The mammary (mam′ ă-rē; relating to breasts) glands are the organs of milk production and are located in the breasts (fig-ure 19.13). The mammary glands are modified sweat glands. Externally, each of the breasts of both males and females has a raised nipple surrounded by a circular, pigmented area called the areola (ă-rē′ō-lă).
In prepubescent children, the general structure of the male and female breasts is similar, and both males and females possess a rudimentary duct system. The female breasts begin to enlarge during puberty, under the influence of estrogen and progesterone. Some males also experience a minor and temporary enlargement of the breasts at puberty. Occasionally, the breasts of a male can become permanently enlarged, a condition called gynecomastia (gı̄′ nĕ-kō-mas′ t ē-ă). Causes of gynecomastia include hormonal imbalances and the abuse of anabolic steroids.
Each adult female breast contains mammary glands con-sisting of usually 15–20 glandular lobes covered by adipose tissue (figure 19.13a,b). It is primarily this superficial adipose tissue that gives the breast its form. Each lobe possesses a single lactiferous duct that opens independently to the surface of the
The duct of each lobe is formed as several smaller ducts, which originate from lobules, converge. Within a lobule, the ducts branch and become even smaller. In the milk-producing, or lactating, mammary gland, the ends of these small ducts expand to form secretory sacs called alveoli. Myoepithelial cells sur-round the alveoli and contract to expel milk from the alveoli (figure 19.13c).
The breasts are supported by suspensory ligaments that extend from the fascia over the pectoralis major muscles to the skin over the breasts (figure 19.13b).
The nipples are very sensitive to tactile stimulation and contain smooth muscle. When the smooth muscle contracts in response to stimuli, such as touch, cold, and sexual arousal, the nipple becomes erect.