Temporal evolution of cell focal adhesions: Experimental observations and shear stress profiles

Live cells create adhering contacts with substrates via focal adhesion (FA) sites associated with the termini of actin stress fibers. These FA 'anchors' enable cells to adhere firmly and locally to a substrate, and to generate traction that enables cell body displacement. Using time-lapse video microscopy, we have monitored the spontaneous anisotropic changes in FA size and shape, resulting from molecular reorganization of the adhesion sites in live, non-motile rat embryonic fibroblasts adhering to fibronectin-coated glass surfaces. The resulting experimental data on FA growth, saturation and decay is compared with predictions from a number of biophysical models. We find that the growth and saturation regimes for all FAs exhibit a consistent, recurring pattern, whereas the disassembly regime exhibits erratic behavior. We observe that the maximum size of FA areas depends on the FA growth rate: larger FA areas are formed as the growth rate increases. Using a composite mechanics model by which the evolution of the shear stress profile along the FA region can be calculated, we suggest that FA saturation is triggered either by the reaching of a minimum shear stress threshold at the FA back edge, or of a maximum difference between the maximum and minimum shear stress along the FA site.