Macroscopic creep behavior of spheroids derived from mesenchymal stem cells under compression

Spheroid culture, where cells are aggregated three-dimensionally, is expected to have applications as a model that better recapitulates invivo environment beyond two-dimensional environments. When human mesenchymal stem cells are subjected to spheroid culture in the presence of osteogenesis supplements, the gene expression of osteocyte differentiation marker is greatly increased within a short period compared to two-dimensional culture. However, how such alterations may be reflected to mechanical properties of the spheroid remains unknown. In this study, using a uniaxial compression system, we evaluated the macroscopic mechanical properties of human mesenchymal stem cell-derived spheroids including viscoelastic behavior. The Young's modulus of spheroids cultured for 2 days was about 18 kPa, whereas that of individual cells is around 1-10 kPa. We also found that creep behavior of the spheroid was greater in 50% strain compression beyond 10 or 30% strain, indicating that they are viscoelastic materials. Upon release from compression, the spheroids tended to revert to their original shape through elastic deformation. However, spheroids in which actin filament formation was inhibited exhibited a remarkably greater plastic deformation, suggesting that the actin filaments play a crucial role in the elastic behavior of spheroids. By understanding the mechanical properties and behavior of spheroids, it provides a framework for predicting and manipulating the development of tissues and organs in the field of morphogenesis.