The ultrastructure of the ependymoblastoma.

The ependymoblastoma is a rare, primitive neuroectodermal tumor morphologically distinct from the ependymoma and the malignant or anaplastic ependymoma. This neoplasm is characterized by uniform neuroepithelial cells, ependymoblastic rosettes, perivascular pseudorosettes and numerous mitotic figures. The fine structure of this neoplasm is characterized by a predominant population of well-differentiated ependymal cells and intermingling mitotic figures. Many cells have an apical surface bearing cilia and microvilli projecting into a lumen and interconnected to adjacent cells by zonulae adhaerentes. The basal surface often forms a basal lamina-lined labyrinth. This tumor also contains a population of cells less differentiated than 3-week embryonal ependymal cells. Intermediate cell forms suggesting multiple lines of differentiation occurring within a single cell are not present in this human ependymoblastoma. The ependymoblast represents a stage in the differentiation of the primitive medulloepithelial cell to the mature ependymocyte.