The Breast-Structure and Function

Anatomy of Breast Cancer - Updated: Structure of the Breast

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Breasts are composed mostly of fat and breast tissue, along with nerves, veins, arteries and the connective tissue that helps hold everything in place. Breast tissue is a complex network of lobules and mammary ducts in a pattern that looks like bunches of grapes on their stems. The main chest muscle (the pectoralis muscle) is located behind the breast and in front of the ribs in the chest wall.

Adult women have 15 to 20 lobes in each breast [1]. Each lobe consists of many round sacs (called lobules) that produce milk. The ducts are the canals that carry milk from the lobules to the nipple openings when a woman is breastfeeding. The smallest ducts are attached to lobules. These ducts join together like the branches of grape stems into increasingly larger ducts. There are between five and ten ductal systems in each breast, each with its own opening at the nipple.

Muscle tissue in the nipple allows it to become erect in response to sexual stimulation or breastfeeding. Muscle tissue around the lobules helps squeeze milk into the ducts. Glands on the areola, the shaded circle of skin around the nipple, secrete small amounts of fluid to lubricate the nipple when breastfeeding [1]. Figure 1.1 shows the different parts of the breast.

Throughout childhood, girls have a small patch of immature breast tissue, which is inactive until puberty. During puberty, the breasts grow due to hormones produced by the ovaries and pituitary gland (a part of the brain that controls growth and other glands in the body). This causes the ducts to stretch out and become more branched. After this, the breast tissue develops into the mature system of lobules and ducts. Though the breast is mature after puberty, the breast tissue remains inactive until pregnancy. During pregnancy, the lobules grow and begin to produce milk. The milk is then released into the ducts so a mother can breastfeed her baby.

After menopause (when the ovaries stop producing hormones and a woman stops having periods), the number of lobules decreases and those remaining shrink in size. With the loss of breast tissue during menopause, breast density also decreases. In postmenopausal women, more of the breasts are made up of fat than breast tissue compared to the breasts of younger women. This natural change makes mammography more effective in postmenopausal women [2]. This is discussed in more detail in the Early Detection and Screening section.

For information on male breast health, Breast Cancer in Men.

Updated 05/28/09

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