Ultrastructural characterization of transitional cells in oligodendrogliomas

In oligodendroglial tumors the expression of glial fibrillary acidic protein (GFAP) is found in cells with an astrocytic morphology representing preexistent or neoplastic astrocytes. In addition, a proportion of the GFAP-positive cells has the morphology of miniature gemistocytes (minigemistocytes) or oligodendrocytes (gliofibrillary oligodendrocytes or GFOC). Both minigemistocytes and GFOC are considered as cells transitional between astrocytic and oligodendroglial lineage. Though minigemistocytes can readily be distinguished in routinely stained histological sections, GFAP immunostaining is obligatory for the identification of the GFOC. In the present study, the GFOC is characterized at the ultrastructural level using an immunogold-silver stain on semithin (1 micron) slides for identification of GFAP immunoreactivity and subsequent processing of the adjacent slide for immunoelectron microscopy. In analogy with the minigemistocytes, the glial filaments in the GFOC are arranged in parallel bundles. The finding of cells with ultrastructural features intermediate between those of GFOC and minigemistocytes suggests a close relationship and a possible interconvertibility between the two transitional cell types in oligodendrogliomas.