High resolution detection of mechanical forces exerted by locomoting fibroblasts on the substrate

We have developed a new approach to detect mechanical forces exerted by loc omoting fibroblasts on the substrate. Cells were cultured on elastic, colla gen-coated polyacrylamide sheets embedded with 0.2-mu m fluorescent beads. Forces exerted by the cell cause deformation of the substrate and displacem ent of the beads. By recording the position of beads during cell locomotion and after cell removal, we discovered that most forces were radially distr ibuted, switching direction in the anterior region. Deformations near the l eading edge were strong, transient, and variable in magnitude, consistent w ith active local contractions, whereas those in the posterior region were w eaker, more stable, and more uniform, consistent with passive resistance. T reatment of cells with cytochalasin D or myosin II inhibitors caused relaxa tion of the forces, suggesting that they are generated primarily via actin- myosin II interactions; treatment with nocodazole caused no immediate effec t on forces. Immunofluorescence indicated that the frontal region of strong deformation contained many vinculin plaques but no apparent concentration of actin or myosin II filaments. Strong mechanical forces in the anterior r egion, generated by locally activated myosin II and transmitted through vin culin-rich structures, likely play a major role in cell locomotion and in m echanical signaling with the surrounding environment.