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Mechanical stimulation of dorsal mouse skin by skin massage or removal of the horny layer results in a mutually comparable increase in DNA-labelling and mitotic activity. However, only after injury such as removal of the horny layer hyperplasia develops. This phenomenon, called 'hyperplastic transformation' is characterized by a transient abolition of the epidermal GI chalone responsiveness. There is some indication that the susceptibility to a heat labile factor, probably the epidermal G2 chalone, is not affected. Skin massage neither interferes with the responsiveness to epidermal G1 chalone nor induces 'hyperplastic transformation'.Mouse tail epidermis shows a 'functional hyperplasia' and responds to the GI chalone. To explain these observations, it is assumed that the epidermal stem cell population is heterogeneous consisting of GI chalone-sensitive and G1 chalone-insensitive cells.

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