Fenestrations in endothelium of rat liver sinusoids revisited by freeze-fracture.

The freeze-fracture appearance of fenestrations of sinusoidal endothelial cells has been reassessed in perfusion-fixed rat liver. Fenestrations, limited to the attenuated portions of the endothelial cell cytoplasm are usually round or oval in shape, with diameters ranging from 40-600 nm. Within a given fenestrated region, they are disposed in clusters forming sieve plates. In addition, the presence of wide, irregularly shape openings suggest that adjacent fenestrations may coalesce and result in the larger openings. Our data thus confirm the existence of both small and large fenestrae in the endothelial wall, in contradistinction to previous studies showing only regular, medium sized (100 nm) openings.