Hematopoietic progenitors derived from RUNX1a hESCs can be expanded through interaction with surrogate niche. (A) Morphology of colonies from bulk long-term culture of CD34⁺ core blood cells or RUNX1a H9–derived HPCs (CD34⁺CD45⁺) on M2-10B4 stromal cells at week 5. Scale bars, 200 μm. (B) Limiting dilution assay on 3- or 5-week expanded CD34⁺CD45⁺ cells from vector or RUNX1a-transduced H9 EBs. (C) (Top) Numbers of CFU generated from 40 000 CD34⁺CD45⁺ cells of vector or RUNX1a EBs derived from H9 cells after 3- to 5-week culture under LTC conditions followed by 2-week expansion in semisolid methylcellulose media. (Bottom) Representative CFU pictures at week 3. Scale bars, 100 μm. (D) Wright-Giemsa staining of RUNX1a H9–derived progenitor cells at week 5 in bulk LTC conditions and 2-week expansion in semisolid methylcellulose media. Scale bar, 100 μm.