SRSF3 depletion leads to MK maturation arrest and macrothrombocytopenia. (A) Representative TEM images depicting the ultrastructure of control and Pf4-Srsf3^Δ/Δ MKs at stage I, II, and III of maturation. Scale bars represent 5, 10, and 10 µm, respectively. On the right, a higher magnification of the boxed areas in stage III MKs. Scale bars represent µm. (B) Quantification of control and Pf4-Srsf3^Δ/Δ MKs at each stage of maturation. (C) Representative TEM images of Pf4-Srsf3^Δ/Δ MKs at stage I, II, and III of maturation displaying emperipolesis. Scale bars represent 5, 10, and 10 µm, respectively. (D) Quantification of the fraction of Pf4-Srsf3^Δ/Δ MKs at each maturation stage displaying emperipolesis. (E) Representative TEM images of control and Pf4-Srsf3^Δ/Δ platelets. Scale bars are 1 μm. (F) Quantification of control and Pf4-Srsf3^Δ/Δ platelet area. (G) Mean platelet volume of control and Pf4-Srsf3^Δ/Δ platelets. (H) Quantification of the number of granules in control and Pf4-Srsf3^Δ/Δ platelets. (I) Quantification of the fraction of resting and activated control and Pf4-Srsf3^Δ/Δ platelets. Platelets with filopodia, rounded shape, and/or centralised granules were classified as activated. The data are presented as mean plus or minus SEM. Two-way ANOVA in panels B and D and 2-tailed unpaired Student t test in panels F through I. ****P ≤ .0001; ***P ≤ .001; **P ≤ .01; *P ≤ .05. Activ, activated; rest, resting.