MESENCHYMAL TUMORS OF THE MOUSE URINARY-BLADDER WITH VASCULAR AND SMOOTH-MUSCLE DIFFERENTIATION

Bifenthrin, a synthetic pyrethroid insecticide/miticide, has been fed to male and female Swiss Webster mice at levels of 0, 50, 200, 500, an d 600 ppm in the diet for between 604 and 644 days. Tumors of the urin ary bladder were observed and initially reported as leiomyosarcomas. S ubsequently, the bladders were reviewed and the tumors showed a patter n of both epithelioid cells and spindle cells forming irregular Vascul ar channels. The tumors appeared to arise from the trigone of the blad der and, in some cases, invaded the bladder wall. No metastases were r ecorded. The tumor is usually considered rare; however, in this study, it was commonly observed in all groups but predominantly in males. Th e histogenesis of the tumor is uncertain, but from its pleomorphic his tological features, including smooth muscle and vascularity, it is pro bably derived from vascular mesenchyme.