Onset of epidermal differentiation in rapidly proliferating basal keratinocytes.

Stratified epithelia such as epidermis are classically considered to comprise 2 cell compartments, one consisting of undifferentiated proliferative cells occupying the basal layer, and the other consisting of differentiated postmitotic cells occupying the suprabasal layers. It is also generally assumed that the 58K basic-50K acidic couple of keratins is expressed in basal cells, while the 67K basic-56K acidic couple appears in suprabasal cells. In the present work we demonstrate that the population of basal keratinocytes is heterogeneous, since 8% of them are found to express the 67-56K "suprabasal" set of keratins. The morphology of these transitional cells suggests that they are in the process of detaching from the basement membrane to move upward to the epidermis. Cytoflow-fluorometric studies showed that the fraction of cells in S plus G2/M phases is 4 times higher in transitional keratinocytes than in basal or suprabasal keratinocytes. Altogether, these results suggest that the onset of terminal differentiation occurs in human epidermis in a subpopulation of keratinocytes which are still located in the basal layer, and that a transient increase in proliferation occurs when the cells engage in terminal differentiation and are ready to move toward the suprabasal layers.