Younger platelets disproportionately contribute to thrombotic response. (A) Population tracking in anticoagulated whole blood after stimulation with a range of concentrations (30-100 μM) of PAR4-specific stimulation to drive ex vivo aggregation resulted in decreasing proportions of younger platelets and corresponding increased proportions of older platelets remaining in the nonaggregated single-platelet portion (n = 4). Data are mean ± SEM or individual replicates. (B) Circulating proportions of labeled younger platelets in vivo decreased following PE challenge with corresponding increase in proportions of circulating older platelets. ∗P < .05, ∗∗P < .01 by 2-way analysis of variance with the Dunnett multiple comparison to their respective basal level or paired t test as appropriate.