Chondrogenically primed mesenchymal stem cell-seeded alginate hydrogels promote early bone formation in critically-sized defects

• Alginate supports chondrogenesis of Mesenchymal Stem Cells.
• Higher levels of mineralization observed in defects treated with engineered tissues.
• Bone formation occurs appositional to the slowly degrading alginate hydrogel.
• Hydrogel degradation appears essential for de novo bone deposition.

Hypertrophic cartilaginous grafts can be engineered in vitro using bone marrow derived Mesenchymal Stem Cells (MSCs). When such engineered tissues are implanted in vivo they have been shown to induce bone formation by recapitulating aspects of the developmental process of endochondral ossification. Alginate, a naturally sourced and biocompatible hydrogel, offers an attractive 3D environment to facilitate the in vitro chondrogenesis of MSCs. Furthermore, such alginate hydrogels can potentially be used to engineer cartilage tissues of scale to promote endochondral bone regeneration in large bone defects. The aim of this study was to investigate the ability of chondrogenically-primed MSC-laden alginate hydrogels to induce healing in two distinct critically-sized defect models. Bone marrow derived MSCs were seeded into alginate hydrogels, chondrogenically primed in vitro in the presence of TGF-β3 and then implanted into either a critically-sized rat cranial or femoral defect. μCT analysis 4 weeks post-implantation revealed significantly higher levels of mineralization within the femoral defects treated with MSC-laden alginate hydrogels compared to untreated empty controls, with similar results observed within the cranial defects. However, any newly deposited bone was generated appositional to the alginate material, and occurred only superficially or where the alginate was seen to degrade. Alginate material was found to persist within both orthotopic locations 8 weeks post-implantation, with its slow rate of degradation appearing to prevent complete bone regeneration. In conclusion, while chondrogenically primed MSC–alginate constructs can act as templates to treat critically-sized defects within bones formed through either intramembranous or endochondral ossification, further optimization of the degradation kinetics of the hydrogel itself will be required to accelerate bone tissue deposition and facilitate complete regeneration of such defects.

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