The generation of in vivo repopulating hematopoietic cells from in vitro differentiating embryonic stem cells has remained a long-standing challenge. To date, hematopoietic engraftment has mostly been achieved through the enforced expression of ectopic transcription factors. Here, we describe serum-free culture conditions that allow the generation of in vivo repopulating hematopoietic cells in the absence of ectopically expressed factors. We show that repopulating activity arises immediately upon the commitment of mesodermal precursors to the blood program, within the first wave of hematopoietic specification. We establish that the formation of these progenitors is extremely transient and exquisitely sensitive to the cytokine milieu. Our findings define the precise differentiating stage at which hematopoietic repopulating activity first appears in vitro, and suggest that during embryonic stem cell differentiation, all hematopoietic programs are unraveled simultaneously from the mesoderm in the absence of cues that restrict the coordinated emergence of each lineage as is normally observed during embryogenesis.

从体外分化的胚胎干细胞产生体内再生造血细胞仍然是一个长期存在的挑战。迄今为止，造血植入主要是通过强制表达异位转录因子来实现的。在这里，我们描述了允许在没有异位表达因子的情况下产生体内再生造血细胞的无血清培养条件。我们表明，在造血规范的第一波中，中胚层前体对血液程序的承诺立即产生再增殖活动。我们确定这些祖细胞的形成是极其短暂的，并且对细胞因子环境非常敏感。