title: The formation of tubular structures by endothelial cells is under the control of fibrinolysis and mechanical factors.
abstract: This study highlights the importance of several factors involved in the formation of capillary-like structure formation (CLS) using Human Umbilical Vein Endothelial Cells (HUVEC) and Bovine Retinal Endothelial Cells (BREC) cultured on fibrin gels. The fibrin concentration inducing (CLS) was 0.5 mg/ml for HUVEC and 8 mg/ml for BREC. The high fibrin concentration required for the latter cells appeared necessary to counterbalance the extensive fibrinolysis of the gel by the BREC. Fibrin degradation products measured in the culture media showed that fibrin degradation was mandatory but not sufficient for CLS formation. Fibrin degradation acted in concert with the mechanical, concentration dependent properties of the gels to induce CLS. For example, HUVEC did not form CLS on a rigid fibrin of 8 mg/ml in spite of fibrinolysis. As cell reorganisation occurred, the fibrin was disrupted (HUVEC) or pleated (BREC) giving indirect proof of the development of mechanical forces. During CLS formation, an increasing amount of latent TGFbeta1 was measured in the medium (1000-1700 pg/ml). The active form of TGFbeta1 was not, however, detected and the addition of anti-TGF-beta1 antibody to the medium did not influence the formation of the CLS network. Yet, added activated TGF-beta1 led to the formation of less organised structures, that were completely abolished by the concomitant addition of the same anti-TGF-beta1 antibody. Thus, it is likely that TGF-beta1 secreted by the endothelial cells remained in its latent form. In conclusion, a balance between the mechanical properties of fibrin and the fibrinolytic activity of each cell type may regulate CLS formation in our models. We think that the high fibrinolitic activity of the BREC may represent a defense mechanism to protect the retina against thrombosis-induced damage in vivo.