Controlled growth factor delivery to engineer vascular networks and enhance transplanted cell survival.

Abstract

Tissue engineering, via cell transplantation, offers the potential to eliminate the donor complications and shortages associated with whole organ transplants. The greatest challenge to this strategy of tissue engineering is the poor survival observed in transplanted cell populations. Transplanted cells are initially dependent on the diffusion of nutrients and waste products between the transplanted cells and the host vasculature. To overcome diffusional transport limitations, a vascular network must be formed throughout an engineered tissue. This process of blood vessel development (angiogenesis) has been shown to be, in part, regulated by protein growth factors. Several growth factor delivery systems, for the purpose of enhancing local angiogenesis, were developed and evaluated in this thesis. Alginate beads were shown to effectively deliver a number of growth factors. Several variations of porous polymer matrices were also developed that could deliver biologically active growth factors with controlled kinetics for several weeks, while acting as scaffolds for cell transplantation. When these matrices were implanted into a mouse model those conditions that included vascular endothelial growth factor (VEGF), a potent stimulator of angiogenesis, were observed to experience a dramatic upregulation in the density of blood vessels in the matrices for 2 weeks. Endothelial cells, hepatocytes, and skeletal muscle myoblasts were transplanted into mice on a number of porous polymer matrices. Transplanted endothelial cells were observed to differentiate into functional blood vessels within 2 weeks following implantation. Released VEGF caused an increase in the density of blood vessels derived from the transplanted endothelial cells. Fourteen days after implantation nearly four times as many hepatocytes continued to survive in matrices releasing VEGF as compared to controls. Myoblasts, however, did not respond to the environmental changes induced by the released growth factors since they proliferated at their maximal rate in all experimental conditions. These results show the powerful effect a relatively small amount of growth factor (e.g., VEGF) can have on some transplanted cells and the surrounding host tissue. In addition these studies demonstrate the importance of a robust vasculature on the survival of more sensitive cells such as hepatocytes.