Histopathological features of a PDGF-induced glioblastoma (GBM)-like tumor (a) and an anaplastic oligodendroglioma (AOL)- like tumor (b). a1 Cellular, diffuse proliferation of tumor cells with hyperchromatic nuclei. a2 Pseudopalisading necrosis (asterisk) and endothelial proliferation (arrowheads) were observed. a3 A subset of the tumor cells are immunopositive for proviral HA-tag. a4 GFAP expression is restricted to entrapped, non-neoplastic astrocytes. a5–a7 Tumor cells are diffusely immunopositive for nestin (a5), NG2 (a6) and Olig2 (a7). a8 PCNA-immunostaining reveals high proliferative activity of the tumor cells. b1 Diffuse proliferation of tumor cells with round, monomorphous nuclei and perinuclear halo. b2 Necrosis of the tumor tissue is frequently seen (asterisk). b3 A subset of the tumor cells are immunopositive for proviral HA-tag. b4 GFAP expression is noted in minigemistocyte-like or gliofibrillary oligodendrocyte-like tumor cells (arrowheads) and a small number of entrapped, nonneoplastic astrocytes. b5 Nestin expression is not as prominent as that in GBM-like tumors. b6, b7 Tumor cells are diffusely immunopositive for NG2 (b6) and Olig2 (b7). b8 PCNA-immunostaining reveals the high proliferative activity of the tumor cells. Scale bar 20 μm (b4), 50 μm (a1–a8, b1–b3, b5–b8)