three major types or tendencies prevail. The first, called fibrosis or mastodynia, is marked by an increase of connective tissue in the breast, without a proportionate increase in glandular epithelium. The second, cystic disease, is characterized by an increase in the glandular and connective tissues in local areas, with a tendency toward formation of cysts varying in size. The third major type is adenosis, in which glandular hyperplasia is predominant. Each major form of cystic hyperplasia has its own clinical characteristics, ages of highest incidence, and distribution. Each is important because the breast masses which occur require differentiation from benign and malignant tumors. These lesions also have been found to predispose to the subsequent development of carcinoma.
Breast cancer is the most significant lesion of the female breast, accounting for 25,000–30,000 deaths in the United States each year. It rarely occurs before the age of 25, but its incidence increases each year thereafter, with a sharper climb noted about the time of menopause. Early breast cancer may appear as a small, firm mass which is nontender and freely movable. Diagnosis at this time carries a more favorable prognosis than later, when immobility, nipple retraction, lymph node involvement, and other signs of extension or spread are noted. Paget's disease of the nipple is a special form of breast cancer, in which there are early skin changes about the nipple. See Hormone