Structure formation on HUVEC cell lines with a low concentration of VEGFR-2 receptors, whereas 10 ng/ml PDGFBB combined with VEGF-A causes an increase in Compound Library capillary length, number area and complexes. A further significant increase in capillary-like structures could not be promoted using 30 ng/ml VEGF and 15 ng/ml PDGF-BB. In the literature, concentrations greater than 10 ng/ml PDGF-BB have not been reported for in vitro co-culture models. For the HUVEC cell lines with a high concentration of VEGFR-2 receptors, the length, number of junctions, and area were positively influenced by the additional growth factors. This positive effect of VEGF supplementation on capillary-like structure formation correlates with the findings observed in other cocultures as well as in in vitro and in vivo models of ischemia. The cell morphology did not change in either supportive or vessel-forming cell monolayer cultures by addition of VEGF or PDGF-BB in the aforementioned concentrations. In the case of PDGF-BB, a concentration of 5 ng/ml was shown to increase the length, number of junctions and area for donors with a high concentration of VEGFR-2 receptors, whereas 10 ng/ ml had no additional significantly positive effect. PDGF-BB is an important growth factor involved in vascular development, as well as the development of kidney, lungs and the central nervous system. Although PDGF-BB is not involved in the initial stages of angiogenesis, it stabilises new vessels, induces anastomosis and recruits pericytes to the neovessel. It has been shown that it is strongly expressed in vivo by endothelial tip cells for the recruitment of supportive cells. PDGF has also been used in other 2D coculture systems where indirect or direct positive effects on capillary network formation could be observed. However, the cell types, cell numbers and media employed in these studies were different to those used in the present study. In the co-culture systems, it can be speculated that the positive effect of PDGF-BB is accomplished by increasing VEGF expression levels in mural cells and fibroblasts. Additionally, it stimulates the production of collagenases, which are important for cell migration.VEGF and PDGF-BB have been used in sustained release systems both in vitro and in vivo, leading to a rapid and sustained increase of a mature vascular system. The combination of the two different growth factors VEGF-A and PDGF-BB in two different ratios was applied to evaluate their effect on the amount of capillary-like structures. Although it is well known that both of these factors have a pro-angiogenic effect, it has not been investigated whether or not there may be a synergistic angiogenic effect of both factors in the co-culture system. Considering the lower cell numbers, the effect of VEGF and/or PDGF-BB supplementation was not significantly different. It seems that the effect of VEGF-A/PDGF-BB on the formation of capillary-like structures in the proposed co-culture system can only be recognized in the system where a higher concentration of cells is used.