Breasts are made up of fat and breast tissue, along with nerves, veins, arteries and connective tissue that helps hold everything in place.
Figure 1.1 shows the different parts of the breast. The main chest muscle (the pectoralis muscle) is found between the breast and the ribs in the chest wall.
Image source: National Cancer Institute (www.cancer.gov)
Breast tissue is a complex network of lobules (small round sacs that produce milk) and ducts (canals that carry milk from the lobules to the nipple openings during breastfeeding) in a pattern that looks like bunches of grapes. These “bunches” are called lobes. Breast tissue may also be called glandular tissue.
Throughout childhood, girls have a small patch of immature breast tissue. During puberty, hormones produced by the ovaries and pituitary gland (a part of the brain that controls growth and other glands in the body) cause the breasts to grow. This causes the milk ducts to stretch out and become more branched. The breast tissue then develops into a mature system of lobules and ducts.
Adult women have 15 to 20 lobes in each breast . Each lobe has 20 to 40 lobules . Small milk ducts are attached to the lobules. These ducts join together like branches of grape stems into increasingly larger ducts. There are about 10 duct systems in each breast, each with its own opening at the nipple .
Muscle tissue in the nipples allows them to become erect in response to 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) release small amounts of fluid during breastfeeding to lubricate the nipple .
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.
The loss of breast tissue during menopause means breast density also decreases. Before menopause, the breasts have more breast tissue than fat (higher breast density). After menopause, the breasts have more fat than breast tissue (lower breast density). This natural change makes it easier to read the mammograms of women after menopause than before menopause (learn more) .
Learn more about breast density and mammography.
Learn about breast density and breast cancer risk.
Boys and girls begin life with similar breast tissue. However, men do not have the same complex breast growth and development as women. At puberty, high testosterone and low estrogen levels stop breast development in men. In men, some milk ducts exist, but they remain undeveloped. Lobules are most often absent.
Learn about breast cancer in men.
Facts for Life: Breast Density
Breast Cancer 101: Structure of the Breast
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