Micromechanical Deformation of Chondrogenic Mesenchymal Stem Cells in 3D Hydrogels is Modulated by Time in Culture and Matrix Connectivity

Mesenchymal stem cells (MSCs) are a clinically attractive alternative to chondrocytes for the development of engineered cartilage tissue owing to their ease of isolation and chondrogenic potential [1]. However, the mechanical properties of MSC-based constructs have yet to match those of native cartilage or of chondrocyte-based constructs cultured similarly [1]. One route for improving these properties may be the application of mechanical stimulation, as normal cartilage development and homeostatic maintenance is dependent on force transduction. In a tissue engineering context, dynamic compression applied to chondrocyte-seeded hydrogels modulates both matrix production and mechanical properties [2, 3]. Similarly, when MSCs are embedded in 3D hydrogels, expression of chondrogenic markers and cartilaginous ECM synthesis are differentially regulated by dynamic compressive loading [4, 5]. Indeed, we have recently shown that long-term dynamic loading initiated after a pre-culture period of 21 days in pro-chondrogenic medium improves matrix distribution and the compressive properties of MSC-seeded constructs [5]. Interestingly, when loading was initiated after a single day of culture, mechanical properties failed to develop [6, 7], suggesting that elaboration of matrix was required prior to dynamic loading in order to positively direct construct maturation. When chondrocytes are embedded in agarose, the initial growth phase is characterized by the establishment of a dense pericellular matrix (PCM). At early times in culture, before these islands of PCM become connected into an interterritorial matrix, cells are protected from bulk deformation applied to the gel [8]. In a recent study, we showed that clonal heterogeneity in stem cell populations determines the rate at which this PCM forms, with some MSC clones rapidly establishing a dense PCM, while others fail to develop a robust PCM (and so continue to deform with gel deformation) through several weeks in culture [9]. To further this investigation, this study charted the culture time-dependent changes in ECM connectivity and MSC deformation under basal and chondrogenic conditions.