The Human Body


Breast, one of a pair of glandular organs in mammals that secrete milk for newborns and infants. Also known as a mammary gland, particularly in nonhuman mammals, the breast is unique to mammals and is not found in any other type of animal. In humans, the female breast has a distinctive rounded shape while the male breast remains flat and undeveloped. Only mature females can produce milk, which is rich in nutrients, antibodies, and other substances that aid in a baby’s growth and development.

The surface of a human breast has a circular, pigmented area in the center called the areola. The nipple, a rounded protruding structure, lies in the center of the areola. In mature women the breast contains a collection of 10 to 15 tubes, called ducts, that connect to the nipple. These ducts branch out from the nipple into the interior of the breast, ending in clusters of rounded cells, called lobules, that produce the milk. In addition to the structures directly connected with the production and outflow of milk, the breast is composed of fatty tissue and ligaments that provide support and shape.

In humans, a woman’s breast size and shape vary widely, depending on her age and whether she is pregnant or going through a menstrual cycle. Breast development begins in girls about 10 to 12 years old, when the ovaries start to produce the hormone estrogen. Completion of breast development, which occurs around 16 to 18 years old, requires the interaction of other hormones such as progesterone, prolactin, and corticosteroids.

After breast growth is completed, the breasts typically undergo monthly cyclic changes in response to fluctuating hormone levels in the blood that occur during the menstrual cycle. Just before the onset of menstruation, the breasts are often swollen and tender because the ductal system expands in preparation for pregnancy. If pregnancy does not occur, the estrogen level falls and the breasts return to normal.

During pregnancy there is a remarkable growth of ducts and lobules in the breast along with a thickening of the nipples. After a baby is born, the hormone prolactin stimulates milk production in the breast. Initially, the breast produces a thick yellow liquid called colostrum, which is particularly rich in the disease-fighting substances called antibodies. Within three to five days, the breast produces milk as the suckling infant stimulates the release of another hormone called oxytocin. This hormone causes contractions in the network of cells that surround the ducts and lobules, so that milk readily flows from the breast and into the mouth of the hungry infant.

As a woman passes the childbearing age and enters menopause, the ovaries stop producing estrogen. The decreased stimulation from this hormone causes the milk-producing ducts and lobules in the breast to be replaced with extra fatty tissue.

Breasts are naturally lumpy, but any area of the breast that becomes more prominent than the surrounding tissue or feels unusually firm requires the attention of a physician. Breast lumps are a common problem in women and may range from a cyst, a fluid-filled structure that does not pose a cancer risk, to breast cancer, the most common type of cancer in women other than skin cancer. Also, in men between 20 and 40 years of age, breast enlargement can be a sign of testicular cancer, and breast lumps in men over 40, although rare, can be an indication of breast cancer.

Often one of the first diagnostic tests performed to evaluate a lump, an ultrasound is able to distinguish a cyst from a solid mass, a possible indication of cancer that requires further investigation. Solid lumps are evaluated with a special X-ray procedure called a mammogram, currently the best tool available for early detection of breast cancer. A mammogram may be followed by needle aspiration, a type of biopsy in which cells are removed from the lump and examined under a microscope. If these test results are normal, the lump is unlikely to be cancer. It may be removed or monitored carefully for signs of growth. If cancer is suspected, or if the lump shows signs of growth later, surgical removal is usually necessary.

As a preventive measure, physicians encourage regular breast self-examinations in which a woman uses her fingers to feel for changes in breast shape and fluid discharge from the nipple. Physicians also urge women to have regular mammograms, which can detect very small cancers before a lump begins to develop. These small cancers typically respond to treatment better than cancers that have grown large enough to produce a lump.