Perivascular pseudorosettes in childhood brain tumours: an ultrastructural and immunohistochemical study.

Perivascular pseudorosettes (PP) in childhood central nervous system tumours were examined with light and electron microscopy and immunohistochemistry for glial fibrillary acidic protein, S-100 protein and albumin. One kind of PP at light microscopy comprised a central thin-walled vessel surrounded by a thick mantle of eosinophilic fibrillary material and rings of usually regular nuclei. Adjacent tumour tissue was compact. This type was correlated closely with ultrastructural evidence of ependymal differentiation. The central vessel showed continuous endothelium in all. Another type of PP comprised a central vessel of varying thickness surrounded by hyaline material, clearly defined tapering processes, and rings of often irregular nuclei. Adjacent tissue showed extensive edema and microcystic change. Ultrastructurally, this type showed no ependymal differentiation except in one myxopapillary ependymoma. Fenestrated vessels were seen in half of the tumours associated with this kind of PP. It is proposed that variation in vascular permeability, rather than in the structure of tumour cells, is the main cause for the difference in histological appearance of the two types of PP. Fenestrated vessels may also be responsible for the "myxoid" change in myxopapillary ependymomas. The amount of extracellular albumin showed no consistent correlation with the presence of fenestrations in vessels. A variable degree of positivity to GFAP and S-100 protein was seen in the tumours associated with both types of PPs with no clear difference in pattern.