Thrombin effects on fibrin clot network and fibrin fiber size. Transmission electron microscopy images under dried conditions of fibrin clots made with human plasminogen-depleted IF-1 purified fibrinogen (1 mg/mL), CaCl₂ (2.5 mM), and thrombin at concentrations of (A) 0.1, (B) 1.0, and (C) 10 U/mL. Scale bars, 500 nm. (D) Fibrin fiber radius obtained by measurement of n = 50 fibrin fibers at 0.1, 1.0, and 10 U/mL thrombin. The individually plotted data represent the dispersion of the fiber size within the clot, and the bar represents the mean value. Statistical significance, using a 1-way analysis of variance (ANOVA), is denoted with ****P < .001 for comparison between 0.1 U/mL and the remaining thrombin concentrations.