Femke Mathot

10 General discussion and future perspectives 169 BACKGROUND Peripheral nerve injuries that have led to nerve gaps not repairable by direct coaptation of both nerve ends need to be bridged by nerve grafts. Ideally these grafts are autografts as they are known to result in the best functional outcomes. 1 However, autografts are not endlessly available and donor site morbidity should be taken into account. 2, 3 The off-the- shelve available nerve allografts currently do not lead to equal results as autograft nerves, hypothetically due to the absence of essential elements like a proficient vascular bed and adequate intrinsic trophic factors. First the role of mesenchymal Stem Cells (MSCs) will be described as they possess trophic capacities and have been hypothesized to be able to fulfill a role in improving the outcomes of nerve allografts. Second our studies to assess vascularization of the allograft will be discussed. The neural regenerative capacities of mesenchymal stem cells (MSCs) have been extensively tested in the past. Although some results were hopeful, their absence in current clinical practice reveals the continuing uncertainties regarding the ideal delivery strategy, their mechanism of action and perhaps most important their capacities to improve functional outcomes of peripheral nerve repair. This thesis aimed to get more insight in the mechanisms of action of MSCs when delivered to decellularized nerve allografts and conduits with a recently described innovative seeding strategy. 4 The effects on gene expression, vascularity and functionality of undifferentiated MSCs were compared to MSCs differentiated into Schwann cell-like cells and the clinical potential was explored using FDA approved nerve graft substitutes in a study model. One of the first proposed mechanisms of the MSCs to aide in neural regeneration was their ability to transdifferentiate into cells essential in this process. An often used hypothesis was that MSCs could differentiate into Schwann Cells and thus support and regulate neural regeneration. Over the years, only a few research groups claimed to have proven that delivered MSCs have a structural function, by differentiating into Schwann cells in vivo and by replacing injured cells in the regenerated nerve. 5-7 However, the beneficial effects of delivered MSCs have not been convincingly demonstrated. When studying MSC- dimensions, one could consider that the MSCs are disproportionally big compared to the single nerve fiber/axon on which the MSCs are supposed to exert their effect. Blockage of ingrowing axons and damage to the inner ultrastructure of nerve substitutes are therefore not unthinkable consequences. 8-10 Also, in previous studies MSCs have been mostly delivered inside nerve substitutes by injection. The smaller the injection-needle diameter, the less harm is caused to the ultrastructure of the nerve substitutes. This is opposite to MSC viability: the smaller the needle, the more pressure is built up in the syringe resulting in a more impaired viability of the MSCs. 11-13 Inefficient delivery and fascicle blockage may explain the numerous studies that did not demonstrate a beneficial effect of MSCs delivered inside nerve substitutes despite their differentiational capacities. 14, 15 Derived from other fields of research, the theory evolved that MSCs can have a trophic effect on regenerating tissues without being built into the new tissue. 16, 17 This trophic theory