Unusual pericytic proliferation in a meningotheliomatous meningioma: an ultrastructural study.

Hemangiopericytomas are controversial tumors of the central nervous system and are regarded by many neuropathologists as variants of meningioma. A unique vascular change which may have bearing on this relationship was encountered in a sphenoid wing meningioma in a 93-year-old man. By light and electron microscopy, the tumor showed classical features of a meningotheliomatous meningioma. In addition, however, there was striking proliferation of small dark cells within vessel walls, extending into the parenchyma, and often dominating the picture. Ultrastructurally, the small cells, in contrast to the meningothelial cells, were totally invested by basal laminae, showed extensive pinocytosis, and subplasmalemmal condensations, i.e., pericytic features. The two cell populations, although intimately intermingled, maintained distinct features. This case may represent an expression of divergent mesodermal differentiation of meningeal cells, either as a unique vascular reactive change or alternatively as an early stage in the development of a hemangiopericytic tumor of the meninges.