Anatomy & Physiology
Each of these accessory reproductive glands lies on the superior surface of the chest wall lying mostly on top of the pectoralis major muscle. The glandular portions are basically modified sweat glands. In women, the breasts are responsible for lactation; in men, they are normally undeveloped and without function.
The breast is made of lobes of glandular tissue with associated ducts for transfer of the milk to the exterior and supportive fibrous and fatty tissue. About 80-85% of normal breast tissue is fat during the reproductive years. The 15-25 lobes are further divided into lobules containing alveoli (small saclike features) of secretory cells with smaller ducts that conduct the milk to larger ducts and finally to a reservoir that lies just under the nipple. In the nonpregnant, nonlactating breast, the alveoli are small. During pregnancy, the alveoli enlarge and during lactation the cells secrete milk substances, i.e. proteins and lipids. Muscular cells surrounding the alveoli contract to express the milk during lactation. Breast tissue is supported by ligaments called Cooper's ligaments that keep the breasts in their characteristic shape and postition. In the elderly or in pregnancy these ligaments become loose or stretched, respectively, and the breasts sag.
Reproductive hormones are important in the development of the breast in puberty and in lactation. Estrogen promotes the growth of the gland and ducts while progesterone stimulates the development of milk producing cells. Prolactin, released from the anterior pituitary gland, stimulates milk production. Oxytocin, released from the posterior pituitary in response to suckling, causes milk ejection from the lactating breast.
The lymphatic system drains the tissues of the breast of excess fluid. Lymph nodes along the pathway of drainage screen for foreign bodies such as bacteria or viruses. The lymph nodes can also become enlarged when migrating cancer cells get lodged in the nodes. This is why lymph nodes located in the arm pit are checked during a breast exam and why they are often cut out along with cancerous tissue in the treatment of breast cancer.