Intracytoplasmic lumina in ependymomas: an ultrastructural study

Intracytoplasmic lumina (ICL) are rarely described in tumors of the central nervous system. The morphogenesis of ICL remains incompletely characterized. Ultrastructural features of ICL in six ependymomas (one from lateral ventricle, three from fourth ventricle, and two from spinal cord) and three myxopapillary ependymomas of the filum terminale were analyzed. Two types of ICL were identified: ICL with both microvilli and cilia, and ICL with only microvilli. Some ICL also contained granulofibrillary or condensed material. Ciliated ICL were common in ependymomas of the ventricles, whereas nonciliated ICL were frequently seen in the myxopapillary variant. Various stages of formation and development were observed in ciliated ICL. They seemed to originate from distended periciliary cisterns, to enlarge by fusion with cytoplasmic vesicles or other ICL, and subsequently to open into the intercellular space. The last process may be the mechanism by which the intercellular microrosettes are formed. Ciliated ICL have not been described in other neoplasms. They may represent a characteristic ultrastructural feature of ependymomas. The morphogenesis of nonciliated ICL remains unknown. They may represent a degenerative form of ciliated ICL or pseudolumina resulting from invagination of the extracellular space within the cytoplasm.