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The breast lies anterior to the chest wall over the pectoralis major muscle and typically extends from the second to the sixth rib in the vertical axis and from the sternal edge to the midaxillary line in the horizontal axis. Bundles of dense fibrous connective tissue, the suspensory ligaments of Cooper, extend from the skin to the pectoral fascia and provide support for the breast. At puberty, estradiol and progesterone levels increase to initiate breast development. The adult female breast consists of a series of branching ducts that terminate in lobules. The arrangement of these structures resembles a branching tree with 5–10 primary milk ducts in the nipple, 20–40 segmental ducts, and 10–100 subsegmental ducts that end in glandular units called terminal-duct lobular units (TDLU) (Figure 17-1). The TDLU represents the functional unit of the breast (Figure 17-2). During lactation, there is a dramatic increase in the number of lobules, and the epithelial cells in the TDLU undergo secretory changes consisting of cytoplasmic vacuoles (Figure 17-3). The accumulated secretions are then transported via the ductal system to the nipple. When lactation ceases, the lobules involute and return to their normal resting appearance. The mammary ducts and lobules are embedded within a stroma composed of varying amounts of fibrous and adipose tissue. The stromal component comprises the major portion of the nonlactating adult breast, consisting of lobular stroma and interlobular stroma. The proportions of fibrous and adipose tissue vary with age and among individuals and may affect the sensitivity and specificity of mammographic studies. During menopause, as a result of reduction in estrogen and progesterone, there is involution and atrophy of the TDLUs associated with loss of the specialized intralobular stroma. The postmenopausal breast is characterized by marked reduction in the glandular and fibrous stroma components, typically with concomitant increase in stromal adipose tissue.


Anatomy of the breast (diagrammatic sagittal section). The ductal system extends from the nipple to multiple lobes of terminal-duct lobular units (TDLUs) branching in a treelike fashion. The TDLU is the site of origin for most breast carcinomas. The ducts and lobular acini are lined by two layers of cells: the inner luminal cells and outer myoepithelial cells. Source: Reproduced with permission from McKinley M, O’Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008.


Terminal-duct lobular units (TDLUs). Multiple TDLUs are displayed with terminal ducts designated by X and surrounded by numerous smaller lobular acini. The TDLU represents the functional unit of the breast.


Lactational change. Lactational changes include increased density of lobular acini with the luminal cells containing abundant clear secretory-type vacuoles with production of pink secretory material seen with the lobular acini and terminal duct lumens.

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