Fibrin network density promotes RBC retention in clots, but FXIIIa mediates RBC retention independent of fibrin network density. (A-B) Recalcified (10 mM, final) plasma spiked with Alexa Fluor 647-labeled fibrinogen was clotted with the indicated TF concentrations in the (A) absence or (B) presence of T101. Images are representative confocal micrographs (z-projections of 30 individual slices) of clots visualized on a Zeiss LSM710 laser scanning confocal microscope with a 63× oil-immersion lens (Carl Zeiss). Scale bar, 30 μm. (C) Serum RBC content and (D) clot weight following clot contraction. Each dot represents an individual clot. Lines connect clots formed from the same blood donor. Horizontal dark lines indicate medians. (E-F) Representative scanning electron micrographs of clots formed in recalcified (10 mM, final) whole blood with the indicated TF concentrations in the (E) absence and (F) presence of T101 (10 μM, final). Clots were visualized at 5010× on a Zeiss Supra 25 Field Emission Scanning Electron Microscope (Carl Zeiss). Scale bar, 10 μm. Micrographs were used to measure fibrin network density (supplemental Methods), which was compared with the (G,I) serum RBC content or (H,J) clot weight following clot contraction in the (G-H) absence and (I-J) presence of T101.