Histological and ultrastructural findings of benign intracranial cysts.

Since benign intracranial cysts have similar clinical and neuroradiological imaging appearances, it is difficult to distinguish between these cysts. Thus an electron microscopic study plays an important role in definite diagnosis of these non-tumorous benign cystic lesions, and also in determination of their origins. Arachnoid cysts occurring in 5 patients, epithelial cysts occurring in 2 patients, 2 cases of Rathke's cleft cysts, and one case of pineal cyst, were electron- and light-microscopically observed, and their characteristic features and differences were described. The structure of the arachnoid cyst wall was similar to that of normal arachnoid membrane. The inner surface of the arachnoid membrane. The inner surface of the arachnoid cyst wall was formed of one or several layers of arachnoid cells with slender processes, which contained large extracellular spaces, but not microvilli. As for epithelial cysts, microvilli and surface-coating material were present on the surface of epithelial cysts, and a well-developed basement membrane was continuously observed under the epithelial cells. Rathke's cleft cysts, which may basically be classified as epithelial cysts, are lined by columnar or cuboidal ciliated epithelium. The wall of the pineal cyst is composed of normal pineal tissue, and there are three layers, which have a lining of glial cells in the inner layer, a few sheets of pineal cells in the middle layer, and a fibrous capsule in the outer layer.