Malignant fibrous histiocytoma. An unusual neoplasm of soft-tissue origin in the rat that is different from the human counterpart.

Malignant fibrous histiocytomas were found in eight F344 rats and four Sprague-Dawley rats. They appeared to originate in skin or peritoneum and to have metastasized to liver and lung. Tumor cells appeared to be histiocytic in the light microscopic studies and fibroblastic after ultrastructural evaluation. The cell of origin was uncertain.