Megakaryocyte-specific loss of Bcl-x and 1 allele of Mcl-1 predisposes megakaryocytes to cell death. (A) Peripheral blood counts from 7-week-old mice. Data represent mean ± SD, 1-way ANOVA with Bonferroni multiple comparison test. MPV indicates mean platelet volume. Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/Pf4Δ mice were firstly compared with Bcl-x^fl/fl Mcl-1^+/+ controls, and secondly to Bcl-x–deficient Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/+ mice. #Morphologically recognizable megakaryocytes were enumerated in H&E-stained sternum sections from: n = 3, Bcl-x^fl/fl Mcl-1^+/+; n = 8, Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/+; n = 4, Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/Pf4Δ mice. Data represent mean number per field of view (×200) from 10 fields per individual mouse. (B) H&E-stained BM sections. Megakaryocytes with abnormal morphology are indicated by dark blue arrows. Normal megakaryocytes are indicated by white arrows. Scale bars: 20 μm. (C) Caspase-3–positive atypical megakaryocytes present in Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/Pf4Δ spleen. Cleaved capase-3 in brown and hematoxylin in blue. Blue arrows point to apoptotic megakaryocytes positive for active caspase-3. Note the dark pyknotic nuclei. White arrows indicate healthy megakaryocytes, negative for caspase-3. Scale bars: 20 μm. (D) Representative transmission electron microscope images of BM sections demonstrating the presence of apoptotic megakaryocytes in Bcl-x^Pf4Δ/Pf4ΔMcl-1^+/Pf4Δ mice. Right panels show high-magnification ultrastructure of the corresponding megakaryocytes shown in the left panels. Scale bars: 2 (white) or 10 (black) μm.