Characterisation of ovine bone marrow-derived stromal cells (oBMSC) and evaluation of chondrogenically inducedmicro-pellets for cartilage tissue repairin vivo

Background Bone marrow stromal cells (BMSC) show promise in cartilage repair, and sheep are the most common large animal pre-clinical model. The objective of this study was to characterize ovine BMSC (oBMSC) in vitro , and to evaluate the capacity of chondrogenic micro -pellets manufactured from oBMSC or ovine articular chondrocytes (oACh) to repair osteochondral defects in sheep. Methods oBMSC were characterised for surface marker expression using flow cytometry and evaluated for tri-lineage differentiation. oBMSC micro -pellets were manufactured in a microwell platform, and chondrogenesis was compared at 2%, 5%, and 20% O 2 . The capacity of cartilage micro -pellets manufactured from oBMSC or oACh to repair osteochondral defects in adult sheep was evaluated in an 8-week pilot study. Expanded oBMSC were positive for CD44 and CD146 and negative for CD45. Results The common adipogenic induction medium ingredient, 3-Isobutyl-1-methylxanthine (IBMX) was toxic to oBMSC, but adipogenesis could be restored by excluding IBMX from the medium. BMSC chondrogenesis was optimal in a 2% O 2 atmosphere. Micro -pellets formed from oBMSC or oACh appeared morphologically similar, but hypertrophic genes were elevated in oBMSC micro -pellets. While oACh micro -pellets formed cartilage-like repair tissue in sheep, oBMSC micro -pellets did not. Conclusion The sensitivity of oBMSC to IBMX highlights species-species differences between oBMSC and hBMSC. Micro -pellets manufactured from oBMSC were not effective in repairing osteochondral defects, while oACh micro -pellets enabled modest repair. While oBMSC can be driven to form cartilage-like tissue in vitro, their effective use in cartilage repair will require mitigation of hypertrophy.