Thrombogenicity of vascular cells. Comparison between endothelial cells isolated from different sources and smooth muscle cells and fibroblasts.

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Abstract
When the endothelial cell layer is damaged, a thrombotic reaction starts on the cells'' subendothelium and on the connective tissue deposited by smooth muscle cells in the deeper layers. When more severe vascular damage occurs, hemostasis will involve the vessel adventitia in which fibroblasts are found. In this article, the influence of in vitro cultured endothelial cells, smooth muscle cells, and fibroblasts on the hemostatic balance was studied. To do so, perfusions were performed with low molecular weight heparin anticoagulated blood over the extracellular matrix of the cells. This method allowed the study of tissue factor-dependent thrombin generation and its influence on formation of fibrin and platelet aggregates. The experiments described in this article show that endothelial cells isolated from different human organs interfere differently in the hemostatic response. Endothelial cells isolated from umbilical veins are nonthrombogenic; they do not synthesize tissue factor under unstimulated conditions. On their extracellular matrix, only adherent platelets are found, but no aggregates and no fibrin. Endothelial cells isolated from omentum and atrium contain tissue factor activity under unstimulated conditions. As a consequence, thrombin is generated on their surfaces, and platelet aggregates and fibrin deposition are found on the extracellular matrices after perfusions with whole blood. The matrix of smooth muscle cells and fibroblasts behaved similarly. Increase in shear rate and perfusion time resulted in an increase in platelet aggregate formation. Polymerized fibrin deposition decreased when perfusions were performed at higher shear. Both platelet aggregation and fibrin deposition were tissue factor dependent and could be blocked more than 70% by an antibody against tissue factor. Based on these results, we conclude that endothelial cells isolated from umbilical veins form the best nonthrombogenic surface in vitro. Moreover, coagulation-dependent hemostasis should be included when thrombogenicity of subendothelium is discussed, especially when it concerns matrix derived from cells present in the deeper layer of the vessel wall.

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