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Title: Tissue engineering of a vascularized bone graft
Keywords: Vascularization, Hyaluronan, BMP-2, Tissue engineering, Arteriovenous loop model, Slow release
Issue Date: 14-Aug-2009
Citation: SUBHA NARAYAN RATH (2009-08-14). Tissue engineering of a vascularized bone graft. ScholarBank@NUS Repository.
Abstract: The emergence of novel biomaterials and cell isolation techniques has revolutionized the field of tissue engineering. The required mechanical and physical properties can be customized with available biomaterials, whereas specific tissue might be fabricated with the application of particular cell type and growth factor. However, if the size of the constructs is increased, the applied number of cells must be proportionately more. Their survival in vivo is crucial in such constructs. The diffusion of micronutrients can be possible for merely few hundred micrometers from the capillary. Therefore, specific tissues were generated only in relatively small tissue sizes or in a fractional part of the constructs. Moreover, the central region gets frequently necrotic due to lack of nutrition. Therefore, it is crucial to provide nutrition for the function and survival of the graft. Current attempts rely on the application of specific growth factors to induce vascular growth or on the use of endothelial precursor cells (EPC) for de novo vasculogenesis. In the former case, the desired vasculogenesis is challenging due to short half-life periods of growth factors; while in the later case, the EPCs need nearby vessels to form a vascular tree. In addition, the arbitrary formation of vascular tubes and their random connection to parent blood vessels are unpredictably slow. The thesis reports a novel way of fabricating a viable construct by making an arterio-venous (A-V) loop by microvascular anastomosis of the femoral vessels in a rat. This results in relatively quick formation of fibro-vascular tissue. The biomaterial constructs are observed to be successfully vascularized. To keep specific cells or growth factors in a bioactive stable form, hydrogels are often used as temporary vehicles to provide hydrated environments. However, the properties differ because of their different degradation kinetics and tissue reactivity. To examine the different rates of vascular growth in the A-V loop model, two hydrogels, fibrin glue and hyaluronan, are tested in the loop supplied composite scaffold. The new vascular network is successfully documented, though the kinetic and magnitude differ. In fibrin glue, the vessels are formed quickly with the degradation of the hydrogel in few days. On the contrary, a uniformly increasing vascular growth is observed in hyaluronan containing scaffolds along with a slowly decreasing residual hydrogel. The A-V loop permits the vessels to grow centripetally from the loop invading the scaffold. However, at the end the number and pattern of blood vessels are comparable between the two matrices. This study demonstrates that hyaluronan-contained composite scaffold permits vascular in-growth slowly along with its slow degradation. This can be further explored for any growth-factor containing graft. The utility of the vascularized scaffold-hyaluronan construct is further demonstrated in vivo by attempting to fabricate a bone tissue-engineered product with a bone-inducing growth factor, BMP-2. Two different amounts of BMP-2 and the addition of osteoblasts are tested, expecting vascularization by A-V loop and bone induction by BMP-2 or osteoblasts. Expression of bone-specific genes is detected by real-time RT-PCR analysis, though no significant amount of bone is detected histologically. The heterotopic isolation chamber setting in combination with the absence mechanical stimulation might explain the insufficient bone formation. However, the scaffold-matrix is vascularized to make a viable graft. Optimization of the interplay of cells and growth factors in the scaffolds might eventually allow generation of different axially vascularized grafts for application in reconstructive surgery. This project depicts a promising approach for a vascularized graft for further exploration.
Appears in Collections:Ph.D Theses (Open)

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