Platelet morphology and thrombopoietin (TPO) signaling are similar, but megakaryopoiesis is increased. (A) TEM sections of representative WT (n = 4) (i) and KO (n = 3) (ii) platelets. (iii) Total platelet section area was measured and α- and δ-granule numbers counted, with granule numbers expressed per unit area of counted section. (B) Platelets at 400 × 10⁹/L were stimulated for 5 minutes with 100 ng/mL TPO under nonstirring conditions at 37°C. Membranes were probed as indicated and imaged on a LI-COR Odyssey CLx. (i) Representative blots of WT (n = 5) and KO (n = 4). Densitometric analysis of pSTAT5^Y694 (ii) and pJAK^Y1007/1008 (iii) expressed as arbitrary fluorescence units as generated by LI-COR Image Studio 5.2. Data were compared by 2-way ANOVA with Sidak’s multiple comparison test. (C) Representative images of WT (i) and KO (ii) hematoxylin and eosin–stained bone marrow imaged using a 40× objective. *Megakaryocytes (MKs). (iii) Megakaryocyte number and bone marrow area were measured using ImageJ 1.46. Megakaryocyte counts were expressed per unit area of counted section. ***P < .001. (D) Washed platelets at 20 × 10⁹/L were labeled with Pyronin Y for 15 minutes at 30°C and immediately analyzed on a BD Accuri C6 Plus. WT (N = 5), KO (N = 6). **P < .01.