A&P review: Breast/axillae

Lymphatic Networks

Each breast contains an extensive lymphatic network, which drains into lymph nodes in several areas. As blood flows through the capillary bed, fluid is forced out into the interstitial space and into the cells. Most of the fluid is immediately resorbed into the capillaries; however, fluid left in the interstitial spaces is eventually absorbed by the lymph system and carried through the lymph nodes. More than 75% of lymph drainage from the breast flows outward toward the axillary lymph node groups and then upward to the subclavicular and supraclavicular nodes. Other routes for lymph drainage include flow through the anterior axillae (pectoral) nodes (above the breast), internal mammary nodes (in the thorax), and subdiaphragmatic nodes (toward the abdomen) and through cross-mammary pathways to the opposite breast.

Female Breast

The breast of the mature female has a distinctive shape; however, the "normal" breast size varies greatly. The breasts extend vertically from the second to the sixth ribs and laterally from the sternal margin to the midaxillary line. To facilitate description (or location of lesions), breasts are divided into quadrants by imaginary vertical and horizontal lines intersecting at the nipple. The female breast is composed of three types of tissue: glandular, fibrous, and subcutaneous and retromammary fat. The glandular tissue is arranged into 15 to 20 lobes per breast, radiating around the nipple in a spokelike pattern. Each lobe is composed of 20 to 40 lobules, or alveoli, containing the milk-producing acini cells. During lactation, milk produced by acini cells empties into the lactiferous ducts. These ducts drain milk from the lobes to the surface of the nipple. The largest amount of glandular tissue lies in the upper outer quadrant of each breast. From this quadrant the breast tissue extends into the axilla, forming the axillary tail of Spence. The breast is supported by a layer of subcutaneous fibrous tissue and by multiple fibrous bands termed Cooper's ligaments. These suspensory ligaments extend from the connective tissue layer and run through the breast, attaching to the underlying muscle fascia. Subcutaneous and retromammary fat surrounds the glandular tissue and composes most of the breast. Centrally located on the breast, the nipple is surrounded by the pigmented areola. The nipples are composed of epithelium intertwined with circular and longitudinal smooth muscle fibers. These muscles contract in response to sensory, tactile, or autonomic stimuli, producing erection of the nipple and causing the lactiferous ducts to empty. A number of sebaceous glands, termed Montgomery's glands, are located within the areolar surface, aiding in lubrication of the nipple during lactation. Throughout the reproductive years the breasts undergo a cyclic pattern of size change, nodularity, and tenderness during the menstrual cycle. The breasts are smallest during days 4 through 7 of the menstrual cycle. Three to four days before the onset of menses, many women experience breast fullness, tenderness, and pain because of hormonal changes and fluid retention. The breasts undergo a dramatic change during pregnancy and lactation in response to luteal and placental hormones. These changes include an increase in the number of lactiferous ducts and the size and number of alveoli.

Male Breast

The male breast undergoes very little additional development after birth, and the gland remains rudimentary. It consists of a thin layer of undeveloped tissue beneath the nipple. The areola of the nipple is small when compared with that of the female. During puberty the male breast may become slightly enlarged, producing a temporary condition termed gynecomastia. Although gynecomastia is usually unilateral, it may occur bilaterally. The older male may also have gynecomastia secondary to a decrease in testosterone.