Murine hematopoietic stem cell activity is derived from pre-circulation embryos but not yolk sacs.

The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is well supported, the yolk sac (YS) contribution to adult hematopoiesis remains controversial. The same developmental origin makes it difficult to identify specific markers that discern between an intraembryonic versus YS-origin using a lineage trace approach. Additionally, the highly migratory nature of blood cells and the inability of pre-circulatory embryonic cells (i.e., 5-7 somite pairs (sp)) to robustly engraft in transplantation, even after culture, has precluded scientists from properly answering these questions. Here we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2-7sp murine embryonic explants (Em-Ex). dHSC are undetectable in 2-7sp YS explants. Additionally, the engraftment from Em-Ex is confined to an emerging CD31+CD45+c-Kit+CD41- population. In sum, our work supports a model in which the embryo, not the YS, is the major source of lifelong definitive hematopoiesis

Multispecies RNA tomography reveals regulators of hematopoietic stem cell birth in the embryonic aorta

The defined location of a stem cell within a niche regulates its fate, behavior and molecular identity via a complex extrinsic regulation far from being fully elucidated yet. To explore the molecular characteristics and key components of the aortic microenvironment, where the first hematopoietic stem cells (HSCs) are generated during development, we performed genome-wide RNA tomography sequencing on zebrafish, chicken, mouse and human embryos. The resulting anterior-posterior and dorsal-ventral transcriptional maps provided a powerful resource for exploring genes and regulatory pathways active in the aortic microenvironment. By performing inter-species comparative RNA-seq analyses and functional assays, we explored the complexity of the aortic microenvironment landscape and the fine-tuning of various factors interplaying to control HSC generation both in time and space in vivo, including the ligand-receptor couple ADM-RAMP2 and SVEP1. Understanding the regulatory function of the local environment will pave the way for improved stem cell production in vitro and clinical cell therapy

In vivo generation of haematopoietic stem/progenitor cells from bone marrow-derived haemogenic endothelium

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