Abstract
Rare hematopoietic stem cells (HSCs) can self-renew, establish the entire blood system and represent the basis of regenerative medicine applied to hematological disorders. Clinical use of HSCs is however limited by their inefficient expansion ex vivo, creating a need to further understand HSC expansion in vivo. After embryonic HSCs are born from the hemogenic endothelium, they migrate to the embryonic/fetal niche, where the future adult HSC pool is established by considerable expansion. This takes place at different anatomical sites and is controlled by numerous signals. HSCs then migrate to their adult niche, where they are maintained throughout adulthood. Exactly how HSC expansion is controlled during embryogenesis remains to be characterized and is an important step to improve the therapeutic use of HSCs. We will review the current knowledge of HSC expansion in the different fetal niches across several model organisms and highlight possible clinical applications.
Highlights
Hematopoiesis is a highly conserved process across many organisms that culminates with the emergence of hematopoietic stem cells (HSCs)
The first wave is characterized by the transient erythro-myeloid precursors (EMPs) that arise in the yolk sac in mice and humans (Bertrand et al, 2005; McGrath et al, 2015), the posterior blood island in zebrafish (Bertrand et al, 2007) and the posterior-lateral ventral blood island in xenopus (Ciau-Uitz et al, 2014)
The second wave consists of HSC specification from the aortic hemogenic endothelium by the highly conserved process of endothelial-to-hematopoietic transition (EHT)
Summary
Hematopoiesis is a highly conserved process across many organisms that culminates with the emergence of hematopoietic stem cells (HSCs). Zebrafish HSCs expand in the caudal hematopoietic tissue (CHT) (Tamplin et al, 2015) and migrate to the kidney marrow (KM) and chicken HSCs expand in the para-aortic foci (PAF) before seeding the BM (Dunon and Imhof, 2000; Jaffredo et al, 2000; Bollerot et al, 2005a,b) This process is different in drosophila where an initial wave of HSCs (derived from head mesoderm) arises early during larvae development, followed by a second wave of HSCs found in the lymph gland (Lebestky et al, 2000; Dey et al, 2016). We will briefly discuss the possible clinical implications of this current knowledge
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