Fig. 1. Typical stress map under a migrating Dictyostelium cell. (A) Wild-type Dictyostelium cells (strain AX2) or myosin-II-null cells (strain HS1) transformed with GFP-myosin-II, E476K mutant myosin II or ABD120k constructs were placed on elastic silicone or gelatin substrata embedded with orange- or red-fluorescent marker beads (200-nm or 20-nm diameter). The fluorescence of GFP and the marker beads was imaged simultaneously under TIRF or confocal microscopy. Migrating cells cause strains in the elastic substratum and displacements of the beads in the substratum (arrows). (B) A typical image of 200 nm marker beads under confocal microscopy. The outline of a migrating cell is superimposed as a white line. Displacements of 300 marker beads under and surrounding a migrating cell were measured. (C) The coordinates of each bead and their displacements were transformed to those of each node of a triangle mesh. The length of the sides of each triangle is 400 nm (=8 pixels). The stresses in the surface of the substratum were calculated using original software based on the triangle finite element method (see Materials and Methods). (D) Stress map for a migrating wild-type cell on a gelatin substratum. The distributions of stresses in the substratum were visualized by pseudocolor. The direction of the strain in the substratum at each small white circle is indicated by a white bar. The length of the white bars is three times as long as the strain. The yellow allow indicates the direction of cell migration. Large rearward traction stresses emerged as `stress spots', indicated by the two white arrows. Forward stresses emerged at the posterior edge, as indicated by the white arrowhead. Numerical values in the kPa scale are indicated near each arrow and arrowhead. Bars, 4 µm (B-D).