Profiles of induced tumors in animals

Chemical carcinogens often induce neoplasms which differ qualitatively and quantitatively from tumors that occur spontaneously in the tissue of interest. While some experimental models have produced neoplasms indistinguishable from their spontaneous counterparts, most cause growths that are histogenetically or morphologically different, larger, more numerous, more aggressive, or of shorter latency. Induced tumors may occur in tissues where spontaneous tumors are uncommon. Cells of induced tumors may express phenotypic characteristics (e.g., membrane antigens) not present in corresponding nonneoplastic or spontaneously neoplastic cells. Conversely, some characteristics present in normal or spontaneously neoplastic cells (e.g., certain enzyme activity) may be absent in induced neoplasms. Not only may carcinogens induce tumors that differ from spontaneous ones, but the effect or a particular carcinogen may vary with the condition of its use. The species of strain, sex, and age of the host as well as the dose and route of administration are major variables. Neoplasia is one result of altered cell regulation. If we postulate a central dogma for oncogenensis, induced tumors should arise from the same molecular mechanisms that produce spontaneous tumors. Differences in "profile" reside not in the ultimate mechanism, but in the events which surround it. They result from the myriad of factors which determine the dose of proximal carcinogen to the cell's sensitive volume, together with those that modify subsequent proliferation and differentiation.