Epidermal keratinocyte stem cells: their maintenance and regulation.

The epidermis is a stratified epithelium consisting of interfollicular regions and appendages (hair follicles, sweat glands, sebaceous glands). The dominant cell type (the keratinocyte) is arranged in groups of cells termed epidermal proliferative units (EPUs), and one centrally-located clonogenic stem cell is ultimately responsible for replacing the remainder of the cells in the unit. Evidence is reviewed which indicates that the epidermal Langerhans' cell (ELC), and the cells comprising the dermis, may modify the keratinocyte microenvironment to create stem cell 'niches' and cellular diversity within the basal layer.