Chordoid glioma of the third ventricle: an ultrastructural study of three cases with a histogenetic hypothesis.

Chordoid glioma is a rare neoplasm occurring in the third ventricle and, as the name implies, having a chordoid appearance. It is currently considered a glial neoplasm of uncertain histogenesis with distinct clinicopathologic features. We report three cases of chordoid glioma with a focus on the ultrastructural appearance. The patients were two men and one woman aged, respectively, 34, 40, and 43 years. Immunohistochemically, all tumors showed strong and diffuse reactivity for glial fibrillary acidic protein and vimentin, whereas immunoreactivity for epithelial membrane antigen and cytokeratin was focal. Ultrastructurally, they showed features of ependymal differentiation for the presence of an apical pole with microvilli and a basal pole characterized, as in normal ependyma, by many hemidesmosomelike structures connecting cell membranes to the underlying basal lamina. Constant features were a submicroscopic cell body zonation (i.e., perinuclear, intermediate, subapical, and apical regions) and the presence of secretory granules. These findings were similar to those described for the secretory ependymal cells of the subcommissural organ, a small structure located in a dorsocaudal region of the third ventricle that undergoes regression after birth in humans. Our observations suggest that chordoid glioma may represent a subtype of ependymoma whose cells resemble the highly specialized ependyma of the subcommissural organ.