Myosin inhibitors block cell division of human CFU-E. Human CFU-E generated from purified CD34⁺ cells were cultured for the indicated periods in the presence of EPO with or without various concentrations of inhibitors for non-muscle myosin II ATPase (blebbistatin) and a myosin activator Rho kinase (Y27632). (A) Effects of inhibitors and vehicle on the proliferation of CFU-E. Results are presented as the mean ± SD of 3 independent experiments. (B) Cell cycle analysis of cells cultured for 24 hours with (red areas) or without (solid lines) 80μM blebbistatin and 200μM Y27632. A representative result of 3 independent experiments is shown and is presented as the mean ± SD. (C) Size distribution analysis of CFU-E before (0 hours) and after 24 hours culture (red) with 80μM blebbistatin (green, middle panel) and 200μM Y27632 (green, bottom panel). A representative result of 3 independent experiments is shown. (D) May-Grünwald-Giemsa staining of cells cultured for 24 and 48 hours with or without 80μM blebbistatin and 200μM Y27632. A representative result of 3 independent experiments is shown. Scale bar = 10 μm.