Platelets are heterogenous in size when formed and do not change size with age in the circulation. Following temporal labeling, serial blood samples were obtained each day for 5 days. (A) Representative data of age-tracked platelets (blue) in an individual mouse. Tracked platelets were detectable in circulation for the full 5 days, and distribution of size was comparable with the counterstained global platelet population (red) as determined by flow cytometric FSC and side scatter (SSC) properties. (B) Decrease in ratio of geometric mean FSC of tracked platelet relative to global populations across circulating lifespan (n = 4). (C) Confocal microscopy of fixed platelets in PRP revealed no difference in area of younger and older platelets. (D) Representative image of younger (blue) and older (blue and red) present in analyzed platelet samples isolated from labeled mouse. Data are individual replicates from 4 mice with overlaid mean ± SEM with ∗P < .05 as analyzed by Friedman test with the Dunn multiple comparison test or mean ± SEM with paired t test.