Activation of latent TGF-β is required for TGF-β bioactivity on HSCs. (A) mRNA expression of TGF-β family member genes was analyzed for total embryo at E12.5, freshly isolated BM CD34⁻KSL HSCs (CD34⁻), and CD34⁺KSL hematopoietic progenitor cells (CD34⁺). (B) Freshly isolated CD34⁻KSL HSCs (CD34⁻) were sorted clonally into 96-well microtiter plates and incubated in the presence of SCF and TPO along with either TGF-β1 or latent TGF-β1, and were allowed to form colonies. After 14 subsequent days of culture, the numbers of colonies were counted and the percentages of starting HSCs that gave rise to colonies were presented.