Functionality of in vivo–released platelets. (A-B) Studies with whole murine blood isolated 4 hours after human megakaryocytes or hdPlts were infused. Mean ± 1 SEM of (A) PAC-1 binding as measured by MFI and (B) CD62P surface expression MFI pre- and post-high dose (10 U/mL) thrombin activation. N = 3 independent studies for hdPlt infusion, and N = ≥6 independent studies for infused megakaryocytes arms. *P < .05 compared with hdPlts by 1-way ANOVA analysis. (C) Representative confocal images of laser-induced cremaster arteriole thrombi formed in mice infused with either hdPlts or the various treated in vitro–grown megakaryocytes. Mouse platelets are labeled with CD41–Alexa 647 (dark blue). Human platelets derived from infused donor-derived platelets or megakaryocytes are calcein-AM labeled (green or cyan blue when overlapped with mouse platelets). Original magnification ×20. (D) Quantification of the incorporation into the mice clot standardized per the percentage of human platelets in the mice circulation and thrombus size after infused hdPlts or megakaryocytes. Individual study results are shown with N ≥ 16 independent studies per arm. Mean value is shown as a single horizontal line in each arm.