Competition assay to reveal polymerization mechanism. High concentrations of fibrin knob A and B mimetic peptides were added to neonatal and adult fibrinogen solutions before initiation of clotting with thrombin. (A) Representative images are shown from confocal microscopy imaging of clots at 63× magnification. (B) Fiber density analysis reveals significant differences between adult and neonatal clots after equivalent peptide incubation times. N = 4 per group. Scale, 10 μm. Mean fiber density ± standard deviation is reported. *P < 0.05, **P < .01, ***P < .001.