Inducing ossification in an engineered 3D scaffold-free living cartilage template

Large and complex bone defects or voids cannot rely on natural bone healing process for recovery. They require natural or engineered grafts to facilitate osteo-progenitor cell recruitment and development. In this study, we have employed an in vitro macro-sized 3D cell-based platform for investigation and application of osteogenesis. The model is based on a porous construct made of engineered living cartilaginous tissue named living hyaline cartilaginous graft (LhCG). It is scaffold-free and is solely made up of living chondrocytes and their extra cellular matrix (ECM). To evaluate the efficiency of LhCG as a viable platform for bone formation, osteoblast and human mesenchymal stem cell (hMSC) were seeded respectively into LhCG constructs, establishing a co-culture system consisting of osteo-progenitors and chondrocytes. The results showed that LhCG could support both osteoblast and hMSC maturation and differentiation to the osteogenic lineage respectively. Successful osteogenesis is also observed after subcutaneous implantation in nude mice model suggesting that bone formation could be achieved both in vitro and in vivo. Additionally, with exposure to osteogenic medium, LhCG construct without any further cell seeding expressed similar levels of osteogenic phenotype markers as the ones with hMSC seeded on. It suggests the existence of an osteoprogenitor sub-population residing within LhCG chondrocytes. Hence, it is demonstrated that LhCG, as a cartilage template, could serve as a dynamic platform to support osteogenesis and its intrinsic phenotypic flexibility may also permit a wide range of applications for stem cell research and processing.