Selective cleave of fibrinopeptides. (A) Fibrin polymerization occurs through the noncovalent binding of exposed fibrin knobs A and B to fibrin holes a and b on adjacent proteins. These mechanisms have not been explored in neonates. (B) Snake venom thrombin-like enzymes were used to selectively cleave fibrinopeptides and activate fibrin. (C) Representative images from confocal microscopy at 63× magnification of adult and neonatal fibrin clots at equal enzyme concentrations reveals differences in fiber density (D) and fiber alignment (E). Scale, 10 μm. Fiber density, N = 5 per group; fiber alignment, N = 4 to 5 per group. Average ± standard deviation is shown. *P < .05, **P < .01, ***P < .001.