Ultrastructure of cerebellar capillary hemangioblastoma. V. Large pinocytic vacuolar bodies (megalopinocytic vesicles) in endothelial cells.

Large pinocytic vacuolar bodies (megalopinocytic vesicles) containing electron-dense granulo-fibrillary material, not previously described in micro-vascular endothelium of brain tumors, were observed in endothelial cells of all five cases of cerebellar hemangioblastoma studied ultrastructurally. They were present in 23% of a total of 132 capillary profiles studied. Some were prominent and aggregated to occupy a large portion of the endothelial cytoplasm. Unlike the ordinary pinocytic vesicles in endothelial cells, they were distributed predominantly in the vicinity of the nucleus and surrounded by abundant organelles. They were irregular and usually several times larger than macropinocytic vesicles. The larger vacuolar bodies were often surrounded by bundles of microfilaments which often anchored on their limiting membrane. They coexisted frequently with Weibel-Palade bodies and occasionally fused with them. Convergence of coated vesicles and micropinocytic vesicles with the vacuolar bodies was present. However, there was no direct contact between the vacuolar bodies and Golgi apparatus, rough and smooth endoplasmic reticulum and mitochondria. The vacuolar bodies were closely associated with pericytic foot processes. It was suggested that they were formed by invagination of the abluminal cytoplasmic membrane with engulfed extracellular material and migrated internally. Discharge of their contents into the vascular lumen and interendothelial space was observed. Some had a disrupted membrane with a suggestion of release of contents into the cytoplasmic matrix. Their function is unknown, but they may serve as a specific vehicle of transport or digestive mechanism in microvascular endothelium under certain pathophysiological conditions, such as neoplasm, to meet the increasing metabolic demands.