Autologous bone is still the gold standard for repairing bone defects due to trauma, congenital defects, and surgical resection. However, autogenous bone grafting has significant limitations, including donor pain, risk of infection, and limited bone volume that can be isolated from the patients1,2,3,4. Numerous biomaterials have been developed as bone substitutes, combining natural or synthetic polymers with mineralized materials such as calcium phosphate or hydroxyapatite5,6. Bone formation in these engineered materials is usually achieved using the mineralized material as a priming material to allow stem cells to differentiate directly into osteoblasts through the intramembrane ossification (IMO) process7. This process lacks the angiogenic step, resulting in insufficient in vivo vascularization of the graft after implantation8,9,10, and therefore, approaches using such a process may not be optimal for treating large bone defects11.
Strategies applied to recapitulate the endochondral ossification (ECO) process, an innate mechanism in skeletogenesis during development, have been shown to overcome significant problems associated with traditional IMO-based approaches. In ECO, chondrocytes in the cartilage template release vascular endothelial growth factor (VEGF), which promotes vascular infiltration and remodeling of the cartilage template into bone12. The ECO-mediated approach to osteogenesis via cartilage remodeling and angiogenesis, which is also activated during fracture repair, uses artificially created cartilage tissue derived from MSCs as a priming material. Chondrocytes can tolerate hypoxia in bone defects, induce angiogenesis, and convert a vascular-free cartilage graft into angiogenic tissue. Numerous studies have reported that MSC-based cartilage grafts generate bone in vivo by implementing such an ECO program13,14,15,16,17,18,19,20,21.
An essential requirement for the clinical application of this ECO-mediated approach is how to prepare the desired amount of cartilage graft in a clinical setting. Preparing clinical cartilage of a size that fits the actual bone defect is not practical. Therefore, graft cartilage must form bone integrally when multiple fragments are implanted22. Hydrogels may be an attractive tool for scaling up tissue-engineered grafts for endochondral ossification. Many naturally derived hydrogels support MSC cartilage formation in vitro and ECO in vivo23,24,25,26,27,28,29,30,31,32; however, the optimal support material to meet the clinical application requirements has remained undetermined. Hyaluronic acid (HA) is a biodegradable and biocompatible polysaccharide present in the extracellular matrix of cartilage33. HA interacts with MSCs via surface receptors such as CD44 to support chondrogenic differentiation25,26,28,30,31,32,34. In addition, HA scaffolds promote IMO-mediated osteogenic differentiation of human dental pulp stem cells35, and scaffolds combined with collagen promote ECO-mediated osteogenesis36,37.
Here, we present a method for preparing HA hydrogels using bone marrow-derived adult human MSCs and their use for hypertrophic chondrogenesis in vitro and subsequent endochondral ossification in vivo38. We compared the characteristics of HA with those of collagen, a material widely applied in bone tissue engineering with MSCs and a useful material for scaling up artificial grafts for endochondral ossification17. In an immunocompromised mouse model, HA and collagen constructs seeded with human MSCs were evaluated for in vivo ECO potential by subcutaneous implantation. The results show that HA hydrogels are excellent as a scaffold for MSCs to create artificial cartilage grafts that allow bone formation through ECO.
The protocol is divided into two steps. First, constructs of human MSCs seeded on hyaluronan hydrogel are prepared and differentiated into hypertrophic cartilage in vitro. Next, the differentiated constructs are implanted subcutaneously in a nude model to induce endochondral ossification in vivo (Figure 1).