The ultrastructure of the lysis of fibrin by endothelium in vitro.

The release of plasminogen activator from endothelial cells results in the lysis of fibrin-plasminogen clots. The ultrastructural morphology of this process was examined. Lysis occurred first over the cytoplasm of endothelial cells between the humps formed by the endothelial cell nuclei. The lytic region next became confluent and a condensed zone of fibrin (and possibly adsorbed products of lysis) was seen giving a distinct edge to the fibrin film. The endothelial cells remained relatively intact throughout the process, though there were indications of injury vesicular blebs were formed on the luminal surface of the endothelium. These were also seen attached to the edge of the fibrin. The edge of the lytic zone was characterized by small dense collections of fibrin fibrils which made a porous rim to the cleared area.