Chemically induced smooth muscle tumors of the mouse urinary bladder.

Occasional submucosal tumors of indeterminate origin and without definite connection to the overlying mucosa have been reported in mice given various carcinogenic agents. Two Swiss mice fed niridazole developed such tumors. Electron microscopy of 1 of the tumors revealed thin myofilaments with oval dense bodies, marginal dense plaques, and gap junctions, identifying the tumor as originating from smooth muscle cells. No desmosomes, tonofilaments, secretory granules, striated muscle, or collagen were present in the tumor cells. Since the biology of these lesions is unknown, classification as benign or malignant cannot be made at this time. Their recognition is important in evaluating possible bladder carcinogens in mice.