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.