Cysts of the third ventricle.

: Cysts of the third ventricle, predominantly being of congenital origin, are often summarized under the heading of ependymal or neuroepithelial cysts, if they contain an epithelial covering. Though surely they form a heterogeneous group, even by histochemistry and electron microscopy a differentiated classification on a histogenetic basis is difficult because of overlaps in structural details. The aim of this study is checking by immunohistochemistry with antibodies to cytokeratins (KL 1), epithelial membrane antigen (EMA), S 100 protein (S 100), and glial fibrillary acidic protein (GFAP). Fourteen colloid cysts showed clear reaction differences of their epithelium in relation to neuroepithelium, which render feasible the verification of the earlier presumed bronchogenic origin of the former. Two other "ependymal cysts" could be classified as diverticulum respectively meningeal glioependymal cyst. A review is given about localization, clinical and neuroradiological aspects of these partly extreme rare lesions together with differential diagnosis.