Abstract
Hemangiopericytoma, transitional cell carcinoma, squamous cell carcinoma and adenosquamous carcinoma developed in the urinary bladder of 6 of 95 guinea pigs of a noninbred stock and 16 of 87 guinea pigs of inbred strains 2 and 13 all gamma or X-ray irradiated. An additional 7 of 66 untreated guinea pigs of the two inbred strains also developed bladder tumors. Twenty-four guinea pigs had multiple tumors of the bladder. In the irradiated animals, the tumors arose significantly earlier but the increase in number of tumor bearing guinea pigs did not reach statistical significance. The tumors extended through the superficial muscularis into the underlying connective tissues and occasionally into lymphatic vessels and veins but none metastasized. Of 29 animals with tumor of the bladder, 21 had papillary proliferation of the renal pelvic mucosa, two papillary transitional carcinoma and one hemangiopericytoma. The EM and light microscopic criteria for the identification of hemangiopericytoma are described.