ABSTRACT
The skin consists of an outer squamous epithelium, the epidermis, and an inner connective tissue, the dermis (also containing pilosebaceous units, nails, and sweat glands). The epidermis fulfills the crucial barrier function of the skin and undergoes continuous self-renewal as a result of mitotic activity of the stem cells in the basal layer that provide new keratinocytes. The keratinocytes (the major cellular skin component constituting approximately 90%–95% of the epidermis) complete a differentiation-induced cell death program (called cornification) while moving upward through the different epidermal layers to become the corneocytes in the outer layers of the epidermis before they are shed from the skin (Figure 6.1).
