Femke Mathot

Chapter 2 36 of BDNF, GDNF, angiopoietin-1 and VEGF-A upon differentiation of MSCs. These increased levels of growth factors resulted in higher total neurite outgrowth, longer neurites and a better angiogenic potency after removal of the factors that stimulate differentiation from the growth medium. 17 Tomita found similar results and showed differentiated human MSCs produced higher levels of neurotrophic factors like BDNF, NGF and GDNF compared to undifferentiated MSCs. The secretion of these neurotrophic factors resulted in a significantly increased percentage of neuron-bearing neurites, and a significant increase in both neurite length and number of neurons. 4 Ladak also demonstrated that co-culture of differentiated MSCs with dorsal root ganglion neurons led to longer and more arborous neurite outgrowth than undifferentiated MSCs. 23 As described previously, the same result was found with differentiated human MSCs. 26 In vitro studies that examined the interaction between undifferentiated and differentiated MSCs with a processed nerve allograft showed persistent enhanced expression of neurotrophic genes that subsequently led to the secretion of neurotrophic growth factors. 52, 53 In general, differentiated MSCs enhance the expression of neurotrophic genes and the secretion of neurotrophic proteins, resulting in increased neurite outgrowth in vitro. These in vitro results are promising and support the hypothesis that differentiated MSCs have a trophic function in nerve regeneration. The remark needs to be made that any agent or growth factor added to the growth medium may become embedded in the extracellular matrix (ECM) and might not be completely washed out after removal of the differentiation media. Thus, the enhanced gene expressions and the increased neurite outgrowth could still be the effect of direct stimulation by the added growth factors instead of being positively influenced by the differentiated MSCs. In vivo research could eliminate this discrepancy. Table 1. Overview of the pros and cons of the described delivery methods of MSCs. Delivery method Efficiency Pros Cons Injection into nerve grafts 10-40% • Delivers a high number of MSCs in the inner and middle nerve zones • Reduced viability of MSCs • Damage to the ultrastructure of the nerve • Leakage of cells (conduits) • Local accumulation of MSCs Intravenous injection 100% • No damage to the ultrastructure of the nerve • No cell leakage • Reduced viability of cells • Entrapment of MSCs in capillaries • Low number of MSCs at regeneration site Intramuscu- lar injection 100% • Locally delivers MSCs • No damage to the ultrastructure of the nerve • No cell leakage • Reduced viability of cells • Low number of MSCs at regeneration site Soaking Unknown • Delivers MSCs in the outer nerve zones • Preserved viability of MSCs • Damage to the nerve (micro- needle roller) Seeding 89.2% • Uniform distribution of MSCs • Preserved viability of MSCs • No damage to the ultrastructure of the nerve • No cell leakage • Interaction between MSCs and extracellular matrix is required