| Live cells are constantly exposed to mechanical forces induced by different physical interactions. One manner by which mechanical forces are sensed by cells is through cell-substrate contacts called focal adhesion (FA) sites associated with the termini of actin stress fibers. These forces play significant roles in many different cell functions. This area of research is currently the subject of extensive experimental and theoretical research, because the physical mechanisms involved in cell mechanics processes are still far from being understood. In the present study we borrow theoretical and experimental tools from the materials science field to further understand the physical mechanisms involved in cell response to mechanical signals.;First, force transfer at cell-substrate adhesion sites (FA sites) was investigated by using a shear-lag type mechanical model classically used in composite material science. The shear stress profile produced along the cell-substrate interface was calculated and analyzed as a function of various material and geometrical characteristics of the adhesion region. It was found that at the front edge of the FA site there is a maximal shear stress. The full shape of the shear stress profile along the cell-substrate interface suggests a likely mechanism for the biochemical feedback activity leading to the growth of the adhesion region.;Second, we studied how the shear stress profile varies during the temporal evolution of a single FA site, which was monitored and analyzed in detail for the first time using time-lapse video microscopy of rat embryonic fibroblasts. From image analysis of FA temporal evolution, it was found that single FA sites exhibit a consistent, recurring pattern of growth and saturation modes. On the contrary, disassembly mode of single FA sites exhibits an erratic behavior. Based on these results, the shear stress profiles were calculated at each step of the FA evolution process, and new aspects of a mechano-sensing mechanism related to FA growth and saturation, were suggested. The resulting experimental data for FA growth, saturation and decay was also compared with predictions from a number of biophysical models.;Third, to further understand how cells respond to an external applied force or stress, we imposed a controlled non-uniform stress field by stretching Polydimethylsiloxane substrates containing an elliptical hole at the center. Specifically, we explored the influence of a non-uniform stress field and a free boundary around an elliptical hole on cell proliferation rate, migration and orientation. First results show that cell response around an elliptical hole is different for mechanically stressed samples, as compared to specimens under no stress. Also, for stressed samples, cell response is different in highly tensile stressed regions, in comparison with cells located in compressive regions. |
