Diagram of a traction force assay
We use traction force assays to detect the pattern of forces that moving cells exert on their substrata. These consist of an elastic sheet, in which microscopic (1 um in diameter) fluorescent marker beads are embedded. Using dual fluorescence microscopy we collect a seqence of paired images showing marker bead displacements (red) and the change in cell position (green) during movement. This information is used to calculate the distribution of traction stresses beneath the cell and is represented as traction vector maps. These reveal the size, direction and distribution of traction forces that are used to drive cell movement. In the example shown here (see movie) a fish epithelial keratocyte is crawling on a thin (~ 30 um in thickness) film of Knox Jello gelatin (unflavored).