[Pathogenesis of human cancer development due to environmental factors]

In spite of a remarkable progress of the experimental research on carcinogenesis, no clear-cut elucidation has been made on pathogenesis of human cancer. Carcinogenesis due to environmental factors may be characterized by the diversity in causal agents, the lower level in the exposure density and the very random exposure chance to the agents. These characteristics are largely different from those employed in the ordinary animal experiment. Therefore, simple deduction from the experimentally obtained knowledges may lead to incorrect understanding of the developmental steps in human cancer. An instance clearly showed this is differences in the morphological features observed between the human and the animal tissues in which the former often shows a single and de novo development but the latter exhibits various precancerous lesions and multicentric development. In human, repeated exposures to carcinogens are primarily essential to induce cancer in the usual conditions, and the more possible way of carcinogenesis would be the synergistic and/or cocarcinogenic process including initiation-promotion steps. These problems were discussed by using the author's own evidences obtained from animal model experiments.