A new model of epidermal differentiation: induction by mechanical stimulation.

In vivo, epidermal cells are committed to terminal differentiation in which they undergo a series of morphological and biochemical changes. In vitro, keratinocytes are able to undergo some steps of this differentiation process only. In view of the fact that in vivo skin is continuously subjected to mechanical stress, we investigated the stimulation of differentiation of transformed keratinocytes by mechanical stimulation. The cells, grown in plastic culture dishes, were periodically treated with weights exerting a pressure of 0.015 Ncm-2. This stimulation lasted from 1 to 4 days. Then keratinocytes were examined using indirect immunofluorescence, 3H-thymidine and 14C-amino acid incorporation, SDS polyacrylamide gel electrophoresis, and Western blotting. Following pressure treatment, the previously monolayered keratinocytes locally grew up to several layers, the number of horny scales increased and, after 4 days, the pattern of cytokeratin was modified. The total amount of keratin increased, forming granular accumulations, while the proliferation rate of the cells decreased. Both the 67 kDa and 49.5 kDa keratin subunits increased in stimulated cells. Moreover, a weak keratin band of 44 kDa appeared that was not present in controls. The results demonstrate that cyclic pressure promotes differentiation of cultivated epidermal cells.