The adult female breast is actually a modified sebaceous gland, located within the superficial fascia of the chest wall (Fig. 31.1). Histologically, the breast is composed primar-ily of lobules or glands, milk ducts, connective tissue, and fat. The relative amounts of these tissue types vary con-siderably with age. In younger women, the breast con-sists predominantly of glandular tissue. With age, the glands involute and are replaced by fat, a process accel-erated by menopause. Differences in palpable consis-tency and in radiographic density between the glands and fat are key components of breast cancer detection programs.
Architecturally, the breast is organized into 12 to 20 lobes, with a disproportionate amount of the glandular or lobular tissue in the upper outer quadrants of each breast. This disproportionate distribution of glandular tis-sue accounts for why breast cancer most commonly arises in the upper outer quadrant. The lobules consist of clusters of secretory cells arranged in an alveolar pattern and sur-rounded by myoepithelial cells. These glands drain into a series of collecting milk ducts that course through the breast, ultimately coalescing into approximately five to ten collecting ducts that lead to and drain at the nipple. Typically, cancer begins at these terminal duct-lobular units of the breast and follows the path of those ducts.
Congenital anomalies of the breast can include absence of the breast as well as accessory breast tissue located any-where along the “milk lines,” which extend from the axilla to the groin in the fetus. Extra nipples (polythelia) are more common than true accessory breasts (polymastia).
The breast has a rich blood supply and lymphatic sys-tem, which support milk production and overall breast health. The blood supply comes from perforating branches of the internal mammary artery, the lateral thoracic artery, the thoracodorsal artery, the thoracoacromial artery, and various intercostal perforating arteries. The lymphatic ves-sels lead to several superficial and deep nodal chains throughout the trunk and neck, including those located in the axilla, deep to the pectoralis muscles, and caudal to the diaphragm (Fig. 31.2). The ipsilateral lymph node and occasionally the internal mammary nodes are the most common route of metastasis.
Breast tissue is very sensitive to hormonal changes, espe-cially the glandular cells. The transition from the immature,pediatric breast to the mature, adult breast is orchestrated by the changes in circulating levels of estrogen and proges-terone that accompany puberty. Estrogen is primarily responsible for the growth of adipose tissue and lactifer-ous ducts. Conversely, progesterone stimulation leads to lobular growth and alveolar budding.