Subarachnoid epithelial cyst of the cerebellum. Immunohistochemical and ultrastructural studies.

This report describes the immunohistochemical and ultrastructural studies of a subarachnoid epithelial cyst of the cerebellum found incidentally at autopsy of a 76-year-old man. The cyst was composed of an inner epithelial layer, a middle connective tissue layer and an outer arachnoid membrane. The epithelial layer was strongly positive for cytokeratin, carcinoembryonic antigen and epithelial membrane antigen, but negative for glial fibrillary acidic protein, S-100 protein, neuron-specific enolase and vimentin. Ultrastructurally, the epithelial layer had four distinct types of cells: ciliated cells, non-ciliated cells with coated microvilli, basal cells with tonofilaments and desmosomes, and cells with dense-core secretory granules (Kulchitsky cells). The latter two types of cells have not been described previously in epithelial cysts of the CNS. The lining epithelium closely resembled the upper respiratory epithelium. The findings suggest that the cyst was of endodermal rather than neuroectodermal origin.