Each breast has 15 to 20 sections, or lobes, that surround the nipple like spokes on a wheel. Inside these lobes are smaller lobes, called lobules. At the end of each lobule are tiny "bulbs" that produce milk. These structures are linked together by small tubes called ducts, which carry milk to the nipples.
The nipple is in the center of a darker area of skin called the areola. The areola contains small glands, called Montgomery glands, which lubricate the nipple during breast-feeding. Fat fills the spaces between the lobes and ducts. There are no muscles in the breasts, but the pectoral or chest muscles lie under each breast and cover the ribs.
Each breast also contains blood vessels, as well as vessels that carry a fluid called lymph. Lymph travels throughout the body through a network called the lymphatic system. The lymphatic system carries cells that help the body fight infections. The lymph vessels lead to the lymph nodes (small, bean-shaped glands).
One group of lymph nodes is located in the armpits, above the collarbone and in the chest. If breast cancer has reached these nodes, it may mean that cancer cells have spread to other parts of the body via the lymphatic system. Lymph nodes are also found in many other parts of the body.
Breast development and function depend on the hormones estrogen and progesterone, which are produced in the ovaries. Estrogen elongates the ducts and causes them to create side branches. Progesterone increases the number and size of the lobules in order to prepare the breast for nourishing a baby.