The relationships among adhesion, stratification and differentiation in keratinocytes

Epidermis is a self-renewing, multilayered tissue composed primarily of keratinocytes. The epidermal keratinocyte follows a terminal differentiation pathway that under normal circumstances is tightly linked to its position within the epidermis and culminates in the formation of the protective barrier (stratum corneum) that constitutes the outermost layer of skin. Strong but pliant adhesive mechanisms are essential for normal functioning of the epidermis. In the epidermis, adhesion is mediated primarily by four structures: hemidesmosomes and focal adhesions, which function in cell-matrix adhesion, and desmosomes and adherens junctions, which function in cell-cell adhesion. In this review we concentrate on the transmembrane components of these structures, which are thought to mediate directly the adhesive function. Members of the integrin family of adhesion molecules comprise the transmembrane components of hemidesmosomes and focal adhesions, although hemidesmosomes also have a second, unrelated transmembrane molecule known as 'bullous pemphigoid antigen 2'. Members of the cadherin family are the transmembrane constituents of desmosomes and adherens junctions. Desmosomes consistently contain two types of cadherins (desmoglein and desmocollin), while adherens junctions may contain only one type of cadherin (E- or P-cadherin). Expression of most of the transmembrane components varies with the position of the keratinocyte within the epidermis and thus may reflect the degree of epidermal differentiation. All of the integrin subunits have been localized predominantly to the basal layer. In contrast, the cadherins show very complex expression patterns throughout the epidermis. Desmogleins and desmocollins (the desmosomal cadherins) are each encoded by three genes, and the expression of each gene is limited to certain epidermal layers. With respect to the cadherins of the adherens junction, it has been shown that E-cadherin is present throughout the epidermis, while P-cadherin is limited to the basal layer. Interestingly, these complex expression patterns of integrins and cadherins within the epidermis may not simply be passive events in differentiation; rather, evidence is accumulating that adhesion molecules can exert a dynamic role in epidermal differentiation/stratification. For example, decreased adhesion to extracellular matrix, induced by changes in one or more integrins, appears to be a signal that induces certain differentiation-related events. Even more profound effects on epidermal morphogenesis have been demonstrated for the cadherins. E- and/or P-cadherin is required not only to initiate normal intercellular junction formation but also for the subsequent development of a stratified epithelium. Thus, the findings to date with both integrins and cadherins suggest that adhesion molecules may function not just as direct mediators of adhesion, but also as regulators of epidermal stratification, differentiation, and morphogenesis.