Access of plasma tracers to the core region is size dependent. (A) Representative photomicrographs showing Alexa-488–labeled albumin infused 25 minutes post-injury. The image at the left shows albumin fluorescence. The brightfield image on the right is overlaid with a pseudocolor image depicting albumin intensity within the hemostatic plug. The black line in the pseudocolor image outlines the P-selectin–positive area. A time-lapse video illustrating albumin penetration into this hemostatic plug is included in supplemental Video 3. (B) The graph shows the porosity measured using fluorescent albumin as a tracer for the shell and core regions. Values are mean ± SEM for n = 8 injuries from 4 mice; statistics were performed using a 2-tailed Student t test. (C) Photomicrographs showing representative hemostatic plugs 20 minutes post-injury following infusion of either 10-kDa dextran (Ci) or 70-kDa dextran (Cii). Images are pseudocolored to depict dextran concentration within the platelet mass (the pseudocolored region depicts the entire platelet mass as defined by CD41 fluorescence; blue corresponds to low dextran, red to high dextran). Note the lack of 70-kDa dextran penetration into the core (dark blue region in Cii). (D) The graph shows platelet mass porosity 20 minutes post-injury calculated using 3 different sized dextrans (3 kDa, open bars; 10 kDa, gray bars; and 70 kDa, black bars). Data are expressed as the mean ± SEM porosity within the core and outer shell regions. N = 13 injuries from 4 mice for the 3-kDa dextran, 8 injuries from 4 mice for the 10-kDa dextran, and 8 injuries from 4 mice for the 70-kDa dextran. Statistical comparisons made using a 2-way ANOVA with Bonferroni post-hoc test. NS indicates not significant. #, indicates apparent porosity because the true porosity should be probe-insensitive and the reduced value obtained with 70-kDa dextran represents inadequate access to the core region due to a transport barrier.