Abstract
During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.
Highlights
In mammals, the hematopoietic system is established during embryogenesis in three consecutive overlapping waves (Dzierzak and Bigas, 2018)
Macrophage but not erythrocyte potential has been experimental confirmed for these so-called early EMP (Wittamer and Bertrand, 2020)
The hematopoietic stem cells (HSCs) subsequently play a central role in HSC-Independent Hematopoiesis maintaining the hematopoietic system for the lifetime of the organism (Medvinsky et al, 1993; Muller et al, 1994)
Summary
The hematopoietic system is established during embryogenesis in three consecutive overlapping waves (Dzierzak and Bigas, 2018). Hematopoietic cells of all waves are generated from the mesoderm, which is known to give rise to both endothelial and hematopoietic lineages (Davidson and Zon, 2000; Dzierzak and Bigas, 2018). Several recent studies suggest that primitive hematopoiesis (wave 1) is generated through a HE(-like) intermediate that has been termed hemogenic angioblast (Lancrin et al, 2009; Stefanska et al, 2017; GarciaAlegria et al, 2018). Despite this potential common cellular origin, not all waves originate from the same anatomical site. The HSC mature and amplify in the fetal liver (FL) before taking up residence in the bone marrow (BM)
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