Femke Mathot

Chapter 2 34 Methods of administration and cell dosage The desired method of cell delivery depends on the intended mechanism of action of MSCs. MSCs need to be delivered within the ultrastructure of nerves to fulfill a structural function or need to be able to migrate to the site of injury. Micro-injection of the MSCs has been described, but the consequences of injection to cell viability and the resulting ultrastructural trauma to the nerve are potential concerns. Jesuraj and colleagues reported the pressure build-up in the syringe and needle during injection reduces viability of cells after needle passage. (46) In contrast, Onishi and colleagues reported that adipose derived MSCs were fairly robust within a range of fluid pressures within the syringe upon expulsion. 37, 38 Studies that examined the viability of bone marrow derived MSCs post-injection have various conclusions ranging from no viability changes to a temporarily affected viability, to a reduced viability. 39-41 Increasing the needle gauge may intuitively reduce cell damage, but inserting a larger needle in a processed nerve graft is practically almost impossible and can easily cause tearing of the epineurium. In addition, uncontrolled micro-injection leads to a non-uniform distribution of cells and may result in local accumulation of clusters of MSCs that potentially block the ingrowth of the regenerating nerve rather than enhancing it. 38 The calibers of myelinated axon fibers (2 to 22µm) in proportion to the average diameter of MSCs (17.9 – 30.4µm) also may be problematic when MSCs are injected in the nerve allograft. 42-44 In case of using hollow nerve conduits, injection of MSCs will not harm the conduit itself, but it can still cause decreased viability of the cells and might obstruct axonal ingrowth. Furthermore, leakage of cells out of the nerve substitute is a recognized problem; the study by Jesuraj and colleagues showed only 10% of cells were successfully transferred after one million cells were injected. 38 The injection of MSCs in nerve substitutes is not clinically applicable due to low and uncontrollable delivery efficiencies and the potential damage to the cells and the nerves. Hence, future studies may consider alternative delivery methods for both differentiated and undifferentiated MSCs. Intravenous injection of MSCs has been investigated as an alternative to MSC-injection that prevents nerve-damage and cell-leakage and focuses on themore likely trophic function of MSCs. Although the vasculature potentially delivers a subset of MSCs to the area of injury, the cells may not accumulate to a critical mass to enhance nerve regeneration. In addition, the relatively large size of MSCs causes entrapment in capillaries. 45 MSCs can also be administered by intramuscular injection which delivers cells locally with preservation of the nerve. Intramuscular injection of MSCs in the gastrocnemius muscle resulted in a significantly improved functional recovery and neuro-conduction velocity compared to intravenous injection of MSCs or sham injection. 46 It has been reported that intramuscular injection of MSCs leads to enhanced nerve regeneration. 47 Even though these findings are promising, the described techniques still require injection of cells which potentially decreases the viability of the cells. The enhanced outcomes after intravenous and intramuscular injection of MSCs, do confirm the previously suggested trophic function of MSCs. Soaking nerve grafts in MSC-solutions is another described method of cell delivery. Thompson and colleagues compared the injection of cells to a soaking technique in which the nerve samples were pretreated with a micro-needle roller. Injection led to a higher number of cells in the inner and middle zones of the nerves, while soaking delivered a higher number of cells