Glioependymal cyst in the posterior fossa.

A large cyst filled with clear fluid was resected from the cerebellum of a 29-year-old man. By light microscopy the cyst was lined by flat epithelial cells. These epithelial cells abutted on a glial layer and immunohistochemical staining revealed that some of them expressed glial fibrillary acidic protein. Corpora amylacea were noted within the glial layer. Ultrastructurally, 2 types of cells lining the cyst were identified. One was characterized by electron-dense cytoplasm, cytoplasmic vacuoles, microvilli without surface-coated material, cilia, and basal bodies. The other was a cell with a smooth cytoplasmic membrane, round nucleus, and clear cytoplasm. Zonulae adherentes was observed between cyst lining cells and the cyst lining cells were shown to rest directly on the glial layer. Surprisingly, myelinated axons were identified in the glial layer, although nerve cells were not identified. These findings are compatible with those of a glioependymal cyst. The origin of glioependymal cysts of the posterior fossa is not understood, but possibilities include neuroglial heterotopia, persistent Blake's pouch (diverticulum of the roof of fourth ventricle), and remnants of a tela chorioidea. The location of the cyst and the presence of myelinated axons in the cyst wall may be best explained by an origin from neuroglial heterotopia.