Hypoxia Longitudinally Correlates with Initial Extrinsic and Subsequent Intrinsic Vascularization of Collagen Elastin Matrix-Embedded Arteriovenous Loops in Experimental Rats

Purpose Tissue engineering techniques that allow the growth of vascularized tissue are currently being developed and characterized with the help of animal models such as the rat arteriovenous loop (AVL), a microvascular vessel loop that is generated microsurgically, and implanted within an isolation chamber. Inside the chamber, the loop can be embedded in different matrices that become considerably vascularized with time. While vascularization of rat AVLs has been studied in the context of various matrix materials, hypoxia and subsequent vascularization have so far not been longitudinally assessed in rat AVL constructs embedded in a collagen elastin matrix. This assessment, however, is important to determine the peak of vascularization and thus the ideal time frame for transplantation. Methods We employed histological and micro-computed tomography analyses with subsequent region-based image segmentation that allows three-dimensional reconstruction of the vascular network 5, 15, 21, and 28 days post collagen elastin matrix-embedded AVL generation in rats. Results The number of newly formed vessels was substantially increased on day 15 compared to day 5 and plateaued at day 21 after surgery. The proportion of hypoxic cells in the loop constructs plateaued at day 15 and correlated significantly with the number of newly formed vessels. Moreover, vascularization was initiated extrinsically at the chamber entrance, and intrinsic sprouting occurred only later. Conclusion Hypoxia contributes to the vascularization of collagen elastin matrix-embedded AVLs, and extrinsic vascularization can occur through the isolation chamber entrance. Lay Summary Tissue survival after transplantation strongly depends on blood supply. Therefore, tissue engineering techniques that allow the growth of pre-vascularized tissue hold the promise to find application in reconstructive surgery. Here, we examined vascularization of a collagen-embedded rat arteriovenous loop (AVL) over time. We found that vessel numbers were substantially increased 15–21 days after AVL generation, suggesting that this may be a suitable time frame for transplantation. Moreover, we observed a correlation between hypoxia levels and vessel growth within the matrix, supporting the hypothesis that hypoxia plays a pivotal role in stimulating blood vessel outgrowth in the AVL model. Description of Future Work The rat arteriovenous loop (AVL) is an ideal model to investigate the influence of various matrix materials on vascularization. However, the effects of other matrix materials on hypoxia-induced angiogenesis, and the influence of angiogenesis-promoting factors on vascularization and survival of the AVL construct post-transplantation remain to be investigated. Graphical Abstract Created in BioRender. Thiele, W. (2025) https://BioRender.com/es3w7ng