The growth fraction of normal human epidermis.

In human epidermis, germinative cell production is determined by three parameters: the cell cycle time, the loss of cycling cells by programmed cell death and the proportion of keratinocytes actively engaged in the cell cycle or growth fraction. The last parameter is still a subject of debate as the identification of a proliferative cell with certainty is impossible. Two indirect methods (grain count dilution of labelled cells and continuous labelling) suggest that non-proliferative cells may exist in the germinative compartment of human epidermis. Results obtained in vitro demonstrate that keratinocytes can be blocked in certain conditions in a state of quiescence similar to a G0 phase. Increasing evidence suggests that, as in mouse epidermis, the germinative compartment of human epidermis is and is divided into a small proportion of stem cells (10%) with a high proliferative potential, a larger proportion of transit-amplifying cells with a limited proliferative potential (50%) and postmitotic cells committed to undergo terminal differentiation (40%). In this hierarchical arrangement, most of the nonproliferative cells belong to the postmitotic compartment. If this model is correct, the germinative cell population in a quiescent or G0 state is a small proportion of the stem cell compartment and has therefore little influence on the kinetics of normal human epidermis.