| Stem cell therapies have shown promise in the treatment of musculoskeletal diseases by integrating with host tissue, remodeling diseased environments, and recruiting other cells through paracrine signaling. However, clinical trials have yielded limited success in part because the role of the extracellular matrix (ECM) has been underappreciated. The physiochemical properties of this protein network, notably the density, composition, and degree of crosslinking, all influence the Young's modulus, i.e. 'stiffness,' of this matrix. Cells 'feel' changes in their local environment by contracting against this material, integrating physical cues that guide cell function. In this thesis, we aim to elucidate how cell-matrix mechanosensing guides stem cell migration and differentiation. Our studies have employed synthetic hydrogels to decouple specific ECM properties, allowing us to study how each component independently regulates cell behavior. After fabricating hydrogels with mechanical gradients, we observed that stem cell migration velocity scaled with gradient strength over a given stiffness range as long as the cytoskeleton remained intact. This mechanical regulation of migration in normal and disease pathologies suggests that stem cells may better contribute to repairs in stiffer regions of tissues where they may preferentially accumulate. Mechanical step gradients provided a means to mechanically confine stem cells, which readily aligned and robustly underwent myogenic differentiation and fused, and could serve as a synthetic platform for micro tissue engineering constructs. Subsequent studies focused on cell-sensing in geometrically constrained environments. Tuning cell shape revealed that focal adhesion formation and cell-generated traction stresses follow opposing trends over a range of stiffness. Our contractility-based observations suggest that differentiation may be possible in non-permissive environments and could prove beneficial to treat fibrotic diseases. Further investigation on a microscopic scale of how cells interact with synthetic materials in vitro revealed that the substrate stiffness of our systems, and not the attached cell-adhesive ligands nor their configuration, was the most important factor to determine differentiation of stem cells. Taken together, these data imply that the mechanics of the environment drive biased cell migration and differentiation and that stem cell clinical trials may have higher chances of success if ECM mechanics are factored into the therapy design. |
