Focal ependymal differentiation in choroid plexus papillomas. An immunoperoxidase study.

Choroid plexus papillomas are usually easily distinguishable from papillary ependymomas by their delicate fibrovascular stroma and their cytologic similarity to normal choroid plexus epithelium. Exceptionally, however, examples are met which give rise to diagnostic difficulty. We therefore tested 22 choroid plexus papillomas for the presence of glial fibrillary acidic (GFA) protein using the immunoperoxidase technique. Positivity for the protein was found focally in epithelial tumor cells in nine of the 22 papillomas. All were in adults ranging from 19-66 years of age. Eight of the nine tumors originated in the 4th ventricle or one of its lateral recesses. In six papillomas showing GFA protein in the cells, intracellular fibrils were found in a small number of elongated epithelial cells with the PTAH and/or Masson trichrome stains; in all these six cases, the GFA protein-positive cells were considerably more numerous than cells containing fibrils. Normal choroid plexus epithelium lacks GFA protein, but pathologically altered ependymal cells are often GFA protein-positive. Our findings therefore suggest that focal divergent glial (presumably ependymal) differentiation may be expressed in neoplastic choroid plexus epithelium, consistent with the origin of this epithelium from primitive neuroepithelial (ventricular) cells.