Pretreatment with croton oil, DNA synthesis. And carcinogenesis by carcinogen followed by croton oil

In the mouse, the number of epidermal cells labelled by tritiated thydidine is reduced 12 hours after a single application of 5% croton oil in mineral oil, increases four‐fold 24 hours affterwards, and returns towards normal 72 hours afterwards. The yield of papillomata induced by a single application of 7,12‐dimethylbenz (α)anthracene followed by repeated applications of croton oil is increased by an application of croton oil given 23 hours before the administration of carcinogen, but not by croton oil given 72 hours beforhand. The yield of papillomata induced by a single injection of urethane followed by croton oil is also increased by an application of croton oil given 23 hours before the carcingogen. These findings are cokmpatible with the theory that the carcinogens tested act on epidermal cells synthesizing DNA. It is also suggested that, following a single application, mineral oil may remain on the skin for at least 24 hours. If a carcinogen is applied within this period, it may be diluted, and so apparently reduced in potency.