Microcystic meningioma: tumors of arachnoid cap vs trabecular cells.

Three unusual meningiomas with extensive microcystic formation were studied by light- and electron-microscopy. The tumor cells in the first and second cases had many morphological characteristics in common with the arachnoid cap cells: prominent and complex interdigitation of the fine cytoplasmic processes, numerous well developed desmosome attachments, and intracytoplasmic microfilaments. The prominent features of these tumors were a marked vascularization and hyaline thickening of the vessel walls, and the microcystic formation was considered to be related to these changes. In contrast, the tumor cells of the last case had characteristic structures in common with trabecular arachnoid cells. They were stellate in shape, and had only a small number of intracytoplasmic filaments. Arborizing relatively thick and long cytoplasmic processes showed simple junctions with occasional desmosomes, and delimitated microcysts. The tumor cells in this case were considered to recapitulate the subarachnoid structure and formed microcysts. Although the histogenesis of microcysts in meningiomas is not considered to be uniform, the biological behavior of these 3 tumors corresponded to those of meningiomas in general.