Neoplastic transformation in the planarian: I. Cocarcinogenesis and histopathology.
Although several investigators have reported that exposure to mammalian carcinogens induces abnormal tumorlike growths and teratogenic remodeling in planarians, there is no general agreement that these, or comparable responses in any other invertebrates, model mammalian carcinogenesis. To investigate this question, freshwater planarians of the species Dugesia dorotocephala were exposed to culture water containing an initiator and a promoter, either alone or in combination. Cadmium, a potent carcinogen, was used as an initiator in the protocol. Treatment with sublethal concentrations of cadmium sulfate produced a benign, but persistent, tumor in a small percentage of the planarians. The addition of 12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester and well-known promoter, to the cadmium-containing solutions resulted in the induction of a progressive, potentially lethal, transplantable tumor in a large proportion of the treated flatworms. Light and electron microscopy revealed this particular tumor to be composed both of immature cells and of a single mature cell type: newly differentiated, but transformed, reticular cells. Further examination of the infiltrating tissue formations elucidated the profile of differentiation, from a population of mitotically active transformed stem cells through the transitional stages in the associated reticuloma. These results suggest that 1) the freshwater planarian displays the major phenomenology of mammalian cocarcinogenesis and that 2) the planarian reticuloma models several important features of a neoplastic stem cell disease.