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Five cases of anaplastic oligodendrogliomas containing numerous GFAP-positive cells have been analysed by electron microscopy to establish the fine structural characteristics of neoplastic cells. Ultrastructurally, all tumours have revealed monotonous appearance typical of oligodendrogliomas, however some structural variability, particularly with reference to astrocytic differentiation, has been observed. The majority of neoplastic cells have shown the fine structural features of oligodendrocytes, accompanied by various numbers of intermediate cytoplasmic filaments. These filaments have been usually distributed in the perinuclear, less often in the peripheral, parts of the cytoplasm. The cells exhibiting features common to both oligodendroglial and astroglial cells might be regarded as an intermediate morphological form between these two cell types. True neoplastic astrocytes could be encountered only sporadically. The present electron microscopic studysupports the opinion that GFAP-positive oligodendroglial tumours contain heterogeneous neoplastic cell populations with the transitional cell types between oligodendroglial and astroglial lineage.

Ultrastructurally, oligodendrogliomas were tumors of rather monotonous appearance, with round and oval nuclei without indentations and cytoplasm of an electron-density lower than that of astrocytic tumors. Cisterns of endoplasmic reticulum and those of Golgi apparatus were occasionally distended. In addition to the usual subcellular organelles, the cytoplasm was rich in ribosomes, microtubules and glycogen granules. Some cellular processes contained branching cisterns, similar to those encountered in human neuroaxonal dystrophies. There are no discernible intermediate filaments. Blood vessels are apparently normal in oligodendrogliomas. Probably, the most characteristic features of oligodendrogliomas are concentric arrays of membranes (so called membrane laminations, whorls or scrolls). A fragment of the cytoplasm sequestrated within a particular whorl may contain mitochondria, lysosomes or abundant glycogen granules. The apparent continuity of membranes of whorls and the channels of endoplastic reticulum is rarely observed.