FXIII_plasma, but not FXIII_plt, mediates clot weight. (A-B) Representative western blot (A) and quantification (B) for FXIII-A in plasma and platelets from F13a^+/+, F13a^+/−, and F13a^−/− mice (n = 3). (C) RBCs were reconstituted with C57BL6/J FXIII-sufficient (wild-type) and deficient (F13a^−/−) plasma and platelets, and clotting was triggered with TF/CaCl₂ (n = 9) or thrombin/collagen/CaCl₂ (n = 6). Data show mean ± SEM. (D) Plasma and platelets from C57BL6/J FXIII-sufficient (wild-type) and deficient (F13a^−/−) mice were recombined to make PRP sufficient or deficient in plasma or platelet FXIII. Clotting was triggered with TF/CaCl₂, and reactions were quenched at the indicated time points and analyzed by SDS-PAGE with western blotting (n = 2). Fibrin crosslinking was detected using anti-fibrin(ogen) antibody, and identity of bands was confirmed by mass spectrometry.