Traction stresses across the cell front. (A) Immunofluorescence of paxillin (red), serine-19–phosphorylated myosin II light chain marking activated myosin II (blue), and phalloiden staining of F-actin (green). Locations of lamellipodium, lamellipodium base, and lamella are indicated; distal and proximal directions are defined. (B) GFP-paxillin (inverted contrast) with traction stress vectors superimposed (Video 2, available). (C) Heat-scale plot of traction stress magnitude; segmented FAs indicated by black outlines (Video 2). White lines delineate boundaries between lamellipodium (LP), FAs, and cell body (CB). (D) Box plots of traction stresses in the lamellipodium, FAs, cell body, and background (BG) measured directly outside the cell. Box and whisker plots in all figures indicate the 25% (lower bound), median (middle line), and 75% (upper bound) nearest observations within 1.5 times the interquartile range (whiskers), 95% confidence interval of the median (notches) and near (+) and far (0) outliers. (E) FSM image of x-rhodamine F-actin with F-actin velocity vectors superimposed (Video 1). (F) Box plots of F-actin speed in the lamellipodium (LP), FAs, and cell body (CB). (G) Histograms of the F-actin speed across the entire cell front (left) and in areas of highest (>95%) traction stresses (right). (H) Heat scale plot of directional coupling, cosine of angle θ, between F-actin velocity and traction stress vectors; segmented FAs indicated by black outlines. Bars: (A) 10 μm; (B, C, and E) 3 μm; (H) 5 μm.