Apoptosis of Mcl-1–deficient megakaryocytes in vivo. (A) H&E-stained sternum sections from ABT-737–treated Mcl-1^+/+ and Mcl-1^Pf4Δ/Pf4Δ mice, 2.5 hours after treatment. Pyknotic nuclei are indicated by white arrows. Scale bar: 20 μm. (Bii-vi) Representative transmission electron microscope images of BM sections showing aberrant megakaryocyte morphology and death in Mcl-1^Pf4Δ/Pf4Δ mice 2.5 hours after ABT-737 treatment. White arrows highlight abnormal characteristics in Mcl-1^Pf4Δ/Pf4Δ megakaryocytes not observed in wild-type controls (i). (i-iii) Examples of megakaryocytes displaying cytoplasmic vacuolization. (iv) Dysmorphic megakaryocyte nucleus. (v) Accumulation of short filamentous bundles within megakaryocyte. (vi) Megakaryocyte displaying fragmentation of plasma membrane. Black scale bars: 5 μm; white scale bar: 2 μm; blue scale bar: 1 μm.