Structural bases of the interactions of thrombin-stimulated iMKs with a fibrin network during contraction of a plasma clot. (A) Representative confocal microscopy images of calcein-labeled iMKs (green) and Alexa fluor–labeled fibrin network (red) before (left) and at 15 minutes (right) of clot contraction. The images are projections of 26-μm z-stacks. Scale bar, 20 μm. See also supplemental Video 2. (B) A representative confocal microscopy image of an individual thrombin-stimulated iMK stained for F-actin within a clot fixed immediately after formation. Arrows show filopodia and blebs on the surface of the iMK. (C, upper panel) A thrombin-stimulated individual iMK embedded into the fibrin network fixed at the 0 point (left) and at 15 minutes (right) of clot contraction. (C, bottom panel) Deformation, radial orientation of fibrin fibers, and compaction of fibrin are caused by the contracting iMK shown on the upper panel (the red fluorescence channel only). Scale bars, 20 μm. (D) Temporal increase of the amount of fibrin accumulated on individual iMKs over the course of contraction, measured as a relative amount of iMK-colocalized fibrin in confocal microscopy z-stacks of clots (n = 20; mean ± 95% CI). (E) A representative diagram showing orientation of fibrin fibers in the selected field in C (dashed rectangle) in the vicinity of a contracting iMK. CI, confidence interval.