Tiam Mana Saffari

227 MICRO CT ANALYSIS AFTER STEM CELL DELIVERY AND ANGIOGENESIS 10 DISCUSSION Nerves are vascularized through two systems: the extrinsic and intrinsic blood supply. Both systems serve as a nutrient systemand are responsible for the cellular metabolism of the nerve 20 . After injury, blood flow is increased to help prevent the nerve from ischemia 21 , resulting in a cascade of biological processes over time including activation of angiogenesis, sprouting of vascular endothelial cells and remodeling of blood vessels 22 . In vascularized nerve grafts, blood flowwas found to be increased compared to non- vascularized nerve grafts, starting from the first postoperative day 23 . This finding is in line with our study, which demonstrated enhanced vascularity in nerve grafts to which surgical angiogenesis via a SIEF flap was applied. The combination of surgical angiogenesis and stem cells, in particular undifferentiated MSCs, resulted in further enhancement of vascularity and has not been previously described. Stem cells are known for their paracrine properties and exert different functions when influenced by their environment 24 . Differentiation of MSCs in Schwann cell-like cells results in specific gene expression profiles and are found to express high gene-levels in the first 72 hours after seeding on nerve samples in vitro , while undifferentiated MSCs are characterized by enhanced gene-levels after one week 25 . Specifically, vascular endothelial growth factor (VEGF) was expressed in higher levels by undifferentiated MSCs compared to differentiated MSCs 25 , potentially explaining the findings in this study. Undifferentiated MSCs combined with the SIEF f lap not only increased vascularity, but also resulted in revascularization of the mid-section of the graft at two weeks (Figure 2E), emphasizing that stimulation of centripetal revascularization is determined by the surrounding tissue 4,26 . Previous studies have reported increased vascularity in decellularized allografts treated with differentiated MSCs versus undifferentiated MSCs at 16 weeks 27 . Vascularity was measured using a two-dimensional method, which lacks details of nerve angioarchitecture compared to a 3D method. Additionally, this study did not evaluate the provision of surgical angiogenesis. With different experimental designs, it is difficult to compare our results at two weeks to their 16 week outcomes 27 . The synergistic effect of stem cells and surgical angiogenesis is hypothesized to be due to the adipofascial flap providing vascular delivery of trophic factors, sufficient nutrients and oxygen, resulting in less hypoxia vital to sustaining survivability of stem cells 28,29 . Stem cells have a neuroprotective and immunomodulatory effect causing less fibrosis