Epidermal Stem Cells and Dermal–Epidermal Junction
Aging of mammalian skin results from two distinct biological processes: intrinsic and extrinsic mechanisms of skin aging. Exposure to solar ultraviolet (UV) radiation (photoaging) is the major extrinsic assault accelerating the normal aging process. The epidermis constitutes the outermost layer of the skin and thus is constantly challenged by harmful environmental assaults. As an integral part of the skin barrier, the epidermis ensures protection by its robust stratified architecture and integrated epidermal appendages. The epidermal tissue is further stabilized by specialized zones connecting the epidermis to the underlying dermal tissue. These dermal–epidermal junctions (DEJs) consist of a network of different intracellular, transmembrane and extracellular proteins that together fulfil crucial biological and structural functions. Above all, the epidermis relies on tissue stem cells to maintain homeostasis and to guarantee tissue repair following damage. Hence, proper stem cell function is pivotal and required to sustain assaults. In this review we present important structural and functional aspects of the DEJ and its role in aging of the skin. We also discuss implications and protective mechanisms of UV-irradiated epidermis, highlighting stem cell-specific surveillance mechanisms and the important function of the stem cell niche.