All mammals have breasts, or mammary glands, that produce breast milk to feed their young.
The two main components of the breast are:
- The lobules, which are lined with cells that, when stimulated by hormones, are responsible for milk production.
- The ducts, which are the pipes of the breast, carrying milk from the lobules.
Together the lobules and ducts form a ductal-lobular unit. These ductal-lobular units may look similar to a bunch of grapes (lobules) on a stem (duct). The smaller stems all travel to the main stem. In the breast, the ducts join forces and increase in size as they approach the nipple. At the nipple 10 to 15 ducts open to the surface. These ducts appear as tiny openings on the nipple surface.
Surrounding the ductal-lobular unit is supportive tissue, or stroma. This fibrous substance allows the ductal-lobular units to remain properly aligned and not collapse. Fat, or adipose tissue, is intermingled around the stroma and ductal-lobular units. The anterior and posterior fascia, an envelope of supportive tissue around all of these structures, keeps the breast in a constant position on the chest wall.
The skin of the breast includes the nipple and areola. Just beneath the nipple-areolar complex are muscle fibers, as well as very sensitive nerve endings. With breastfeeding and latching, the nerves are stimulated, which signals the muscles to contract. These muscles then control the release of breast milk from the ducts.
The areola contains hair follicles and sebaceous glands. The glands, called Montgomery glands, appear as small bumps on the areola. They serve to lubricate the skin during breastfeeding.
Prior to menopause, breasts are quite dense because estrogen promotes the ductal-lobular units and stroma. The breast tissue of women of childbearing age contains many ductal-lobular units and stroma but not much fat. However, as a woman ages and enters menopause, the stroma regresses and the ductal-lobular units shrink and collapse. Tissue is replaced with fat, leading to softer and less “perky” breasts.