Stacey Mills_Histology for Pathologists_9781496398949

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CHAPTER 1:  Skin

The eccrine glands develop from the fetal epidermis independent of the hair follicles (21). Initially, they are seen as regularly spaced undulations of the basal layer. At 14 to 15 weeks, the tips of the primordial eccrine glands have reached the deep dermis, forming the eccrine coils (26). At the same time, the eccrine epithelium grows upward into the epidermis. The primordial eccrine epithelium acquires a lumen by the 7th to 8th fetal month, and thus the first eccrine unit is formed. Both ducts and secretory portions are lined by two layers of cells. The two layers in the secre- tory segment undergo further differentiation; the luminal cells into tall columnar secretory cells, and the basal layer into secretory cells or myoepithelial cells. The first glands are formed on the palms and soles by the 4th month, then in the axillae in the 5th month, and finally on the rest of the hairy skin (27). Epidermis The epidermis is a stratified and keratinizing squamous epi- thelium that dynamically renews itself maintaining its nor- mal thickness by the process of desquamation. The cells in the epidermis include (a) keratinocytes, (b) melanocytes, (c) Langerhans cells, (d) Toker cells (in certain anatomic locations), and (e) Merkel cells. In addition, the epidermis contains the openings for the eccrine ducts (acrosyringium) and hair follicles. Recent immunohistochemical studies have demonstrated that the epidermis contains free nerve axons in association with Langerhans cells (28). Keratinocytes The keratinocytes of the epidermis are stratified into four orderly layers from bottom to top: (a) the basal layer (stratum basale, germinativum), (b) the squamous layer (prickle cell layer or stratum spinosum), (c) the granular layer (stratum granulosum), and (d) the cornified or horny layer (stratum corneum) (Fig. 1.1). In histologic sections, the dermoepi- dermal junction has an irregular contour because of the upward extension of the papillary dermis to form the dermal papillae. The portion on the epidermis separating the dermal papillae are the rete ridges (Fig. 1.2). The transcription factor p63 plays an important role in this orderly arrangement and continuous development of the pre- and postnatal skin (29). ❯ T he B asal L ayer Basal cells are the mitotically active cells that give rise to the other keratinocytes. Histologi- cally, basal cells are seen as a single layer of cells above the basement membrane that show minor variations in size, shape, and melanin content. Basal cells are columnar or cuboidal, with a basophilic cytoplasm. The nucleus is round or oval, with coarse chromatin and indistinct nucleolus. Basal cells contain melanin in their cytoplasm as a result HISTOMORPHOLOGY

FIGURE 1.1  Electron micrograph of normal epidermis and portion of papillary dermis ( × 2,100) ( 1 , papillary dermis; 2 , basal cells; 3 , squamous layer; 4 , granular layer; 5 , cornified layer).

of pigment transfer from neighboring melanocytes. Basal cells are connected to each other and to keratinocytes by specialized regions (known as desmosomes) located in the plasma cell membranes. They are aligned perpendicular to the subepidermal basement membrane and attached to it by modified desmosomes, hemidesmosomes. Certain dermatitides involving the basal layer produce vacuolar alteration of the basal cells, which may progress to the formation of subsequent subepidermal vesicles as seen in diseases such as graft-versus-host disease, lupus erythe- matosus, and erythema multiforme. ❯ T he S quamous L ayer The squamous layers are com- posed of approximately 5 to 10 layers of cells with keratino- cytes larger than the basal cells. The suprabasal keratinocytes are polyhedral, have a somewhat basophilic cytoplasm, and a round nucleus. Again, melanin is seen scattered in many of these keratinocytes, where it provides protection from the damaging effect of ultraviolet light. The more superficial

FIGURE 1.2  Normal skin showing stratified epidermis with rete ridges, papillary dermis, and reticular dermis (H&E). Copyright © 2020 Wolters Kluwer Health, Inc. Unauthorized reproduction of this content is prohibited.

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