AbstractThe dynamic mechanical force transmitted through microenvironments during tissue formation and regeneration continuously impacts the mechanics of cells and thereby regulates gene and protein expression. The mechanical properties are altered during the process of stem cells differentiating into different lineages. At different stages of differentiation, stem cells display different mechanical properties in response to surrounding microenvironments, which depend on the subcellular structures, especially the cytoskeleton and nucleus. The mechanical properties of the cell nucleus affect protein folding and transport as well as the condensation of chromatin, through which the cell fate is regulated. These findings raise the question as to how cell mechanics change during differentiation. In this study, the mechanical properties of human bone marrow mesenchymal stem cells (hBMSCs) were determined during adipogenic and osteogenic differentiation by atomic force microscopy (AFM). The cytoskeletal structure and the modification of histone were investigated using laser confocal microscope and flow cytometry. The mechanical properties of cell nuclei at different stages of cell differentiation were compared. The stiffness of nuclei increased with time as osteogenesis was induced in hBMSCs. The H3K27me3 level increased during osteogenesis and adipogenesis according to flow cytometry analysis. Our results show conclusively that AFM is a facile and effective method to monitor stem cell differentiation. The measurement of cell mechanical properties by AFM improves our understanding on the connection between mechanics and stem cell fate.