Spreading defects in Jak2^Plt−/− platelets.Jak2^Plt+/+ and Jak2^Plt−/− platelets were left to adhere to 50 µg/mL type 1 collagen or to 100 µg/mL fibrinogen and activated with 1 µg/mL CRP or 0.01 U/mL thrombin glass coverslips for 30 minutes at 37°C as indicated. Fixed platelets were stained for β-tubulin and F-actin (phalloidin) and analyzed by structured illumination microscopy. (A) Representative immunofluorescent micrographs of fixed platelets. Bars represent 3 µm (activated) and 7 µm (resting). (B-D) Morphologic quantification of platelet spreading as determined by resting, extension of defined filopodia, or full spreading in response to collagen (total platelets scored, 549 Jak2^Plt+/+ and 311 Jak2^Plt−/− in 4 independent experiments) (B); CRP (total platelets scored, 888 Jak2^Plt+/+ and 1017 Jak2^Plt−/− in 5 independent experiments) (C); or thrombin (total platelets scored, 932 Jak2^Plt+/+ and 796 Jak2^Plt−/− in 5 independent experiments) (D). Results represent the mean ± SD and were compared by 2-way ANOVA. *P < .05; **P < .01; ***P < .001. (E-G) Violin plots of platelet surface area in response to collagen (total platelets scored, 684 Jak2^Plt+/+ and 374 Jak2^Plt−/− in 4 independent experiments) (E); CRP (total platelets scored, 918 Jak2^Plt+/+ and 1174 Jak2^Plt−/− in 5 independent experiments) (F); or thrombin (total platelets scored, 958 Jak2^Plt+/+ and 842 Jak2^Plt−/− in 5 independent experiments) (G). Results were compared by 2-way ANOVA. *P < .05; ***P < .001.