Simultaneous quantification of cell elasticity and internal pressure in adherent animal cells

Understanding the mechanical properties of animal cells is essential for fundamental biological processes. Two key quantities for these properties are the effective cell stiffness (E) and intracellular pressure (P). Their combined influence is significant in understanding cell deformation under external forces. However, there is no general method to simultaneously quantify E and P in animal cells. In this study, we used atomic force microscopy (AFM) to obtain topographic images and force-indentation curves of adherent animal cells, which were then analyzed by applying elastic shell theory (EST). AFM is widely used to estimate Young's modulus based on Hertz contact theory, but it cannot independently evaluate both E and P. EST treats the cell membrane-cortex complex as a thin-shell structure and analyzes local changes in cell shape under external loading, enabling quantitative estimation of both E and P. We tested Madin-Darby canine kidney (MDCK), mouse fibroblast (NIH3T3), and human brain tumor (U87) cell lines. Among them, only NIH3T3 cells exhibited a clear linear response to cell indentation that was applicable to the EST model. E and P were estimated to be 0.5 ± 0.3 MPa and 7 ± 3 kPa, respectively. In contrast, MDCK and U87 cells showed nonlinear responses, indicating that their deformation could not be described by a thin-shell structure. Fluorescence imaging results revealed that NIH3T3 cells possessed a denser actin meshwork than MDCK and U87 cells. This structural difference likely causes the occurrence or absence of the shell-like mechanical response. These results clarify that EST can reveal the mechanical state of animal cells associated with the structural actin network. Our approach provides a comprehensive framework for evaluating cellular mechanical states and advances the interpretation of AFM measurements in living cells.