The fully-developed female breast is a well-differentiated apocrine sweat gland originating in the ectoderm that secretes milk during lactation. Each breast is cone-shaped, particularly in younger nulliparous females, extends from the sternum to the midaxillary line, and lies anterior to the pectoral muscle. A thin outer dermal layer covers a subdermal layer of adipose tissue that varies in thickness from several millimeters to 1 cm. Cooper's ligaments (Figure 2.1) are strings of fibrous connective tissue extending from the prepectoral fascia to the skin to support the glandular tissue. Cooper's ligaments also support blood vessels, lymph channels, and varying quantities of adipose tissue (Figures 2.2a and 2.2b).
Traditional anatomic dissections show that the glandular tissue consists of 15 to 20 lobes or segments containing ducts that branch and subdivide into smaller ducts as they extend into the deeper glandular tissue. The end units of the smallest ducts are composed of milk-forming lobules that drain radially through the ducts toward the nipple. Each lobe has its own segmental duct into which all the ducts from that lobe drain. There are wide lactiferous sinuses in the subareolar region; each of these receives the drainage from one or more segmental ducts. Each lactiferous duct then narrows as it passes through the nipple. This narrowed duct in the nipple is called a collecting duct. Sartorius (1986) has demonstrated that the nipple contains only five to seven collecting ducts.
Males and prepubescent females have only rudimentary glandular tissue. In the western world, a young woman's glandular tissue begins to proliferate early in her second decade, although maturation may be earlier or later. By the time a woman has completed puberty, her glandular tissue usually has developed to its maximum size. Hormonal variations related to menstrual cycles, pregnancy, and lactation cause the size of the glandular tissue to wax and wane. At menopause, glandular tissue gradually recedes, causing the breast to flatten somewhat, and become pendulous and less firm.
Although hormones cause the glandular tissue to become more dense, a woman's genetic predisposition and her ratio of total body
adipose tissue to total body weight also influence her ratio of glandular tissue to adipose tissue in her breast. For this reason, some young women have breasts consisting primarily of adipose tissue, while some elderly women have breasts with exceedingly dense glandular tissue.
Glandular tissue can extend throughout the entire breast; only a thin layer of retromammary adipose tissue separates it from the pectoral muscle. The upper outer quadrant, which extends towards the axilla, is known as the axillary tail or the tail of Spence. It is the thickest portion of the glandular tissue and the part reaching furthest from the nipple.
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