The skin's barrier.

The skin provides an effective barrier between the organism and the environment, preventing the invasion of pathogens and fending off chemical and physical assaults, as well as the unregulated loss of water and solutes. In this review we provide an overview of several components of the physical barrier, as well as how barrier function is regulated and altered in association with dermatoses. The physical barrier localized primarily in the stratum corneum (SC) and consists of protein-enriched cells (corneocytes with cornified envelope and cytoskeletal elements, as well as corneodesmosomes) and lipid-enriched intercellular domains. The nucleated epidermis, with its tight, gap and adherens junctions, additional desmosomes and cytoskeletal elements, also contributes to the barrier. Lipids are synthesized in the keratinocytes during epidermal differentiation and are then extruded into the extracellular domains, where they form lipid-enriched extracellular layers. The cornified cell envelope, a robust protein/lipid polymer structure, is located below the cytoplasmic membrane on the exterior of the corneocytes. Ceramides A and B, forming the backbone for the subsequent addition of free ceramides, free fatty acids and cholesterol in the SC, are covalently bound to cornified envelope proteins. Filaggrin is cross-linked to the cornified envelope and aggregates keratin filaments into macrofibrils. Cytokines, cAMP and calcium influence the formation and maintenance of barrier function. Changes in lipid composition and epidermal differentiation lead to a disturbed skin barrier, which allows the entry of environmental allergens, immunological reaction and inflammation in atopic dermatitis. A disturbed skin barrier is an important component in the pathogenesis of contact dermatitis, ichthyosis, psoriasis, and atopic dermatitis.