[Origin of colloid cyst: immunoperoxidase study].

Despite their consistent anatomic location and histological features, the histogenesis of colloid cysts has remained obscure. We report immunohistochemical data on surgically excised cysts in attempt to define the origin of the lining epithelial cells. Paraffin-embedded tissues from colloidal cysts (7 in the 3rd ventricle and one in the sella) and two similar epithelial cysts (one in the cerebellopontine angle and one in the sella) were investigated. After examination by conventional stains, immunoperoxidase studies were carried out using primary antisera against epithelial membrane antigen (EMA), tissue polypeptide antigen (TPA), epidermal keratin (EK) and S-100 protein. The apical plasma membrane, surface microstructures were consistently EMA positive. Columnar cells demonstrated cytoplasmic staining for TPA. Basal cells of lining epithelia expressed EK. All epithelia studied were negative for S-100 protein. These findings suggest an endodermal origin of colloid cysts.