Brain Pathology Case of the Month - September 2002   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