The human breast is formed from the milk stream, an ectodermally derived tissue, which eventually atrophies, leaving behind the nipple bud. The glandular part of the organ develops subcutaneously. Breast ducts extend down from the nipple into the glandular portion of the breast. The mature breast lies between the deep and the superficial layers of the superficial pectoral fascia. Within the axillary space lie multiple groups of nodes, which are of great interest to the surgeon. Knowledge of the lymphatic drainage of the breast allows prediction of sites of metastatic spread.
Three tissue types make up the mature breast: epithelium, fibrous stroma, and fat. The fat composition of the breast increases with age, allowing improved radiographic detection of abnormalities by mammography. Cooper's ligaments are strands of dense connective tissue that extend from the skin through the fat to the underlying deep fascia. Any contraction or impingement on these ligaments, as commonly occurs with cancer, causes a dimpling pattern of the skin. The glandular portion of the breast is composed of ducts that extend downward from the nipple areolar complex and branch radially, terminating in spaces called ductal glandular units, the milk-producing glands of the lactating breast. At the surface of the nipple are 15 to 30 orifices lined with low columnar or cuboidal epithelium that meets with squamous epithelium at the surface.
In prepubescence, the breast tissue is composed of dense fibrous stroma and scattered ducts. Increase in adipose tissue and glandular development are under hormonal influence. The development of breast buds in a prepubescent female unaccompanied by other changes of puberty is called prepubertal gynecomastia, and this entity should not be confused with puberty. During phases of the menstrual cycle, the breast undergoes cyclic changes, including hypertrophy of the stroma and the epithelium and intralobular fluid accumulation corresponding to perimenstrual breast engorgement. During pregnancy there is an increase in lobular units; this is termed adenosis of pregnancy. In lactating females, prolactin stimulates milk production. Milk expulsion is under the control of oxytocin stimulation of myoepithelial cells.
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