Combining "waste utilization" and "tissue to tissue" strategies to accelerate vascularization for bone repair.

Background: A pivotal determinant for the success of tissue regeneration lies in the establishment of sufficient vasculature. Utilizing autologous tissue grafts from donors offers the dual advantage of mitigating the risk of disease transmission and circumventing the necessity for post-transplant immunosuppression, rendering it an exemplary vascularization strategy. Among the various potential autologous donors, adipose tissue emerges as a particularly auspicious source, being both widely available and compositionally rich. Notably, adipose-derived microvascular fragments (ad-MVFs) are a promising candidate for vascularization. ad-MVFs can be isolated from adipose tissue in a short period of time and show high vascularized capacity. In this study, we extracted ad-MVFs from adipose tissue and utilized their strong angiogenic ability to accelerate bone repair by promoting vascularization. Methods: ad-MVFs were extracted from the rat epididymis using enzymatic hydrolysis. To preserve the integrity of the blood vessels, gelatin methacryloyl (GelMA) hydrogel was chosen as the carrier for ad-MVFs in three-dimensional (3D) culture. The ad-MVFs were cultured directly on the well plates for two-dimensional (2D) culture as a control. The morphology of ad-MVFs was observed under both 2D and 3D cultures, and the release levels of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP-2) were assessed under both culture conditions. In vitro studies investigated the impact of ad-MVFs/GelMA hydrogel on the toxicity, osteoblastic activity, and mineralization of rat bone marrow mesenchymal stem cells (rBMSCs), along with the examination of osteogenic gene and protein expression. In vivo experiments involved implanting the ad-MVFs/GelMA hydrogel into critical-size skull defects in rats, and its osteogenic ability was evaluated through radiographic and histological methods. Results: ad-MVFs were successfully isolated from rat adipose tissue. When cultured under 2D conditions, ad-MVFs exhibited a gradual disintegration and loss of their original vascular morphology. Compared with 2D culture, ad-MVFs can not only maintain the original vascular morphology, but also connect into a network in hydrogel under 3D culture condition. Moreover, the release levels of VEGF and BMP-2 were significantly higher than those in 2D culture. Moreover, the ad-MVFs/GelMA hydrogel exhibited superior osteoinductive activity. After implanting into the skull defect of rats, the ad-MVFs/GelMA hydrogel showed obvious effects for angiogenesis and osteogenesis. The translational potential of this article: The utilization of autologous adipose tissue as a donor presents a more direct route toward clinical translation. Anticipated future clinical applications envision the transformation of discarded adipose tissue into a valuable resource for personalized tissue repair, thereby realizing a paradigm shift in the utilization of this abundant biological material.