Ultrastructural studies on macromolecular permeability in relation to endothelial cell turnover

Our previous light microscopic studies demonstrated the correlation of focal arterial uptake of macromolecules with the mitosis or death of endothelial cells (ECs). To investigate horseradish peroxidase (HRP) permeability associated with the clefts surrounding these ECs at the ultrastructural level, experiments were performed on rat thoracic aortae by using transmission electron microscopy. In en face preparations of aortic specimens, light microscopy was used first to detect mitotic ECs by hematoxylin staining prior to electron microscopy. Dying (or dead) ECs containing cytoplasmic immunoglobulin G (IgG) were identified by an indirect immunogold technique, HRP was found to permeate from the vessel lumen through the widened junctions around the mitotic and dying cells, as well as some non-widened junctions and the plasma membrane of dying cells. The transiently open junctions during cell turnover lead to an increased transendothelial permeability to macromolecules. In addition to its enhanced passage through the leaky junctions around EC turnover and through the damaged membrane of dying cells. HRP can also traverse many normal intercellular clefts into the subendothelial space of the aorta. These observations show that normal intercellular junctions can provide a significant pathway for the transport of macromolecules with the size of HRP, and that HRP transport is enhanced in transiently open junctions surrounding ECs undergoing turnover. The widened junctions around the mitotic and dying cells provide the pathway for macromolecules larger than HRP, e.g., the low density lipoproteins (LDLs).