Mills Ch3 Breast

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CHAPTER 3:  Breast

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FIGURE 3.6  The mammary ductal-lobular system is lined by a dual cell population, an inner epithelial cell layer and an outer layer of myoepithelial cells. A: High-power view of a lobule. The myoepithelial cells sur- rounding the acinar epithelial cells are variably conspicuous. B: High-power view of an extralobular duct, showing distinct epithelial and myoepithelial cell layers.

The epithelium throughout the ductal-lobular system is bilayered, consisting of an inner (luminal) epithelial cell layer and an outer (basal) myoepithelial cell layer. The importance of this double cell layer cannot be overempha- sized because it is one of the main guides used to distinguish benign from malignant lesions (19). The luminal epithelial cells of the resting breast ducts and lobules are cuboidal to columnar in shape and typically have pale eosinophilic cyto- plasm and relatively uniform oval nuclei. These epithelial cells express a variety of low—molecular-weight cytokera- tins, including cytokeratins 7, 8, 18, and 19 (20–24). The outer (or myoepithelial) cell layer, although always present, is variably distinctive (Fig. 3.6). Myoepithelial cells range in appearance from barely discernible, flattened cells with compressed nuclei to prominent epithelioid cells with

abundant clear cytoplasm. In some cases, the myoepithelial cells have a myoid appearance featuring a spindle cell shape and dense, eosinophilic cytoplasm, reminiscent of smooth muscle cells (Fig. 3.7). Even when inconspicuous on hema- toxylin- and eosin-stained sections, myoepithelial cells can readily be demonstrated using immunohistochemical stains for a variety of markers, including actins, calponin, smooth muscle myosin heavy chain, p63, CD10, and p75 among others (Fig. 3.8) (25–30). However, these markers vary in both sensitivity and specificity for myoepithelium as well as in their expression by location within the terminal duct lobular unit (TDLU). Myoepithelial cells express high— molecular-weight cytokeratins 5/6, 14, and 17 (20–24,31), but the expression of cytokeratin 14 is restricted to the myoepithelial cells of the large ducts and terminal ducts;

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FIGURE 3.7  Myoepithelial cells can vary in their histologic appearance. A: Myoepithelial cells in this lobule show prominent cytoplasmic clearing. B: In this lobule, the myoepithelial cells show myoid features.