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 program med cell death and the proportion of keratinocytes actively engaged in the cell cycle or growth fraction. The last parameter is still a subj ect of debate as the identification of a proliferative cell with certa inty is impossible. Two indirect methods (grain count dilution of labe lled cells and continuous labelling) suggest that non-proliferative ce lls may exist in the germinative compartment of human epidermis. Resul ts obtained in vitro demonstrate that keratinocytes can be blocked in certain conditions in a state of quiescence similar to a G(0) phase. I ncreasing evidence suggests that, as in mouse epidermis, the germinati ve compartment of human epidermis is heterogeneous and is divided into a small proportion of stem cells (10%) with a high proliferative pote ntial, a larger proportion of transit-amplifying cells with a limited proliferative potential (50%) and postmitotic cells committed to under go terminal differentiation (40%). In this hierarchical arrangement, m ost of the nonproliferative cells belong to the postmitotic compartmen t. If this model is correct, the germinative cell population in a quie scent or G(0) state is a small proportion of the stem cell compartment and has therefore little influence on the kinetics of normal human ep idermis.