The effect of MEK inhibition on megakaryocyte endomitosis in CD41-selected cell culture: Analysis of polyploid cells. Murine bone marrow cells were harvested and CD41⁺ cells were selected using a high gradient magnetic system (MiDiMACS). Purified cells were grown in serum-free media containing 35 ng/mL murine TPO for 84 hours. In 1 culture, PD 98059 at final concentration of 25 μmol/L was added initially and a similar dose was added 24 hours later (B). An equal volume of DMSO was added in the control culture. Cells were then stained with FITC-conjugated anti-CD41, solubilized, and stained with PI before being analyzed using flow cytometry. Only viable cells, defined by forward and side light scattering characteristics, were analyzed for fluorescent intensity. After culture, there was an 80% reduction in the total number of CD41⁺ cells in the presence of the inhibitor compared with control. However, equal number of cells were counted in each group. The histograms show the ploidy distribution of cells with DNA contents of 4N or higher. The consecutive peaks represent the frequencies of cells with DNA contents, as indicated by bars. These data are the representative results from 3 separate experiments.