Stacey Mills_Histology for Pathologists_9781496398949

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SECTION I : CutaneousTissue

FIGURE 1.12  Electron micrograph of a Langerhans cell containing Birbeck granules ( arrows ) and multisegmented nucleus ( × 8,000).

FIGURE 1.13  Cytokeratin 20 staining a Merkel cell in the basal layer of the epidermis.

Merkel cells are not recognized in routine histologic preparations. Electron microscopy and immunostaining are required for their identification. By electron microscopy, MCs are attached to adjacent keratinocytes by desmosomes. They have scant cytoplasm, invaginated nuclei, a parallel array of cytokeratin filaments in the paranuclear zone, and the characteristic membrane-bound dense core granules that are often, but not always, related to unmyelinated neurites. By immunostaining techniques, normal and neoplastic MCs may express neuron-specific enolase, chromogranin, synaptophysin, neural cell adhesion molecule, and various neuropeptides and other substances (63–65). However, the expression of these substances in MCs is heteroge- neous and variable. The constant pattern seen in MCs is the presence of paranuclear aggregates of cytokeratins (15,65,66), which include low–molecular-weight keratins 8, 18, 19, and 20. The most specific cytokeratin is CK20 because, in addition to MCs, they are expressed in simple epithelial cells and not in adjacent keratinocytes (67,68) (Fig. 1.13). Pilar Unit The pilar unit is composed of the hair follicle, sebaceous gland, arrector pili muscle, and (when present) eccrine and apocrine glands. ❯❯ H air F ollicle The hair follicle is divided into three segments from top to bottom: (a) the infundibulum,

which extends from the opening of the hair follicle in the epidermis to the opening of the sebaceous duct; (b) the isthmus, which extends from the opening of the sebaceous duct to the insertion of the arrector pili muscle; and (c) the inferior segment, which extends to the base of the follicle. The inferior segment is bulbous and encloses a vascularized component of the dermis referred to as follicular (dermal) papilla of the hair follicle (Fig. 1.14). The microanatomy and function of the hair follicle are very complex. The cells of the hair matrix differentiate along six cell linings. Beginning from the innermost layer, they are (a) the hair medulla; (b) the hair cortex; (c) the hair cuticle; and (d) three concentric layers of the inner root sheath, which are the cuticle of the inner root sheath, Huxley layer, and Henle layer. The inner root sheath of the hair follicle is surrounded by the outer root sheath (Fig. 1.15), which is composed of clear cells. These glycogen-rich cells are seen in some of the neoplasm with hair follicular differentiation (e.g., trichilem- moma). A PAS-positive basement membrane separates the outer root sheath from the surrounding connective tissue. Thus, the hair shaft is formed from the bulb region that occupies the hair follicular canal. Dendritic melanocytes are present only in the upper half of the bulb, whereas inactive (amelanotic) melanocytes are present in the outer root sheath. These melanocytes can become active after injury, migrating into the upper portion of the outer root sheath and to the regenerating epidermis.

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