The internal structure of the breast
Each breast has 15 to 20 subdivisions, or lobes, that surround the nipple, like spokes on a wheel. Within these lobules, the smaller ones are called mammary lobules. At the end of each lobule are tiny “vesicles” that secrete milk, and these structures are linked together by small ducts called ducts that carry milk to the nipple.
The nipple is located in the center of a darker area called the areola. The areola contains a small gland called the Montgomery gland, which lubricates the nipple during breastfeeding. Fat fills the space between the lobes and ducts of the breast. There is no muscle in the breast, but the pectoralis and chest muscles are located on the deep side of each breast and cover the rib cage.
Lymphatic system of the breast
Each breast also includes blood vessels and lymphatic vessels that contain lymph fluid. The lymph fluid is transported to all parts of the body through a network called the lymphatic system. The lymphatic system contains cells that help the body fight infection. The lymphatic vessels lead to lymph nodes (small, bean-shaped glands).
There are separate sets of lymph nodes located in the armpits, above the collarbone, and in the chest. If breast cancer cells have metastasized to these lymph nodes, it may mean that the cancer cells have moved to other parts of the body through the lymphatic system. There are lymph nodes in other parts of the body.
The role of estrogen and progesterone
Breast development and function depend on the estrogen and progesterone produced in the ovaries. Estrogen promotes lengthening of the ducts and causes them to produce lateral branches. Progesterone increases the number and size of lobules so that the breast is ready for feeding the baby.
After ovulation, progesterone causes breast cells to grow and blood vessels to enlarge and fill with blood. At this time, the breasts tend to fill with fluid and may become soft and swollen.