Evidence for differentiation in medulloblastomas appearing primitive on light microscopy: an ultrastructural study.

Surgical biopsy specimens from five children with cerebellar medulloblastoma were examined for ultrastructural evidence of differentiating activity. At light microscope level, all the tumour samples appeared primitive and featureless, and could not be classified as showing either neuroblastic or spongioblastic differentiation. Using electron microscopy, the major part of each specimen examined likewise showed largely primitive and undifferentiated appearances typical of medulloblastoma. In all the cases however, scattered groups of cells and related cytoplasmic processes were found with more specific, differentiated features. Moreover, in each tumour sample, these cells and processes were found to show two morphologically distinct appearances. One cell type had ultrastructural features reminiscent of primitive neuronal morphology, and a similar appearance was found in cell perikarya forming rosettes. The other cell type showed evidence of spongioblastic differentiation, with bundles of glial-like filaments in the cytoplasm. Cilia were also found in one case. Whilst in keeping with current views on the neuroectodermal origin of the tumour, these observations suggest that all medulloblastoma cells may have bipotential differentiating capacity, and that at least some evidence of this is likely to be found ultrastructurally, even when there is only a primitive and completely undifferentiated appearance on light microscopy.