Inhibition of actomyosin-driven contractility inhibits fibrin clot retraction in vivo. Fibrin-dependent clot retraction was visualized over 120 minutes in vivo, as described in “Methods.” (A) Confocal maximum-intensity projection of platelets (Dylight 649 anti-mouse GP1ba antibody; red) and fibrin (Alexa Fluor 546 anti-fibrin antibody; white) within the same thrombus after 20 minutes (left column) or 120 minutes (middle column) in the absence/presence of the myosin II inhibitor blebbistatin. The right column depicts the size of the fibrin network after 20 and 120 minutes. The direction of blood flow (arrow) and scale bars (20 μm) are shown. (B) Quantification of the decrease in fibrin network size (red bars; left y-axis) and increase in fibrin amount (blue bars; right y-axis) over 120 minutes of thrombus maturation was performed, and data expressed as a percentage change from the fibrin size/amount at 5 minutes. Results depict the mean ± SEM (DMSO, n = 9; blebbistatin, n = 7); **P < .01 by unpaired 2-tailed Student t test.