Fig. 1.

Measurement of contractile forces in a fibroblast cell on a bed of microneedles. The actin fibers are stained in green. The arrows show the deflection of the posts, with the lengths of the arrows proportional to the force exerted by the cell on the posts. There seems little correlation between the orientations of the visible stress fibers and the directions of the force vectors (figure courtesy of C. Chen, University of Pennsylvania, Philadelphia, PA).