High Resolution Mapping of Stresses at the Cell-Substrate Interface During Locomotion of Fibroblasts
Wednesday, January 6, 1999 - 11:00am - 12:00pm
We have developed a new approach to map mechanical forces exerted by locomoting fibroblasts on the substrate. 3T3 fibroblasts were cultured on elastic, collagen-coated polyacrylamide sheets embedded with 0.2 micron fluorescent beads. Forces exerted by the cell cause deformation of the substrate and displacement of the beads, which was measured by tracking positions of the beads before and after cell removal with trypsin. To calculate forces, the projected area of the cell was divided into quadrilaterals and traction forces at each node determined by a maximum likelihood algorithm and appropriate boundary conditions. The results were checked for statistical significance and for the agreement with the deformation data following reverse calculation. The calculated field had a spatial resolution of ~5 microns and a maximum magnitude of ~100,000 dynes/square cm. Strong propulsive forces were always located within 15 microns of the leading edge, with a net propulsive thrust on the order of 0.2 dyn. Forces along the anterior periphery were directed centripetally, however most forces under the lamella were weak and along a direction resisting forward cell movement. At the trailing edge, most cells exerted weak, diffuse retarding traction, likely reflecting resistive forces associated with the peeling of adhesive bonds. Combination of force mapping and immunofluorescence indicated co-localization of vinculin plaques with strong traction forces along the frontal periphery, however no strong force s were located at focal adhesions in the interior regions. Moreover, there was no apparent concentration of myosin II at sites of strong traction forces, despite pharmacological evidence for the dependence of such forces on myosin II. Our data provide the first high resolution maps of the traction stresses under a cultured fibroblast and demonstrate that fibroblasts locomote via a frontal contraction mechanism coupled to anterior-posterior differential adhesion.