Mesenchymal stem cells-derived cartilage micropellets: A relevant in vitro model for biomechanical and mechanobiological studies of cartilage growth.

The prevalence of diseases that affect the articular cartilage is increasing due to population ageing, but the current treatments are only palliative. One innovative approach to repair cartilage defects is tissue engineering and the use of mesenchymal stem/stromal cells (MSCs). Although the combination of MSCs with biocompatible scaffolds has been extensively investigated, no product is commercially available yet. This could be explained by the lack of mechanical stimulation during in vitro culture and the absence of proper and stable cartilage matrix formation, leading to poor integration after implantation. The objective of the present study was to investigate the biomechanical behaviour of MSC differentiation in micropellets, a well-defined 3D in vitro model of cartilage differentiation and growth, in view of tissue engineering applications. MSC micropellet chondrogenic differentiation was induced by exposure to TGFβ3. At different time points during differentiation (35 days of culture), their global mechanical properties were assessed using a very sensitive compression device coupled to an identification procedure based on a finite element parametric model. Micropellets displayed both a non-linear strain-induced stiffening behaviour and a dissipative behaviour that increased from day 14 to day 29, with a maximum instantaneous Young's modulus of 179.9 ± 18.8 kPa. Moreover, chondrocyte gene expression levels were strongly correlated with the observed mechanical properties. This study indicates that cartilage micropellets display the biochemical and biomechanical characteristics required for investigating and recapitulating the different stages of cartilage development.