SR1-derived CD34⁺CD41^low population has an increased proplatelet and platelet-producing capacity. (A) Evolution of CD34 and CD41 expression. Representative flow density plots of CD34 and CD41 expression at day 10. CD34⁺ cells were cultured as in Figure 4A, and analyses were performed on day 10. (B) Flow sorting and culture protocol of CD34⁺CD41^low cells. CD34⁺ cells cultured for 10 days in the presence of SR1 were sorted according to their CD34⁺CD41^low phenotype (G1) using a FACS Aria II flow cytometer and then cultured for 7 days in a medium containing TPO with or without SR1. Representative DIC microscopy photographs of culture wells at day 7 showing extensive proplatelet development in the presence of SR1. Scale bar, 50 μm. (C) Proportion of MK extending proplatelets at day 7 (10.0 ± 6.6% for the control vs 91.0 ± 2.4% with SR1; mean ± SEM in 5 experiments; Student t test, ***P < .001). (D) Amount of culture-derived platelets at day 7 (1.3 ± 0.2 × 10⁵ platelets per well in the control vs 8.3 ± 0.9 × 10⁵ with SR1; mean ± SEM in 3-4 experiments; Student t test, ***P < .001).