Identification of a mesodermal progenitor for the pro-definitive angio-hematopoietic lineage

Mammalian hematopoietic cells arise from mesodermal progenitors in a close developmental relationship with endothelium, and along three distinct cell lineages known as primitive, pro-definitive, and definitive hematopoiesis. However, the developmental hierarchy between mesodermal progenitors, endothelium, and blood cell lineages is incompletely understood. We report here the identification in mouse gastrula and human (h)iPSC cultures of a population of CXCR4 ⁺ primitive streak stage mesodermal progenitors that give rise to yolk sac endothelium, blood islands, yolk sac hematopoiesis, and resident macrophages, corresponding to the pro-definitive lineage. Strikingy, this progenitor does not give rise to primitive erythropoiesis or to caudal endothelium and definitive hematopoiesis. Interestingly however, the pro-definitive progenitor population also gives rise to rostral endothelium that persists in adults. Finally, pro-definitive progenitor-derived endothelium is the first and main source of macrophages in embryo and hiPSC cultures, both directly and via previously described multipotent Erythro-Myeloid Progenitors. The identification and isolation of this mesodermal progenitor defines a revised pro-definitive angio-hematopoietic lineage and provides a framework for resident macrophage and endothelial differentiation relevant to disease pathophysiology and their manipulation for therapeutic purposes.

One sentence summary

Identification in gastrulating embryo and hiPSC cultures of a mesodermal progenitor for yolk sac and rostral endothelial cells, early macrophages, and Erythro-Myeloid Progenitors characterize pro-definitive hematopoiesis and provide a revised framework for early angio-hematopoietic development.

Schematic represents the progeny of angio-hematopoietic progenitors at the posterior primitive streak. Developmental timing of CXCR4 expression distinguishes i) CXCR4 ^- primitive hematopoiesis, ii) Early C X CR4 ⁺ a ngio-hematopoietic p rogenitors (EXAP) (purple) which express CXCR4 between E6.5 and E7.5 and give rise to rostral vessels and resident macrophages through YS hemogenic endothelium, and iii) late CXCR4 ⁺ intraembryonic angio-hematopoietic progenitors (blue) which acquire expression of CXCR4 around E8 and give rise to definitive hematopoiesis through an intraembryonic hemogenic endothelium. Abbreviations: E (Mouse Embryonic Day); YS (Yolk Sac); Hemog. (Hemogenic); Endo. (Endothelium); Defi. (Definitive).