TGF-β signaling is transiently upregulated through the transition from early to late BFU-Es. (A) Expression of 3 types of TGF receptors in purified TβRIII^10%hi and TβRIII^10%lo BFU-E, CFU-E, and Ter119⁺ cells from mouse fetal livers. (B) Colony-forming assays were conducted to determine large BFU-E colony numbers from 100 TβRIII^10%lo BFU-E cells cultured under the indicated conditions. Colony-forming assays were performed at indicated time points. Only large BFU-E colonies with >12 clusters were counted (**P < .01; ***P < .001). (C) Colony-forming assays of BFU-E, CFU-E, and CFU–granulocyte, erythrocyte, monocyte, megakaryocyte in indicated FACS-isolated populations from human CD34⁺ cord blood cells, based on TβRIII expression as described in “Materials and methods” (*P < .05.). (D) Dynamics of TβRIII expression on cell surface as determined by flow cytometry. Human TβRIII^10%lo BFU-E cells were isolated and cultured in medium supplemented with stem cell factor, Epo, interleukin 3, interleukin 6, and DEX during the first 8 days before switching to differentiation medium. (E) Purified human TβRIII^10%hi or TβRIII^10%lo CD34⁺ cells were cultured in the presence or absence of 100 nM DEX or 100 nM galunisertib. Galunisertib treatment of significantly increased erythroblast production compared with the TβRIII^10%lo CD34⁺-untreated group. The production of erythroblasts was quantified over time. Error bars represent mean ± standard deviation from 3 biological replicates (*P < .05).