The breast is divided into quadrants or described in comparison to a clock face for ease of communication of any findings. Cooper's ligaments are connective tissue that attach perpendicularly to the dermis that help to support the breast. The breast contains 15 to 20 lobes which are further divided into smaller functional lobules. The posterior breast tissue lies on the pectoralis major fascia. The physical boundaries of the breast are the clavicle superiorly, the sternum medially, the insertion of the rectus abdominis muscles inferiorly, and the serratus anterior muscles laterally. The adult breast is roughly conical, the base of which overlies the pectoralis muscles in the upper portion of the chest. At this time, connective tissue becomes more dense and adipose tissue gradually replaces breast parenchyma. During menopause, decreases in circulating estrogen and progesterone cause the lobular tissue to undergo involution. Once this ceases, the buildup of pressure of unexpressed milk in the ductal system results in epithelial atrophy. Lactation occurs as long as there is neural stimulation of the nipple areolar complex through nursing. Complete breast maturity is not reached until pregnancy and delivery, during which time the epithelial tissue proliferates, and milk production is initiated. During puberty, various hormonal signals initiate further maturation of the female breast, most notably inducing ductal and connective tissue proliferation through the effects of estrogen and progesterone. Supportive connective tissue derives from epithelial cells. Breasts develop similarly between males and females in utero and are identical until the onset of puberty. These subsequently proliferate into 15 to 20 secondary buds, which later develop into the lobes found in the adult breast. Two buds of ectoderm penetrate the mesenchyme along the ventral ridges overlying the pectoral tissue, which are referred to as primary buds. These buds are bilateral ridges that extend from the future sites of the axilla to the inguinal region, the so-called "milk line," along which accessory nipples and breast tissue may occasionally be found in the adult. The breasts originate from ventral buds of ectoderm in the fifth to sixth week of fetal development. Many diseases of the breast arise from a derangement of normal function and follow along a spectrum of mild, benign abnormality to a malignant process. Knowledge of the basic anatomy and physiology of the breast provides a framework for understanding the pathophysiology of breast disease.
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