Femke Mathot

2 Stem cell differentiation in peripheral nerve repair 29 INTRODUCTION To achieve successful repair of peripheral nerve segmental defects, nerve autografts still supersede the results of all commercially available nerve graft substitutes (bioabsorbable conduits, vessels or processed allografts). 1 Nerve autografts are limited in availability and their harvest from patients automatically generates donor side morbidity. The application of MSCs has been actively considered for in vitro and in vivo studies seeking to improve outcomes of peripheral nerve reconstruction. MSCs potentially provide the necessary biological support for nerve substitutes to equate results obtained by autografts. 2, 3 Prior studies have also evaluated the application of Schwann cells to nerve graft substitutes and have demonstrated active expression of neurotrophic factors with encouraging outcomes. 4 However, clinical application of this technology is impractical, as it would require harvest of autologous nerve tissue to obtain autologous Schwann cells and extensive time to culture and grow the requisite number of Schwann cells for adequate seeding of the nerve graft substitutes. Alternatives to autologous Schwann cells would be the differentiation of autologous MSCs from the patient into Schwann-like cells. In vitro targeted stimulation of autologous MSCs has resulted in differentiation into Schwann-like cells without having to sacrifice autologous nerves. 4, 5 Hence, autologous MSCs can be harvested from the patient, differentiated into Schwann-like cells and be delivered to the site of nerve repair or seeded onto nerve graft substitutes to improve the regenerative environment. Important topics addressed in this review include methods for how to differentiate MSCs into Schwann-like cells, how to deliver naive or differentiated MSCs and the regenerative potential of differentiated MSCs compared to undifferentiated MSCs. MESENCHYMAL STEM CELLS MSCs can be isolated from a variety of tissues from the stromal vascular fraction that are extrinsic to blood vessels. They are most frequently obtained from either bone marrow or adipose tissue. Multiple studies have compared bone marrow and adipose MSCs and both sources yield viable MSCs that comply with minimal criteria for MSCs as defined in 2006 by the International Society for Cellular Therapy. 6 Key properties of MSCs include that they are plastic adherent, multi-potent and express canonical mesenchymal stem cell markers (CD44 and CD90), while other markers are absent (CD34 and CD45). 6, 7 In contrast to bone marrow, adipose tissue is more easily accessible and requires only minimally invasive methods (liposuction vs bone marrow harvest) to obtain adequate quantities of MSCs, while having a similar effect on nerve regeneration. 8, 9 MSCs that are derived from the stromal vascular fraction of adipose tissue are easily expanded and differentiated. 10, 11 These properties render adipose derived MSCs of particular interest for clinical applications compared to less accessible bone marrow derived MSCs. A well-established method to derive MSCs from adipose tissue consists of mechanically disrupted and enzymatically digested tissues. The fat tissue obtained by liposuction is minced and enzymatically digested using collagenase type I. The undigested tissue is removed by filtration and the filtered solution is suspended in standard culture media containing a-MEM.