Bracken fern-induced bladder tumors in guinea pigs. A model for human neoplasia.

We have induced tumors by feeding guinea pigs with a diet containing 25 or 30% dried bracken fern for 100 or 150 days. A high incidence of bladder tumors was obtained. All but one animal had preneoplastic or neoplastic lesions after 4 months; after one year, 24 or 25 exposed animals had carcinoma. Bladder tumors obtained were essentially pure transitional cell carcinomas, although 4 cases (7% of the exposed animals and 10% of the 39 transitional cell carcinoma observed) showed areas of focal squamous metaplasia. Immunohistological detection of cytokeratins 10, 13, and 18 confirmed the transitional nature of these tumors. Tumor development can be followed by ultrasonography and cytology. Bladder tumors arose through several steps. Dysplasia and preneoplastic hyperplasia were seen after 4 months and papillary carcinomas appeared after 6 months, whereas muscle-invasive carcinomas required 1 year. Thus this model reproduces the full spectrum of preneoplastic and neoplastic bladder lesions observed in humans. Interestingly, when tumors were induced in older guinea pigs, none of them progressed to a muscle-invasive stage. This phenomenon should provide the opportunity to study the molecular mechanisms associated with these two different growth patterns, a major issue in understanding human bladder tumor progression.