The morphogenesis of 1,2-dimethylhydrazine (DMH)-induced lesions in the colon of outbred NIH Swiss mice was determined for up to 5 months of treatment. The effect of hyperplasia on DMH carcinogenesis was also evaluated by introducing a transient hyperplastic stimulus to the colon during the chronic weekly treatment regimen of DMH. The hyperplastic stimulus was a naturally occurring disease of mice, transmissible murine colonic hyperplasia, which is caused by a variant of Citrobacter freundii. In control mice, those not receiving the bacterium, weekly injections of the carcinogen induced neoplastic changes first detectable at two months of treatment in all segments of the colon and in both sexes. The changes increased in frequency and severity with time. Diffuse mucosal hyperplasia and chronic inflammatory and degenerative changes were also associated with DMH after prolonged treatment. The hyperplastic stimulus of C. freundii reduced the latent period for appearance of early DMH tumors, but it had no influence on already established DMH tumors.

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Supported by the National Large Bowel Cancer Project USPHS Research Grant 5 R26 CA 15405, National Cancer Institute.

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