An ultrastructural study of transport pathways across rat hepatic lymph vessels

Hepatic lymph vessels in the rat were examined by qualitative and quantitative analyses in order to obtain data pertinent to the mechanism of lymph formation. The ultrastructually visible transport pathways across these vessels appeared to be by way of intracytoplasmic vesicles (89.6 micron mean diameter) and normal channels (22.6 micron wide) between endothelial cells. Three types of intercellular contacts were seen, end-to-end, overlapping, and interdigitating. Only one open junction (greater than 30 nm) was seen in 226 contacts examined. Specialized junctional complexes, either fasciae occludentes or fasciae adherentes, were seen in 65% of the contacts. Approximately one-third of the contacts had a dilatation along part of their length separating the opposing endothelial cells. Vesicles occupied 3.5% of the endothelial cytoplasmic volume and were distributed as follows: 40% opening onto or touching the luminal membrane, 34% without visible connection to either surface, 23% opening onto or touching the abluminal membrane, and less than 3% associated with membranes forming intercellular contacts. It was concluded that the mechanism of lymph formation in the liver is similar to that in the kidney and different from that in the dermis or diaphragm.