Figure 1419

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Photomicrograph of a sagittal section of distal phalanx with a nail. A

nail is a keratinized plate located on the dorsal aspect of the distal phalanges. Under the free edge of the nail is a boundary layer, the hy-ponychium, which is continuous with the stratum corneum of the adjacent epidermis. The proximal end, the root of the nail, is overlapped by skin, the eponychium, which is also continuous with the stratum corneum of the adjacent epidermis. Deep to the nail is a layer of epithelium with underlying dermis. The proximal portion of this epithelium is referred to as the nail matrix. The bone in this section represents a distal phalanx. Numerous Pacinian corpuscles are present in the connective tissue of the palmar side of the finger. Note that even at this low magnification, the stratum lucidum in visible in the epidermis of the fingertip. x10.

zone, or matrix. The matrix contains a variety of cells including stem cells, epithelial cells, melanocytes, Merkel's cells, and Langerhans' cells. The stem cells of the matrix regularly divide, migrate toward the root of the nail, and there differentiate and produce the keratin of the nail. Nail keratin is a hard keratin, like that of the hair cortex. Unlike the soft keratin of the epidermis, it does not desqua-

Epidermal repair is effected by basal cell proliferation or, in extensive trauma, by hair follicle and sweat gland epithelia. The repair of an incision or laceration of the skin requires stimulated growth of both the dermis and the epidermis. Dermal repair involves (a) removal of damaged collagen fibers in the wound site, primarily through the effort of macrophage activity; and (b) proliferation of fibroblasts and subsequent production of new collagen and other extracellular matrix components. Application of sutures reduces the extent of the repair area through maximal closure of a wound, minimizing scar formation. Surgical incisions are typically made along cleavage lines; the cut tends to parallel the collagen fibers, thus minimizing the need for excess collagen production and the inherent scarring that may occur.

Repair of the epidermis involves the proliferation of the basal keratinocytes in the stratum germinativum in the undamaged site surrounding the wound (Fig. 14.20). Mitotic activity is markedly increased within the first 24 hours. In a short time, the wound site is covered by a scab. The proliferating basal cells of the stratum germinativum begin migrating beneath the scab and across the wound surface. The migration rate may be up to 0.5 mm/day, starting within 8 to 18 hours after wounding. Further proliferation and differentiation occur behind the migration front, leading to restoration of the multiiayered epidermis. As new cells ultimately keratinize and desquamate, the overlying scab is freed with the desquamating cells, which explains why a scab detaches from its periphery inward.

In cases in which the full thickness of the epidermal layer is removed either by trauma or in surgery, the deepest parts of hair follicles and glands that remain as islands of epithelial cells in the dermis will divide and produce cells that migrate over the exposed surface to reestablish a complete epithelial (epidermal) layer. Massive destruction of all of the epithelial structures of the skin, as in a third-degree burn or extensive full-thickness abrasion, prevents reepithelialization. Such wounds can be healed only by grafting epidermis to cover the wounded area. In the absence of a graft, the wound would, at best, reepithelialize slowly and imperfectly by ingrowth of cells from the margins of the wound.

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