Angiogenesis, the sprouting of new blood vessels from existing vasculature, is a complex biological process of interest to both the treatment of numerous pathologies and the creation of thick engineered tissues. In the context of tissue engineering, one potential solution to the diffusion limitation is to create a vascular network in vitro that can subsequently anastomose with the host after implantation, allowing the implantation of thicker, more complex tissues. In this study, the ability of endothelial cells to sprout and form stable vascular networks in 3-dimensional (3D) fibrin matrices was investigated as a function of matrix density in a prevascularized tissue model. The results demonstrate that while increasing matrix density leads to a nearly 7-fold increase in compressive stiffness, vascular sprouting is virtually eliminated in the most dense matrix condition. However, the addition of human mesenchymal stem cells (HMSCs) to the denser matrices reverses this effect, resulting in an up to a 7-fold increase in network formation. Although the matrix metalloproteinases (MMPs) MMP-2, MMP-9, and MT1-MMP are all upregulated early on with the addition of HMSCs, MT1-MMP appears to play a particularly important role in the observed angiogenic response among these proteases. This study provides a means to design suffer prevascularized tissues utilizing naturally derived substrates, and its results may yield new mechanistic insights into stem cell-based angiogenic therapies.
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