Endothelial gaps as sites for plasma leakage in inflammation

Objective: In 1961, Majno and Palade proposed that plasma leakage in acute inflammation caused by histamine, serotonin, or bradykinin results via gaps that form between endothelial cells of postcapillary venules. Now the rele vance of endothelial gaps in plasma leakage is being questioned. The purpos e of this review is to summarize experimental evidence from our studies sho wing that endothelial gaps participate in plasma leakage in inflammation. Methods: Using neurogenic inflammation as a model of plasma leakage in acut e inflammation, we compared five methods to determine whether endothelial g aps form in the microvasculature of the rat trachea. 1) Endothelial cell bo rders and gaps were stained with silver nitrate and visualized by light, sc anning, and transmission electron microscopy. 2) The luminal surface of end othelial cells was examined by scanning electron microscopy. 3) The luminal surface of endothelial cells was stained with a biotinylated lectin and av idin-biotin-peroxidase histochemistry, and then was examined by differentia l interference contrast microscopy. 4) Endothelial junctions were reconstru cted from serial sections photographed by transmission electron microscopy. 5) Leakage was measured after perfusion of lectins or tracers through alde hyde-fixed vessels in situ. Results: The results from the five methods used in this system were consist ent with the formation of gaps between endothelial cells. Endothelial gaps were rare or absent under baseline conditions, but appeared with the onset of plasma leakage and had a distribution that matched the distribution of l eakage. Gaps had a complex morphology and were accompanied by fingerlike ce ll processes, which may anchor adjacent endothelial cells to one another an d participate in gap closure. In contrast to normal vessels, vessels that w ere leaky in life continued to leak after aldehyde fixation, in evidence th at, once formed, the leakage pathway did not require energy-dependent membr ane movement or vesicle shuttling. Holes through endothelial cells were les s than 1% as frequent as intercellular gaps. Conclusions: Taken together, the results show that endothelial gaps are a c onsistent feature of leaky vessels in the model system we studied, and are not an artifact of a particular method. The morphological complexity of the openings and accompanying fingerlike cell processes and overlapping endoth elial cell borders make gaps difficult to distinguish from transcellular ho les in thin sections viewed by transmission electron microscopy. However, s canning electron microscopic observations show that most of the openings in leaky venules are intercellular gaps, not transcellular holes. The formati on and closure of gaps are likely to be energy-dependent, but the process o f plasma leakage is not, provided there is adequate driving force for extra vasation. The cellular mechanisms of gap opening and closure remain to be e lucidated.