Spontaneous Rat Malignant Tumors of Fibrohistiocytic Origin: An Ultrastructural Study

Abstract
The ultrastructural features of a spontaneous malignant fibrous histiocytoma and three malignant histiocytomas in Sprague-Dawley rats are described. The ultrastructural features of the malignant histiocytomas (or histiocytic sarcomas) support origin from cells of the monocyte series. The histogenesis of more fibrous tumors (fibrous histiocytomas) remains uncertain.