Myosin inhibitors block enucleation of human erythroblasts. Human CFU-E were cultured with EPO for an additional 4 days (day 11 cells) and differentiated to the level of mature erythroblasts incipient of enucleation. Mature erythroblasts were then cultured 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 enucleation of day 11 mature erythroblasts. Results are presented as the mean ± SD of 3 independent experiments. (B) Cell cycle analysis of cells cultured for 24 hours with (red lines) or without (black lines) 80μM blebbistatin and 200μM Y27632. A representative result of 3 independent experiments is shown and is presented as the mean ± SD. (C) May-Grünwald-Giemsa staining of day 11 cells cultured for 24, 48 and 72 hours with or without 80μM blebbistatin and 200μM Y27632. A representative result of 3 independent experiments is shown. Scale bar = 10 μm.