Engineering functional vasculature with spatially controlled VEGF delivery

Growth factor driven neovascularization is a major strategy to form new vessels for the treatment of ischemic diseases and for tissue engineering. The successful development of this approach into meaningful clinical therapies will likely require the coordinated delivery of growth factor signals to locally control and spatially guide the complex process of angiogenesis. This thesis utilizes a porous polymeric scaffold for controlled growth factor delivery to create vasculature that efficiently alleviates ischemic tissue conditions. The hypothesis underlying this work is that localized and sustained delivery of a potent angiogenic initiator, vascular endothelial growth factor (VEGF), can form functional neovasculature. Furthermore, spatially controlled creation of a VEGF gradient can direct neovascular organization and enhance perfusion to an ischemic site.; The first set of studies determined the functionality of VEGF-induced vasculature in the context of local ischemia and host immune competence. Controlled VEGF delivery in both subcutaneous pockets and ischemic hindlimbs of immune deficient SCID and immune competent C57/B16 mice resulted in similar initial increases in vessel density, but the resulting increases in perfusion depended on site-specific tissue conditions. Hosts with a complete immune response were able to efficiently remodel neovasculature towards full recovery of hindlimb perfusion, while immune deficient animals lacked this capacity. This demonstrated that improved functionality of VEGF induced vasculature is desirable in hosts with a compromised immune system to prevent tissue necrosis due to ischemia. In order to understand the VEGF delivery parameters necessary to engineer vascular networks with enhanced function, an in vitro three-dimensional model of angiogenesis was developed to determine a quantitative relationship between VEGF binding and endothelial cell sprouting. In vitro analysis of sprouting in the presence of a spatial VEGF gradient validated that the growth of a sprout depends on the concentration of VEGF, while the direction of sprout extension requires the presence of a VEGF gradient. Based on the capacity of a spatial VEGF gradient to direct the process of angiogenesis, a system capable of spatially controlled VEGF delivery was developed and evaluated. Spatially controlled delivery of VEGF led to amplified and complete recovery from ischemia in hindlimbs of immune deficient animals. These results highlight the value of spatially controlled VEGF delivery in strategies to control neovascularization. Overall, this work provides novel model systems to understand the process of growth factor driven neovascularization and to promote the formation of functional blood vessels in a variety of therapies.