The breast is a modified sweat gland located in the superficial fascia of the anterior chest wall. The mature female breast has a distinctive protuberant, mound-shaped, or conical form and covers the area from the second or third rib to the sixth or seventh rib (Fig. 1.1). The nipple projects from the anterior surface and consists mainly of dense fibrous tissue covered by hyperpigmented skin and contains bundles of smooth muscle fibres. The skin immediately surrounding the nipple, called the areola, is also more pigmented than the rest of the breast skin. The areola contains sebaceous glands and numerous sensory nerve endings but lacks pilosebaceous units and hair (Figs. 1.2 and 1.3).

The breast is made up of glandular and ductal elements embedded within fibrofatty tissue with a ratio of glandular to fibrofatty tissue that varies among individuals (Fig. 1.4) [1]. In addition, with the onset of menopause and decreased oestrogen levels, the relative amount of fatty tissue increases as the glandular tissue diminishes. The breast ductal system consists of 15–20 branching ducts, which radiate from the nipple to continue to the functional units of the breast, the terminal ductal lobular units (TDLUs) (Fig. 1.5). The TDLUs consist of the intralobular ducts and round saccules called ductules, which differentiate into the secretory units or acini during pregnancy and lactation. The terms ductules and acini are often used interchangeably. Individual TDLUs vary greatly in size and typically enlarge to become functional during lactation (Fig. 1.6). The TDLUs are embedded in specialised, hormonally responsive connective tissue stroma called intralobular stroma (Fig. 1.7) [2–8]. The largest amount of breast parenchyma is located in the upper outer quadrant, where the majority of cancers develop. An axillary tail of breast tissue often extends into the axilla. Before puberty, female and male breasts have the same appearance. The structure of the breast is under the influence of hormones, growth, and differentiation factors. When puberty begins in females, mammary ducts branch out, terminal duct buds are formed, and the stromal component (mainly adipose tissue) of the breast proliferates. Both stroma and epithelium undergo changes during the menstrual cycle, pregnancy, lactation, and menopause. During male puberty, breast development is limited to rudimentary large duct development without breast enlargement.

The mammary epithelium is ectodermally derived. Small segments of lactiferous duct orifices at the nipple are lined by squamous epithelium, while the rest of the breast ductal system is lined by two cell layers, inner luminal cells and outer myoepithelial cells, surrounded by the basement membrane (Fig. 1.8). Extension of squamous epithelium beyond lactiferous duct orifices represents squamous metaplasia. The luminal cells are usually low columnar to cuboidal, and myoepthelial cells are located between luminal cells and the basement membrane. The myoepithelial cells are often ovoid to spindle shaped and have scant cytoplasm. The mammary ducts and lobules are embedded in fibrofatty stroma. Interlobular stroma contains adipose tissue, fibroblasts and elastic fibres. Scant inflammatory cells, including lymphocytes, plasma cells, mast cells, and histiocytes, are commonly seen. Rarely, stromal cells show prominent myoid differentiation (Fig. 1.9). The intralobular stroma is usually loose and more cellular than the interlobular stroma, and unlike the interlobular stroma, it usually does not have adipose tissue. Intralobular stroma is hormone sensitive and shows cyclic histologic changes.