Scanning electron microscopy of α251 and control fibrin. Thrombin at a final concentration of 0.9 IU/mL was added to recombinant control or Aα251 fibrinogen at a 4.4-μM final concentration in 20 mM HEPES, pH 7.4, 0.15 M NaCl, 5 mM CaCl₂. (A) Fibrin made from control fibrinogen. (B) Fibrin made from control fibrinogen in the presence of 1 IU/mL factor XIIIa. (C) Fibrin clot made from Aα251 fibrinogen. (D) Fibrin clot made from Aα251 fibrinogen in the presence of 1 IU/mL factor XIIIa. Magnification bar equals 1000 nm.