Proplatelet formation is promoted by nuclear accumulation of MKL1. (A) Confocal microscopy images of WT MKs after 3 days of culture in a liquid or 2% MC medium. The DMS and plasma membrane are visualized in green by immunolabeling with anti-GPIbβ; MKL1 immunolabeling is depicted in red, and nuclei appear in white (DAPI staining). The dotted lines delineate cell and nuclear margins. Images are representative of 3 independent experiments. (B) Bar graphs represent the MKL1 fluorescence in the cytoplasm and nucleus and the nuclear/cytoplasmic ratio, in MKs in liquid and 2% MC cultures. Results are the mean ± SEM in 3 cultures, with a total of 189-192 cells analyzed per condition. ***P < .0001 using Student t test. (C, D) Class distribution and proplatelet formation in MKs grown in a liquid or 2% MC medium, in the presence or absence of 10 µM CCG-1423. Results are the mean ± SEM in 3 independent cultures, with a total of 90-94 cells examined per condition (C), or in 4 independent cultures, with a total of 510-800 cells examined per condition (D). *P < .05, **P < .01 using ANOVA analysis and a Newman-Keuls posttest.