Scanning electron microscope observations on the structure of portal veins, sinusoids and central veins in rat liver.

Portal veins can be recognized in SEM by the presence of an accompanying element such as the bile ductule. Endothelial cells within these vessels possess small marginal microvilli. Portal veins show few sinusoidal inlets (direct connections to sinusoids) and no attached "wandering" cells. Sinusoids in periportal areas are narrower and more tortuous than the wider and straighter central ones. Periportal endothelial fenestrae have a larger diameter (111 nm) than central ones (105 nm), but their number per square micron, and therefore the porosity (% open area), is higher in centrolobular sinusoids (7.94 vs 5.96%). Central veins have smoother endothelial cells and show numerous sinusoidal outlets (direct sinusoidal connections). Cells are seen attached to the wall and are probably migrating out of the liver. In the orifices of some sinusoidal outlets, traversing bars can be observed, which obstruct the flow of blood from the sinusoids into the central vein. From our observations and measurements the following conclusions can be drawn: a. In the narrower periportal sinusoids, moving blood cells will be forced against the wall, causing a massage of the space of Disse. This will influence the exchange of fluids and particles through the endothelial fenestrae, and it will cause stirring of the fluid in the space of Disse. b. Centrolobular sinusoids have a larger perimeter and an endothelial lining with a higher degree of porosity. These structural factors will favour transport and uptake processes in the centrolobular parenchyma. c. Sinusoidal blood flow might be expected to be slower in central areas, since the number of sinusoids appear to equal that of the portal region, but their lumen is wider.