M1 protein affects fibrin clot structure. Porosity (Darcy’s constant, Ks (×10⁻⁹ cm²) of (A) purified and (B) plasma fibrin clots formed with thrombin in the presence of rM1 (0-60 μg/mL); n ≥ 3. Fibrin crosslinking by FXIIIa was investigated using (D) densitometry analysis of alpha-polymer chains from (C) SDS-PAGE analysis (n = 3) and (E) D-dimer quantification of lysed fibrin clots in the presence of rM1 (0-60 μg/mL). An FXIIIa inhibitor, T101, was included to mimic no fibrin clot crosslinking by FXIIIa. FXIIIa activity in the presence of rM1 (0-60 μg/mL) was calculated using an (F) AbCam Factor FXIIIa activity kit (N = 5). Fibrin clot architecture was visualized using (G) SEM in the presence of rM1 (0, 3.75, and 60 μg/mL). Fibrin clots, incorporating rM1 (0, 3.75, or 60 μg/mL), were formed in perforated Eppendorf lids and then fixed, dehydrated, critically point dried, and coated with 10-nm iridium. Clots were imaged at a minimum of 3 areas using Hitachi SU8230 (field emission scanning electron microscope) SEM. Arrows indicate the formation of irregular fiber bundles and compacted fibrin. Scale bar: 10 μm. n = 3. Error bars represent standard error of the mean. (∗P < .05, ∗∗P < .01, ∗∗∗P < .001, ∗∗∗∗P < .0001, 1-way ANOVA with Dunnett post hoc test).