Signet-ring cell ependymoma: Case report with implications for pathogenesis and differential diagnosis

We describe light microscopic, immunohistochemical and ultrastructural feat ures of a signet-ring cell ependymoma (WHO grade II) identified in a surgic ally resected left cerebellar cystic tumor from a 64-year-old man. Part of the tumor showed clear-cell differentiation. Immunohistochemical coexpressi on of glial fibrillary acidic protein and epithelial membrane antigen, char acteristic of ependymoma, was detected in both components. Sinuous intermed iate junctions, cytoplasmic lumina, and scant astroglial filaments were dem onstrated by electron microscopy. Signet-ring cell change was shown to be i nduced by disproportionate cavitation of either microvillus-bearing cytopla smic lumina or microrosettes. The staining qualities of clear cells were ma inly due to paucity and degeneration of subcellular organelles. Therefore, signet-ring cell ependymomas represent a unique anomaly of intra- and extra cellular compartmentalization to be distinguished from various unrelated fo rms of cytoplasmic volume increase, resulting in an optically similar "empt y" appearance of tumor cells. As a clinically relevant consequence, signet- ring cell ependymoma must be included in the differential diagnosis of prim ary or metastatic neoplasms of the central nervous system, having in common a phenotype characterized by overdeveloped optically lucent cell bodies.