Notch ligand Dll4 impairs the recruitment of hemogenic cells into intra-aortic clusters and limits hematopoietic stem cell production.

Hematopoietic stem cells (HSCs) develop from the hemogenic endothelium in the embryonic aorta. In most vertebrates, hemogenic cells protrude into the aortic lumen forming clusters where the hematopoietic features are acquired. Although much is known about the molecular characteristics of the hematopoietic cells, we still lack basic understanding on the origin and 3D-organization of intraortic hematopoietic clusters (IAHC) in the mouse embryo and how they evolve over time. Here we use advanced live imaging techniques of organotypic slice cultures, clonal analysis, and mathematical modelling to show that IAHC formation is a two-step process. First, a hemogenic progenitor buds up from the endothelium and undergoes division forming the monoclonal core of the cluster. Next, cells from the surrounding hemogenic endothelium are recruited into the IAHCs, increasing their size and heterogeneity. We found that the recruitment phase of IAHC formation is negatively regulated by Notch signalling via the Delta-like-4 (Dll4) Notch ligand. We show that blocking of Dll4 promotes the entrance of new hemogenic Gfi1+ cells into the IAHC and increases the number of cells that acquire HSC activity. Mathematical modelling based on our data provides estimation of the cluster lifetime and the average recruitment time of hemogenic cells to the cluster under physiologic and Dll4-inhibited conditions. Single cell transcriptomic analysis indicates that cell identities are preserved after Dll4 blockage. Our data provides for the first time a detailed characterization of the aortic hematopoietic stem cell niche and its maturation dynamics, identifying the Notch ligand Dll4 as a major molecular player governing dynamics of cluster formation and HSCs production.