Spontaneous tumors of the meninges in rats

Histologic brain sections from 107 rats reported to have granular cell tumors or meningiomas in 2-year carcinogenicity studies conducted by the National Toxicology Program (NTP) or the National Cancer Institute (NCI) were reexamined microscopically. There were 62 rats with granular cell tumors, 26 with benign meningiomas, and 19 with meningeal sarcomas. Granular cell tumors were compatible with previous descriptions of this tumor. Meningeal sarcomas were subclassified into nine spindle cell sarcomas and ten fibrosarcomas. Among the rats with benign meningiomas, five were typical meningiomas (three fibroblastic meningiomas, and two meningothelial meningiomas) and 21 were meningothelial meningiomas containing cells with granules identical to those in granular cell tumors. There was a transition from epithelial-like cells of the meningothelial meningiomas to granular cells in these 21 cases. Based upon anatomic location, cytomorphologic similarities, and the occurrence of transitional or mixed forms of meningothelial meningiomas and granular cell tumors, it is suggested that these two tumors are related and may both originate from a common progenitor meningothelial arachnoid cell.