Stacey Mills_Histology for Pathologists_9781496398949

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CHAPTER 1:  Skin

Histologic observation of this layer can provide key find- ings in certain entities such as increase (e.g., lichen planus) and decrease (e.g., psoriasis) in the thickness of the granular layer. Keratinocytes, located between the squamous layer and the granular layer, contain small membrane-coating gran- ules known as lamellar granules (also called Odland bodies or keratinosomes). They are composed of the acid hydro- lase and neutral sugars conjugated with proteins and lipids. These granules, present both intra- and extracellularly, are approximately 300 nm in diameter and are not visible by light microscopy. Their functions are to provide epidermal lipids, increase the barrier property of the cornified layer against water loss, and aid in the desquamation process. This interface between the squamous and the granular layer is also the site of synthesis and storage of cholesterol (41). ❯❯ T he C ornified L ayer The cornified layer is composed of multiple layers of polyhedral eosinophilic keratinocytes that lack a nucleus and cytoplasmic organelles. These cells are the most differentiated cells of the keratinization system. They are composed entirely of high–molecular-weight kera- tin filaments. In formalin-fixed section, the cornified layers are arranged in a basket-weave pattern (Fig. 1.5). These cells eventually shed from the surface of the skin. The process of keratinization takes 20 to 45 days. In histologic sections taken from the skin of the palms and soles, a homogenous eosinophilic zone, known as the stratum lucidum is present in the lowest portion of the cor- nified layer (above the granular layer). This additional layer is rich in extracellular elements such as energetic enzymes and SH groups adding to the normal functional barrier of the skin (42). Common abnormalities of the cornified layer are (a) hyperkeratosis—increased thickness in the cornified layer (e.g., ichthyosis), (b) parakeratosis—presence of nuclei in the cornified layer (as usually seen in actinic keratosis), and (c) presence of fungal organisms (superficial dermatophytosis).

FIGURE 1.6  PAS-positive basement membrane.

Basement Membrane Zone The basement membrane zone separates the epidermal basal layer from the dermis. It is seen by light microscopy as a continuous, undulating, and thin periodic acid–Schiff (PAS)-stained layer (Fig. 1.6). By electron microscopy, the basal cells are attached to the basal lamina by hemidesmo- somes. Ultrastructurally, the basement membrane zone is composed of four distinct structures, from top to bottom (Fig. 1.7) (43): 1. The plasma membrane of the basal cells containing the hemidesmosomes. Bullous pemphigoid antigen 1 is local- ized in the intracellular component of hemidesmosomes. 2. The lamina lucida, an electron-lucent area with anchoring filaments containing various laminin isoforms (44). Bullous pemphigoid antigen 2 (type XVII collagen) is associated with the transmembrane component of hemidesmosome- anchoring filament complexes in the lamina lucida. It is also the site of the blister in dermatitis herpetiformis (45). 3. The lamina densa, an electron-dense area composed of mainly type IV collagen. 4. The sublamina densa zone, or pars fibroreticularis, contains mainly the anchoring fibrils (46) (type VII col- lagen) that attach the basal lamina to the connective tissue of the dermis. Antibodies against epidermolysis bullosa acquisita react with the carboxy terminus of type VII collagen (47,48). Inflammatory conditions of the basement membrane can be seen by light microscopy as thickening (e.g., discoid lupus erythematosus) or by the formation of subepidermal vesicles (e.g., bullous pemphigoid). Melanocytes Melanocytes are dendritic cells that derive from the neu- ral crest. During migration from the neural crest, mela- nocytes may localize in other epithelia. In the epidermis,

FIGURE 1.5  Basket-weave pattern of the cornified layer (also in Fig. 1.2). Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

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