Nipple erection is produced with stimulation. The 15 to 25 lobes of each breast are further divided in lobules that are separated and supported by fibrous tissue. Each lobule contains small saclike aveoli surrounded by milk producing cells and small muscular cells. The muscular cells contract to express the milk during lactation. The lobules are drained by ducts that empty into a larger reservoir that lies just below the nipple.
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, stimulates milk production. Oxytocin, released from the posterior pituitary in response to suckling, causes milk ejection from the lactating breast. The first change to herald the coming of reproductive capability in females is the development of breasts. The age at the time of puberty is variable.