The ultrastructure of a cerebellar hemangioblastoma is presented. The tumor consists of a single cell type characterized by numerous intracytopiasmic fibrils, dilated endoplasmic reticulum containing dense granular material, lipid bodies, and unusual myelin forms. The latter are found as aggregates of membranous whorls and spirals with a 60 Aring; radial periodicity. Particulate thickenings, 30 Aring; in diameter, are present at 60 Aring; intervals along the membrane. Release of myelin forms from the cell is demonstrable after the aggregates become enclosed by membranes. A possible origin of the myelin figures is presented.
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