The Role of Hemodynamics in Angiogenesis

The cardiovascular system is the first functional organ to form during embryonic development. Blood flow provides mechanical and chemical signals that are required for proper vascular development. The vasculature adapts to the onset of blood flow in part by forming new blood vessels through a process called angiogenesis. Angiogenesis is not limited to embryonic development, but also occurs after ischemic injuries, during wound healing as well as during tumour growth in cancers. A basic understanding of angiogenesis and the physiological cues that regulate the process is therefore an important therapeutic target for diseases such as stroke, myocardial infarction, and cancer. Angiogenesis also has application in regenerative medicine, and tissue engineering to provide nutrient transport to tissue. Blood flow provides biomechanical stimuli by exerting forces on the surrounding tissue including a tangential force on the luminal surface of the endothelium, called shear stress. Additionally, interstitial flow exiting or entering the vessel walls produces physical forces normal to the endothelium. Angiogenesis is known to be controlled by a range of signals, but the role of blood flow and biomechanical signals are not well understood. One of the greatest difficulties in studying the interplay of flow dynamics and vascular remodelling is that few tools are available to analyse flow dynamics in real time in vivo. Therefore, the initial objective of this thesis was to develop a method to concurrently visualise vascular remodelling and blood flow dynamics. We used an avian embryonic model and injected an endothelial-specific dye, to image the vasculature, and fluorescent microspheres, to track fluid motion. Microsphere motion was analysed via an optical technique called micro-particle image velocimetry (μPIV). µPIV measurements are associated with large errors in complex geometry such as vessel branch points. As a result, we limited μPIV measurements to straight segments and applied computational fluid dynamics (CFD) to obtain the blood velocity in all other locations in the region of interest. The CFD analysis also allowed us to calculate other hemodynamic parameters such as the pressure, the vorticity and the shear stress. We then used our technique to investigate the role that hemodynamic signalling plays in angiogenic sprouting. We found that flow dynamics mediates the location of sprout initiation, direction of sprout elongation, and the rate of sprout elongation during vascular development. Using the developed method and obtained parameters, we demonstrated that sprout location can be predicted based on flow dynamics. Moreover, the rate of sprout elongation is proportional to the pressure difference across the interstitium. Our results suggested that cues from the flow dynamics are important mediators of vascular homeostasis and morphogenesis. In the last part of this work, we extended our technique to model interstitial flow passing through the porous matrix of the mesenchymal tissue. We modelled how VEGF transport within the tissue is altered by the presence of interstitial flow. This allowed us to simultaneously study the real-time interaction of luminal and transmural shear stress, interstitial flow, and VEGF distribution in angiogenesis. Interstitial flow strongly regulates the distribution of vascular endothelial growth factor (VEGF) within the tissue. We found that interstitial flow created regions of high VEGF in the location of sprouting, but did not alone indicate the exact sprouting location. We also showed that the sprout elongated against the direction of interstitial flow, and that a strong relationship was present between the elongation rate and the interstitial flow rate. Our results underscore the interplay between hemodynamics and VEGF distribution that regulates the development of vascular network to meet its many functional demands.nrpages: 156status: accepte