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Adult astrocytes have been described in several types of gliomas, being accepted as high differentiated cells. Their presence is specially important concerning the concept of undifferentiated neuroectodermal tumours (PNET). We have studied two series of brain tumors and compared and contrasted them with silver impregnation (89 cases) and GFAP (127 cases). These are our conclusions: these astrocytes show the same morphology not only in neuroectodermal tumours, but also in CNS parenchyma around meningiomas, metastasis and brain lymphomas; many of these astrocytes are mature, normal cells with involutive features, lying among tumoral cells without transitional stages; their presence is directly related to a prominent peritumoral gliosis, a high proliferation rate and an infiltrating growth. On this basis, it is suggested that most of them are astrocytes belonging to the invaded CNS tissue and not true tumoral cells.

Four ethyl-nitrosourea (ENU)-induced oligodendroglioma-like tumors of the rat showing large rosettes on haematoxylin-eosin stain were studied by means of immunohistochemistry and electron microscopy, and their features compared with six human intraventricular neurocytomas. The similarities between the experimental and human tumors studied support the hypothesis that most of the so-called ENU-induced oligodendrogliomas in the rat are primitive neuroectodermal tumors with the tendency to differentiate toward a neuronal phenotype, and also suggest that the ENU-model of neurocarcinogenesis is useful for the induction of experimental neurocytomas.