Differential involvement of myosin IIA in MC-mediated MK culture. (A) Myosin IIA immunolabeling (red) along with GPIbβ-positive membranes (green) and nucleus (DAPI, white). (B) Left, confocal microscopy images of Myh9^−/− MKs in liquid or 2% MC cultures. Right, bar graphs represent the proportions of Myh9^−/− cells in the different MK classes in liquid and 2% MC cultures, as compared with WT cells. Results are expressed as the mean ± SEM and are from 3 independent experiments, with a total of 86-93 MKs examined per condition. (C) Upper panel, electron microscopy images of Myh9^−/− MKs in 2% MC cultures (left), as compared with in situ MKs (right). Images are representative of at least 6 different cultures and 4 different Myh9^−/− BMs. Lower panel, bar graphs of the circularity of WT and Myh9^−/− MKs in situ and in liquid and 2% MC media. Results are the mean ± SEM in 3 independent experiments, with a total of 37-59 cells examined per condition. ***P < .0001 using an ANOVA analysis and a Newman-Keuls posttest; ns, not significant. (D) Bar graphs represent the percentages of MKs bearing proplatelets in Myh9^−/− liquid and 2% MC cultures, as compared with WT cells. Results are the mean ± SEM in 3 independent experiments, with a total of 428-557 MKs examined per condition. *P < .05 using an ANOVA analysis and a Newman-Keuls posttest. KO, Myh9 knockout.