FV-labeled MKs give rise to functional CD42b⁺platelets in vitro. Day 11 MKs were pulse-labeled with FV, washed, and resuspended in fresh medium. In vitro platelets were harvested 24 hours later for analysis. (A) CD42b and FV expression of CD42a⁺ platelet-sized particles harvested from nonlabeled (left) and FV-labeled (right) MKs. CD42b⁺FV⁺, CD42b⁻FV^high, and CD42b⁻ particles were gated as shown. (B) Relative sizes of platelet-sized particles in vitro compared with human donor platelets. (C) Quantification of Annexin V staining on platelet-sized particles in vitro compared with human donor platelets. (D) In vitro platelets and human donor platelets are stimulated with PAR1-activating peptide (25 μM) for 20 minutes at 37°C. Platelet activation is indicated by the increase in surface CD62P expression. (E) Graph quantifying the fold change in CD62P expression over baseline when in vitro platelet-sized particles and human donor platelets are stimulated.