Stacey Mills_Histology for Pathologists_9781496398949

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SECTION I : CutaneousTissue

cells are larger, flattened, eosinophilic, and oriented parallel to the surface. The keratinocytes contain one or two con- spicuous nucleoli and tonofilaments within the cytoplasm. The squamous layer is also called the spinous or prickle cell layer because of the characteristic appearance by light microscopy of short projections extending from cell to cell. These projections are the result of retraction of the plasma membrane during tissue processing, whereas the desmosomes remain relatively fixed and correlate with intercellular bridges. Desmosomes are composed of a variety of polypeptides, desmogleins and desmocollins as transmembrane constitu- ents and the desmoplakin, plakoglobin, and plakophilin as cytoplasmic components. In addition, other intercellular junctions (such as gap junctions and adherens junctions) are distinct from desmosomes in composition and distribution and provide alternative cell-to-cell adhesion mechanisms (30). An intercellular space of constant dimension is present between each cell; acid and neutral mucopolysaccharides are present in the intercellular spaces as indicated by spe- cial stains. The pemphigus antigens are localized in the cell membranes (31) or in the desmosomes of these cells (32). Occasionally, Toker cells with clear or pale cyto- plasm are seen in the squamous layer. It is important to distinguish these cells from the neoplastic cells of Paget disease. Benign clear cells have a pyknotic nucleus sur- rounded by a clear halo and a narrow rim of clear cytoplasm (Fig. 1.3). They lack the pleomorphism, nuclear morphol- ogy, and intensity of the chromatin staining seen in Paget cells (Fig. 1.4). Regardless of gender (33), these benign

FIGURE 1.4  Paget cells in extramammary Paget disease.

clear cells are often seen in the epidermis of the nipple, the accessory nipple (34,35), and the pubic regions or in the milk-line distribution (36). In the nipple, these clear cells, also called Toker cells, have been considered to be non- neoplastic ductal epithelial cells, although some authors hypothesized that these cells might be the precursors of mammary or extramammary Paget diseases (35,37). Those outside of the nipple are considered to be the result of either abnormal keratinization or aberrant derivatives of eccrine or apocrine sweat gland epithelial cells (38–40). They may present as hypopigmented macules or papules in a rare dis- order called clear cell papulosis. The immunohistochemical staining pattern of benign clear cells may resemble that of Paget cells in that they react with the cytokeratin 7 (CK7) but differ from Paget cells in that they are usually negative for GCDFP-15. However, emphasis should be made that morphologic distinction is the most important manner to differentiate both cells. Common inflammatory changes seen in the squamous layer are (a) spongiosis—intercellular edema (e.g., allergic con- tact dermatitis), (b) acanthosis—thickening of the epidermis (e.g., psoriasis), (c) atrophy—thinning of the epidermis (e.g., discoid lupus erythematosus), (d) acantholysis—detachment of keratinocytes because of changes involving intercellular junctions (e.g., pemphigus), and (e) dyskeratosis—abnormal keratinization (e.g., squamous carcinoma). ❯❯ T he G ranular L ayer The granular layer is composed of one to three layers of flattened cells lying parallel to the skin surface. The cytoplasm contains intensely basophilic- stained granules known as the keratohyalin granules. In contrast, trichohyalin granules (produced by the inner root sheath of hair follicles) are stained red on routine hema- toxylin and eosin (H&E)-stained sections. The keratohya- lin granules are histidine rich and are the precursors to the protein filaggrin, which promotes aggregation of keratin filaments in the cornified layer.

FIGURE 1.3  Clear cells of the nipple epidermis. opyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

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