Filamentous aggregates in a meningioma.

The light microscopic and ultrastructural appearances of unusual filamentous aggregates in a right parietal meningioma in a 14-year-old boy are described. The tumor showed prominent meningothelial as well as fibroblastic components and was graded as an atypical meningioma. By light microscopy, eosinophilic, PAS-positive, granular, irregularly shaped Rosenthal fiber-like structures were widespread within the tumor, in both an intra- and an extracellular location. By immunohistochemical staining, similar location of positivity was obtained for vimentin, laminin, and collagen type IV. The inclusions were nonreactive for keratin, lysozyme, alpha-1-antitrypsin, and ubiquitin. Ultrastructurally, these aggregates were composed of an irregular tangle of filaments with electron dense condensations, sometimes with a lattice pattern. The intracellular aggregates were membrane-bound, and some were found within dilated rough endoplasmic reticulum, while extracellularly, they filled up spaces between adjacent tumor cells. Less prominently, flocculent osmiophilic nonfilamentous material was also seen within the inclusions. These observations suggest that these novel inclusions in a meningioma are composed of intermediate filaments (vimentin) and extracellular matrix proteins, with active synthesis in the rough endoplasmic reticulum and subsequent extrusion from the tumor cells into the extracellular spaces.