Abnormal proplatelet formation in cultured bone marrow–derived MYH9Δ megakaryocytes. WT (A,D) and MYH9Δ (B,E) megakaryocytes after 4 days of culture as observed by scanning electron microscopy (SEM) (A,B) and DIC microscopy (D,E). (C) Quantification by DIC microscopy of the number of megakaryocytes (MKs) extending proplatelets after 4 days of culture. The proportion of megakaryocytes forming proplatelets is increased in MYH9Δ cells. Data are expressed as the percentage of cells extending proplatelets and are the mean of 3 independent experiments. **P < .01 using the Student t test. (F) Measurement of the proplatelet buds in WT and MYH9Δ megakaryocytes forming proplatelets from 3 independent cultures. The size of the proplatelet buds is significantly increased in MYH9Δ cells. ***P < .001, unpaired Student t test.