SUMMARY Hematopoietic stem cells (HSCs) are produced by a small cohort of hemogenic endothelial cells (ECs) during development through the formation of intra-aortic hematopoietic cell (HC) clusters (HCs). The Runx1 transcription factor plays a key role in the EC to HC and HSC transition. We show that Runx1 expression in hemogenic ECs and the subsequent initiation of HC formation are tightly controlled by the sub-aortic mesenchyme, although the mesenchyme is not a source of HCs. Runx1 and Notch signaling are involved in this process, with Notch signaling decreasing with time in HCs. Inhibiting Notch signaling readily increases HC production in mouse and chicken embryos. In the mouse however, this increase is transient. Collectively, we show complementary roles of hemogenic ECs and mesenchymal compartments in triggering aortic hematopoiesis. The sub-aortic mesenchyme induces Runx1 expression in hemogenic-primed endothelial cells and collaborates with Notch dynamics to control aortic hematopoiesis.

The embryonic aorta produces hematopoietic stem and progenitor cells from a hemogenic endothelium localized in the aortic floor through an endothelial to hematopoietic transition. It has been long proposed that the Bone Morphogenetic Protein (BMP)/Transforming Growth Factor ß (TGFß) signaling pathway was implicated in aortic hematopoiesis but the very nature of the signal was unknown. Here, using thorough expression analysis of the BMP/TGFß signaling pathway members in the endothelial and hematopoietic compartments of the aorta at pre-hematopoietic and hematopoietic stages, we show that the TGFß pathway is preferentially balanced with a prominent role of Alk1/TgfßR2/Smad1 and 5 on both chicken and mouse species. Functional analysis using embryonic stem cells mutated for Acvrl1 revealed an enhanced propensity to produce hematopoietic cells. Collectively, we reveal that TGFß through the Alk1/TgfßR2 receptor axis is acting on endothelial cells to produce hematopoiesis.

Since the era of the ancient Egyptians and Greeks, the avian embryo has been a subject of intense interest to visualize the first steps of development. It has served as a pioneer model to scrutinize the question of hematopoietic development from the beginning of the 20th century. It's large size and easy accessibility have permitted the development of techniques dedicated to following the origins and fates of different cell populations. Here, we shall review how the avian model has brought major contributions to our understanding of the development of the hematopoietic system in the past four decades and how these discoveries have influenced our knowledge of mammalian hematopoietic development. The discovery of an intra-embryonic source of hematopoietic cells and the developmental link between endothelial cells and hematopoietic cells will be presented. We shall then point to the pivotal role of the somite in the construction of the aorta and hematopoietic production and demonstrate how two somitic compartments cooperate to construct the definitive aorta. We shall finish by showing how fatemapping experiments have allowed the identification of the tissue which gives rise to the subaortic mesenchyme. Taken together, this review aims to give an overview of how and to what extent the avian embryo has contributed to our knowledge of developmental hematopoiesis.