Signet-ring cell oligodendroglioma--report of two cases and discussion of the differential diagnosis.

Two cases of oligodendroglioma consisting largely of signet-ring cells were analyzed histopathologically, immunohistochemically and at the ultrastructural level. The signet-ring cells were negative for a panel of tumor lineage markers including glial fibrillary acidic protein, and were negative for Ki-67 (MIB-1 immunohistochemistry). In contrast with the abundance of lysosomal structures reportedly present in the so-called eosinophilic granular cells in oligodendrogliomas, degenerating mitochondria were mainly seen in the cytoplasm of the signet-ring cells. The differential diagnosis of the oligodendroglial signet-ring cell tumors occurring in children and in adults is discussed.