Community Research and Development Information Service - CORDIS


HSCORIGIN Report Summary

Project ID: 309361
Funded under: FP7-IDEAS-ERC
Country: Netherlands

Final Report Summary - HSCORIGIN (From mesoderm to hematopoietic stem cell commitment: cellular and molecular events occuring during mouse embryonic development.)

Hematopoietic Stem Cells (HSCs) are responsible for all blood cell production throughout the life of an individual. HSCs are also the key cell type capable to replace the defective hematopoietic system of patients with blood related diseases (e.g. anemia, some cancers). The increasing need of HSCs for transplantation procedures and the difficulties to find compatible donors are major issues in the clinic. A solution would be to generate large quantities of fully competent HSCs in vitro (e.g. by reprogramming of pluripotent stem cells). Despite decades of research, it remains extremely difficult to achieve to date because not all steps leading to HSC generation in vivo have been elucidated yet. Without this knowledge, the in vitro production of HSCs will remain challenging.
Adult HSCs are initially generated during embryonic development. They are first detected in the main arteries of vertebrate embryos, such as the aorta. However, no definitive conclusion could be drawn yet on the intra- and/or extra-embryonic origin of the HSC potential. To answer this fundamental question (objective 1 of the project), we used the chicken animal model. Human and chicken embryos share features that position the chicken as a reliable and accessible alternative model to study developmental hematopoiesis. We first established a complete cartography and quantification of hematopoietic (stem) cell emergence in the aorta during chicken embryonic development. We then demonstrated the existence of bona fide HSCs, originating exclusively from the aorta, by using an in vivo chorio-allantoic membrane transplantation assay. Our study has thus confirmed the intra-embryonic origin of chicken HSCs and has shed new light on the avian model as a valuable system to study HSC production and regulation in vivo.
Clusters of cells attached to the wall of the main embryonic arteries were first observed a century ago in the chicken embryo and since in almost all vertebrate species that have been looked at. Such clusters derive from specialized endothelial cells named hemogenic endothelial cells, which are incorporated in the vessel walls. We and others have shown that the first HSCs reside in these clusters. For the Objective 2 of the project, we determined the exact cell composition and function of the clusters. We demonstrated that beside very few HSCs and committed progenitors, the clusters mainly contain a continuum of HSC precursors (pre-HSCs) that mature into HSCs in vivo (as shown by their capacity to long-term multilineage reconstitute primary neonates and secondary adult recipients upon transplantations).
Clusters are located all around the aorta in the mouse embryo. Transcriptome comparison of clusters, according to their location inside the aorta (ventral versus dorsal side) at different time points in development, was impossible thus far. The challenge was to access the clusters in situ and to detach them from the wall of the aorta. For the objective 3 of the project, we developed a mechanical picking technique allowing the isolation of single and pure clusters inside the aorta of mouse embryos. Molecular analysis at the RNA level revealed that the clusters have very similar transcriptomes, independently of their position inside the aorta. Going beyond the initial objectives of the project, we also identified the genes and transcription factor networks activated during endothelial specification (to acquire a hemogenic potential) and cluster formation by performing single-cell RNA-sequencing.
Overall, our project provides a better cellular and molecular understanding of HSC formation as it occurs physiologically in the developing embryo. It will pave the way for improving HSC production in vitro to address the growing need for tailor-made HSCs to treat patients with blood-related disorders.

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