At low magnification, the tumor appeared relatively sharply demarcated from the surrounding tissue (Fig. 2a). Under high power, however, some scattered tumor cells could be identified within the surrounding white matter. Adjacent to the tumor reactive gliosis was observed, and in some areas abundant formation of Rosenthal fibers (Fig. 2b), as well as occasionally hemosiderin-laden macrophages. The tumor was moderately cellular, and two different tumor components could be discerned, one astrocytic and the other neuronal. The astrocytic cells were arranged in a pseudopapillary fashion around blood vessels as a single or pseudostratified layer of small cells with round nuclei and scant cytoplasm. Prominent blood vessel hyalinization was not observed. Astrocytic tumor cells were strongly immunoreactive for GFAP (Fig. 2c), S-100 protein and vimentin (Fig. 2d), whereas the cells in between the pseudopapillae were GFAP- and vimentin immunonegative.
In the spaces between pseudopapillae the tumor cells displayed neuronal differentiation, and lay in a fine, fibrillary matrix (neuropil). Most cells were small and of neurocytic morphology with scant cytoplasm, dark, round nuclei, sometimes with prominent nucleoli and occasional perinuclear halo formation (Fig. 2e). In between these cells, larger ganglioid cells could be identified, which displayed brightly eosinophilic polygonal cell bodies with prominent processes, and sometimes prominent nucleoli (Fig. 2e, arrow). Mature ganglion cells were not detected. The neuronal cells were strongly immunoreactive for synaptophysin (Fig. 2f) and NSE, and showed weaker staining for chromogranin, and S-100 protein; occasionally, ganglionic cells were immunoreacitive for NF. The astrocytic cells adjacent to blood vessels lacked immunoreactivity with the neuronal markers. The MIB-1 labeling index focally reached 3% (Fig. 2g).
Electron microscopy was only available from re-embedded paraffin-material. Despite considerable artifacts, glial filaments could be identified in tumor cells adjacent to blood vessels (Fig. 2h, arrowheads). Synapses were not detected.