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INTRODUCTION
The main function of the epidermis is to provide a barrier between the organism and the environment. This barrier is the result of a complex terminal differentiation program in which the progeny of epidermal stem cells undergo a series of molecular and biochemical changes that lead to the generation of the morphologically distinct layers of the epidermis (Fig. 56.1). This chapter reviews the molecular and cellular mechanisms that control this terminal differentiation program. It also discusses inherited skin diseases that are caused by a failure of normal epidermal differentiation.
Fig. 56.1 Epidermal differentiation. As keratinocytes are transformed from mitotically active cells in the basal layer to fully differentiated, anucleated squames in the cornified layer, specific proteins are expressed at particular locations within the epidermis. Keratohyalin (profilaggrin- and loricrin-containing) and lamellar (lipid-containing) granules extrude their contents in the granular layer, leading to bundling of keratin filaments and replacement of the plasma membrane with the highly cross-linked, lipid-covered cornified cell envelope (see inset and Fig. 56.2). Courtesy Julie V. Schaffer, MD.