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Epidermal stem cells.

Stem cells are by definition present in all self-renewing tissues and are believed to play a central role in cell growth and differentiation. Existing evidence suggests that a subpopulation of epidermal basal keratinocytes represents stem cells; however, these cells have never been positively identified. In this paper we review evidence that in monkey palm epidermis there exist two morphologically distinct subpopulations of basal keratinocytes that are spatially segregated. One population, located in the shallow rete ridges, is characterized by a cytoplasm filled with tonofilaments and a highly convoluted ("serrated") dermal-epidermal junction; these cells may play a role in anchoring the epidermis to the dermis. In contrast, the other population, located at the tips of deep rete ridges, is characterized by a "primitive" cytoplasm containing abundant melanosomes and a relatively flattened ("nonserrated") dermal-epidermal junction. Tritiated thymidine labeling experiments suggest that the nonserrated basal keratinocytes are slow-cycling; however, a highly proliferative population of keratinocytes can be identified immediately above these basal cells. These findings are consistent with the concept that the nonserrated basal keratinocytes may represent stem cells that give rise to suprabasally located, transient amplifying cells before undergoing terminal differentiation. Monkey palm epidermis provides a model system for further studies of primate epidermal stem cells.