Reversibility of 20-methylcholanthrene-induced bronchial cell atypia in dogs.

The sequence of cellular alterations in the bronchial epithelium of individual dogs was studied by means of combined morphologic and cytochemical techniques during a period of exposure to 20-methylcholanthrene (20-MC) and during a subsequent period without treatment. In agreement with earlier observations, 20-MC was found to induce progressive cellular changes which in four of seven dogs resulted in the occurrence of cells morphologically diagnosed as cancer cells. When the 20-MC treatment was stopped these cellular alterations were found to regress in three of the four dogs. Only in one dog did the advanced cellular atypia persist and cause the death of the animal. The results indicate that cells exhibiting cytomorphologic and cytochemical characteristics of cancer cells may reflect reversible lesions which regress on withdrawal of the carcinogen.