Fig. 1. Dual calcium and traction force imaging in a keratocyte moving on a gelatin substratum. (A) Fluorescence image of a fish keratocyte (white outline) loaded with the calcium indicator, Calcium Green-1-dextran moving in the direction indicated (open arrow). (B) Image of fluorescent beads (0.19-0.20 µm diameter) in their displaced positions within a 3.0% gelatin substratum, upon which the keratocyte in A is moving. (C) Image of fluorescent beads in their undisplaced positions (red) superimposed on panel B (blue beads) and the cell outline in panel A. Comparison of red and blue bead images represents the size and direction of bead displacements caused by the cell in its current position. Beads that do not undergo displacement appear white. The largest bead displacements occur perpendicular to the lateral cell edges (large open arrows) while smaller displacements can be seen within the leading lamella in the direction indicated (small open arrows).