DIAGNOSIS: EXTRAVENTRICULAR NEUROCYTOMA
The differential diagnosis of neurocytomas includes oligodendroglioma, ganglioglioma, and ependymoma. On radiological, surgical and histopathological examination, extraventricular neurocytomas most frequently resemble oligodendroglioma, but as in the present case, they are generally less infiltrative than glial tumors. The defining marker for the identification of neurocytomas has been synaptophysin, since neurocytic tumors show strong immunoreactivity, but glial tumors are mostly negative . Extraventricular neurocytomas are histologically identical to the more common central neurocytomas, but are distinguished by their location outside the lateral ventricles. Neurocytomas have been reported in locations as diverse as the spinal cord, cerebellum, and cerebral cortex [1,2,4,5,7,8].
NeuN immunoreactivity has not been studied much in neurocytomas, central or extraventricular. In this case, we found that the mature neuron marker NeuN was expressed by some neurocytoma cells. NeuN was introduced in 1992 as an antibody that recognized the nuclei (and to a lesser extent the cytoplasm) of differentiated, postmitotic neurons . As a marker of neurocytic differentiation in tumors, NeuN has been of questionable value, as NeuN immunoreactivity has sometimes been found in tumors that are considered to be glial, such as oligodendrogliomas [3,10]. On the other hand, it has also been suggested that traditional tumor classifications may need to be revised to reflect molecular expression data .
Since NeuN is normally found only in postmitotic neurons [6, 9], we hypothesized that even in tumors such as neurocytoma, NeuN might likewise be expressed only by cells that have exited the mitotic cycle. The double labeling results supported this hypothesis, since no neurocytoma cells expressed both NeuN and the proliferative cell marker Ki-67. This finding is consistent with the general idea that differentiation and proliferation tend to be alternative cellular states.
Contributed by Chris Englund, Daniel Silbergeld, Ellsworth C. Alvord, Jr., Randy Small, and Robert F. Hevner