Making sense of the epithelial barrier: what molecular biology and genetics tell us about the functions of oral mucosal and epidermal tissues.

The epidermis of skin and the oral mucosa are highly specialized stratified epithelia that function to protect the body from physical and chemical damage, infection, dehydration, and heat loss. To maintain this critical barrier, epithelial tissues undergo constant renewal and repair. Epithelial cells (keratinocytes) undergo a program of terminal differentiation, expressing a set of structural proteins, keratins, which assemble into filaments and function to maintain cell and tissue integrity. Two types of cell adhesion structures, desmosomes and hemidesmosomes, function to glue keratinocytes to one another and to the basement membrane, and connect the keratin cytoskeleton to the cell surface. Keratinizing epithelia such as the epidermis and oral gingiva that have to withstand severe physical and chemical forces produce a toughened structure, the cornified cell envelope. This envelope is a major component of the epithelial barrier at the tissue surface. This article summarizes our current understanding of the structure and function of these different cellular components and discusses various genetic and acquired diseases that alter tissue integrity and barrier function. We also highlight recent work demonstrating how loss or attenuation of certain proteases can lead to early onset periodontitis and tooth loss as well as other epithelial abnormalities.