[Reaction of epidermis after long-term action of urea]

The epidermis thins in response to short-term contact with urea, and the results of extensive enzymological and autoradiographical studies suggest that urea acts on processes involved in epidermal proliferation. After long-term exposure of skin to urea, lasting more than 2, 4, or 6 weeks, no further thinning occurs, and there is no tendency for atrophy to develop during this period. This assertion is made on the basis of biometrical, autoradiographical, and cytophotometrical data. It is likely, however that a reduced number of cells synthesizing DNA is not the only change in normal epidermal proliferation leading to the peidermal thinning that was measured. The urea could also alter factors regulating either cell entry into DNA synthesis or the synthetic process itself, or both. These findings clearly are of significance not only in industrial medicine but also in the use of urea in topical dermatological therapy.