Observations on the nature of subependymoma: an electron microscopic study.

Surgical biopsy material from four cases of symptomatic fourth ventricle subependymoma, all with typical light microscopic features, was studied by electron microscopy. The ultrastructural appearance of each case was similar, with small groups of tumour cell perikarya widely separated by a meshwork of cell processes containing glial filaments. Large cells lacking specialised features were prominent in all cases and showed similarities with ependymoglial precursor cells described in the adult subependymal layer of animals. Other cells showed either astrocytic or poorly developed ependymal features, and transitional forms between these two appearances were common, suggesting the tumour to be of a single cell type. Pockets of microvilli were present, but they differed from ependymal-type microrosettes in lacking tight junctional complexes. Unlike low grade astrocytomas, the extracellular space was small and there was no close association of tumour cell processes with blood vessels. These observations suggest that the subependymoma may be regarded as a separate entity from other fourth ventricle neoplasms and possibly takes origin from ependymoglial cells normally present in the adult subependymal layer.