The human female breast consists mainly of fat and breast tissue, along with the various nerves, veins, arteries and connective tissue that hold everything together. Because their purpose is to produce milk, the breast tissue itself is a complex network of small round lobule sacs and duct canals that carry the milk from the lobules to the nipple openings during breastfeeding. One thing most people remember after seeing an x-ray or illustration of the interior of a breast is that the network of lobule sacs is gathered in bunches called lobes. Normal adult women will have about 15 to 20 lobes in each breast and each lobe will have has 20 to 40 lobules. The ducts are connected like the branches of a tree, and lead to larger ducts. In total, each breast has about ten duct systems, and each duct has its own opening at the nipple.
The muscle tissue in the nipple is responsible for its ability to become erect when exposed to sexual stimulation or stimulation from breastfeeding. The stiffening muscle tissues around the lobules act to squeeze the milk into the ducts, and small glands on the areola around the nipple secrete lubricating fluid to the nipple during breastfeeding.
Human breasts don’t become fully mature until adolescence, and during childhood young girls will have inactive breast tissue. At the time of puberty however, hormones produced by both the ovaries and pituitary gland will act together to stimulate the breasts to grow more tissue. As the breasts get larger the ducts will lengthen and stretch out to produce more complex branches. In time, the breast will then develop into a fully active system of milk-producing lobules and ducts, but only during pregnancy. The breast tissue is mature but inactive until pregnancy, when the lobules will begin to grow larger and ultimately produce milk allowing a mother to breastfeed her baby.
When menopause occurs later in life, the ovaries will stop producing as many hormones and a woman will stop having monthly periods. After menopause, the number of lobules in the breast tissue decreases and those that do remain will begin to shrink in size. This means that post-menopausal breasts are composed of less actual breast issue and contain more fat. Conversely, before menopause the breasts have a higher density and are made up of more breast tissue than fat. The changes in post-menopausal breasts that cause them to consist of less dense breast tissue also make them easier to scan during regular mammography.
