Role of dying endothelial cells in transendothelial macromolecular transport

There are focal areas in the aorta with an enhanced endothelial permeability to macromolecules, as indicated by the focal uptake of the protein-binding azo dye Evans blue in vivo. These areas exhibit high rates of endothelial cell turnover and a number of structural characteristics in en face endothelial morphology. To determine the relationship of endothelial cell death to macromolecular leakage at the level of individual endothelial cells, thoracic aortas of 12 adult male Sprague-Dawley rats were studied at 3 to 5 minutes after intravenous administration of Evans blue-albumin (EBA). Leakage of EBA around individual endothelial cells in en face preparations of the aorta was visualized by fluorescence microscopy. Dying or dead endothelial cells were identified by indirect immunoglobulin G (IgG) immunocytochemistry. Although endothelial cell death is uncommon in normal aortic endothedium (i.e., an average frequency of 0.48%), a high percentage (63%) of IgG-containing dying or dead endothelial cells was found to be associated with EBA leakage. These dying or dead endothelial cells were responsible for 37% of total EBA leaky foci. The results suggest that, in addition to mitotic endothelial cells, the dying or dead endothelial cells also make significant contributions to the local enhancement in aortic endothelial permeability. The present findings lend further support to the "cell turnover-leaky junction" hypothesis for the localization of atherosclerosis.