FXIII is not required for platelet-mediated clot contraction. (A-D) PRP from F13a^+/+, F13a^+/−, and F13a^−/− mice was clotted with TF/CaCl₂ at 400 (A), 200 (B), 50 (C), and 10 × 10⁹ platelets/L (D). Clot area over time was assessed as a percentage of initial clot area. Dots show mean ± SEM (n = 4); curves were fit to a nonlinear regression with a 1-phase decay equation. (E) Photographs of contracted clots at 24 hours. (F) Transglutaminase activity was measured in platelets from F13a^+/+ and F13a^−/− C57BL/6J mice and F13a^−/− 129Ola/CBACa mice. Data show mean ± SEM (n = 3-5).