Presidential address: the histopathology of meningiomas. A reflection of origins and expected behavior?

Meningiomas and their principal cells of origin, the arachnoidal "cap" cells are unique in their morphology with multiple and sometimes seemingly contradictory features related to their origins and basic character. Some meningiomas express mesenchymal features either in histologic pattern (fibroblastic, lipo-myxochondro-osteoblastic differentiation), participation in other disease processes, e.g. taking part in the formation of rheumatoid nodules, or in storage phenomena shared with other mesenchymal cells of the body. At the same time they may display epithelial features, such as well-formed desmosomes ultrastructurally, papillary formations and intracellular lumina in cells that stain positively for various cell markers usually considered to characterize epithelial cells. Histologic similarities of meningiomas to various gliomas, schwannomas, neuroblastomas, fibrous histiocytomas, myxomas, chordomas, metastatic carcinomas, and in the cases of meningiomas with marked inflammatory infiltrates, to benign or malignant lymphoproliferative disorders involving the meninges may pose serious diagnostic problems. The localization and resectability of meningiomas are important factors related to long-term prognosis. Of the histologic features hemangiopericytomatous pattern, papillary formations, high cellularity (focal or diffuse) and invasion of the brain appear to correlate with potentially aggressive behavior, whereas cytologically aneuploidia, large number of mitoses, prominent nucleoli and cell necrosis suggest a guarded prognosis. However, some meningiomas with no detectable histologic features of malignancy may nevertheless metastasize to distant sites.