EPIDERMAL STEM-CELLS - MARKERS, PATTERNING AND THE CONTROL OF STEM-CELL FATE

Within the epidermis, proliferation takes place in the basal layer of keratinocytes that are attached to an underlying basement membrane. Ce lls that leave the basal layer undergo terminal differentiation as the y move towards the tissue surface. The basal layer contains two types of proliferative keratinocyte: stem cells, which have unlimited self-r enewal capacity and transit amplifying cells, those daughters of stem cells that are destined to withdraw from the cell cycle and terminally differentiate after a few rounds of division. Stem cells express high er levels of the beta(1)-integrin family of extracellular matrix recep tors than transit amplifying cells and this can be used to isolate eac h subpopulation of keratinocyte and to determine its location within t he epidermis. Variation in the levels of E-cadherin, beta-catenin and plakoglobin within the basal layer suggests that stem cells may also d iffer from transit amplifying cells in intercellular adhesive ness. St em cells have a patterned distribution within the epidermal basal laye r and patterning is subject to autoregulation. Constitutive expression of the transcription factor c-Myc promotes terminal differentiation b y driving keratinocytes from the stem cell compartment into the transi t amplifying compartment.