Femke Mathot

Chapter 10 174 extent in the differentiated MSC-group in particular. This might explain the very subtle differences in functional and histological outcomes between both MSC-groups (in benefit for undifferentiated MSCs) and the autografts (in benefit for the autografts). Future studies should focus on ways to quantify vessel diameters and vessel alignment in order to confirm the hypothesis that alignment of vasculature is of major importance for nerve regeneration efficiency. GENE EXPRESSION & FUNCTIONAL OUTCOMES Differences in functional outcomes between groups cannot be completely traced back to the level of neoangiogenesis, so other processes and genes must have affected nerve regeneration as well. Since the baseline gene expression of both MSC-groups was significantly altered upon interaction with processed nerve allografts, leading to production of trophic factors, it is very plausible that the MSCs not only have caused enhanced angiogenesis, but improved axon regeneration in its entirety as well. It is impossible to solidly relate the barely differing functional outcomes of differentiated and undifferentiatedMSCs to each of the individual gene expression curves. Expression curves of PTN, GAP43, NGF, FBLN1 and CCNB2 particularly differed between undifferentiated and differentiated MSCs in vitro, mostly in benefit for differentiated MSCs. Although these differences apparently do not lead to significantly enhanced nerve regeneration, one should not jump to the conclusion that these genes don’t significantly affect nerve regeneration. The subtle discrepancies between in vitro gene expression and functional outcomes in vivo are the consequence of limited numbers per group, enormous differences in signals from surrounding tissues between in vitro and in vivo settings and the completely different time frames of both studies. The different effective phases of both cell types suggested by their gene expression in chapter 4 , might be relatable to the demonstrated tibial muscle area recovery over time. The degeneration of tibial muscles in the groups treated with differentiated MSCs only seems to occur in the first two weeks after surgery after which muscle area increases again, while in the undifferentiated MSC-group the muscle degeneration continued until four weeks after surgery. This might be due to differentiated MSCs being able to interact faster with the ECM than undifferentiated MSCs, although none of the muscle area differences between groups were significant. In contradiction to undifferentiated MSCs 22 , in vivo survivability of dynamically seeded differentiated MSCs has not been studied yet. Although differentiation of MSCs sensitizes MSCs to toxic effects 55 , it has been suggested that differentiated MSCs possess even greater survivability than undifferentiated MSCs. 6 The significant improvement of functional outcomes of nerve allografts by both cell types and the absence of significant differences between undifferentiated and differentiated MSCs described in chapter 7 are in accordance with these findings, suggesting at least sufficient survivability of both cell types to exert their effect. Nevertheless, it would be interesting to analyze the relation of vitality and survivability of both cell types and functional outcomes in future in vivo studies.