Preservation of CD34 expression and increased production of proplatelets and platelet-like elements in the presence of SR1. (A) Peripheral blood CD34⁺ cells were cultured in the absence (Ctrl) or presence of SR1 (1 µM) in a serum-free medium containing a CC220 cytokine cocktail from days 0 to 7 and with TPO (50 ng/mL) from days 7 to 14. (B) Proportion of CD34⁺ cells. The proportion of CD34⁺ cells vs total cells was determined on the indicated days by flow cytometry. Experiments were performed ≥3 times (mean ± SEM; 1-way ANOVA and a Bonferroni posttest, P > .05, not significant [n.s.]; and ***P < .001). (C) Representative DIC microscopy photographs of culture well at day 7. Zoomed images of the boxed region show extensive proplatelet development in the presence of SR1. Scale bar, 50 μm. (D) Proportion of MK extending proplatelets at day 14 (11.5 ± 4.5% in the control vs 34.6 ± 2.1% with SR1; mean ± SEM in 3 experiments; Student t test, **P < .01). (E) Amount of culture-derived platelets. The cell suspension was subjected to multiple pipetting on day 14 of culture, and platelet-like elements were detected and counted by flow cytometry (2.7 ± 0.7 × 10⁵ in the control vs 9.8 ± 2.8 × 10⁵ with SR1; mean ± SEM in 5 experiments; Student t test, *P < .05).