Ck2β^−/−MKs exhibit abnormal ultrastructure, resulting in premature MK fragmentation in the BM and impaired proplatelet formation. (A) Representative TEM images of ck2β^fl/fl (left) and ck2β^−/− (middle and right) MKs in the BM showing increased fragmentation, decreased presence of demarcation membranes, and large granule-free zones on ck2β deficiency. Upper, overview (bar represents 3 μm). Lower, detail (bar represents 1 μm). (B) Representative confocal microscopy images of immunostained BM sections (left) and arithmetic means ± standard errors of the means (n = 6; right) of MK fragmentation in the BM of ck2β^fl/fl and ck2β^−/− mice (n = 6). Green, MKs (GPIb); red, sinusoids (CD105); gray, nuclei (4′,6-diamidino-2-phenylindole). Bar represents 20 μm. (C) Quantification of the percentage of proplatelet-forming fetal liver cell–derived MKs from ck2β^fl/fl and ck2β^−/− mice in vitro on day 4 of culture. Ck2β^fl/fl and ck2β^−/− MKs extending proplatelets were counted 18 hours after bovine serum albumin gradient and expressed as percentage of total MKs. Arithmetic means ± standard errors of the means (n = 10) are shown. (D) 2P-IVM revealing MK instability and reduced proplatelet formation in ck2β^−/− MKs in vivo. Platelets and MKs were stained with anti-GPIX antibodies (green), and the vessel lumen was labeled using fluorescein isothiocyanate-bovine serum albumin and anti-CD105 antibodies (red). Proplatelet-forming MKs (white arrows indicated proplatelets) were counted, and the ratio per mouse was assessed (n = 5). MK morphology was categorized by a blinded experimenter in normal and fragmented (blue arrows). Representative images from the BM of ck2β^fl/fl and ck2β^−/− MKs (left) and arithmetic means ± standard errors of the means (n = 5; right) of proplatelet-forming MKs and of MKs with altered morphology are shown. Bar represents 50 µm. Unpaired Student t test in panels C and D. *P < .05; **P < .01.