Tracing the Progeny of the Aortic Hemangioblast in the Avian Embryo

Abstract

A population of hematopoietic progenitors becomes committed within the embryo proper in the floor of the aorta (P-Sp/AGM in the mouse). In birds, this first aspect of intraembryonic hematopoiesis is prominent during embryonic day 3 (E3) as endothelium-associated "intra-aortic clusters." Between E6 and E8, diffuse hematopoiesis then occurs as "para-aortic foci" located in the dorsal mesentery ventral to the aorta. These foci are not associated with endothelium. Whether these two hematopoietic cell populations arise from distinct or common progenitors is not known. We could recently trace back the origin of intra-aortic clusters in the avian embryo by labeling aortic endothelial cells (EC) in vivo with acetylated low-density lipoproteins. This approach established the derivation of early intraembryonic hemopoietic cells from the endothelium, but did not indicate how long during ontogeny such a relationship may exist, since the progeny of EC labeled at E2 could be traced for 1-2 days at most. Here we report that, when E2 aortic ECs were infected prior to the formation of intra-aortic clusters with a nonreplicative LacZ-bearing retroviral vector, numerous cells were labeled in the para-aortic foci at E6. In contrast, when the retroviral vector was inoculated at E4 rather than E2, that is, after the disappearance of intra-aortic clusters, no cells in the para-aortic foci were labeled. Taken together, our results demonstrate that ECs from the aortic floor seed the two aspects of aorta-associated hemopoiesis and that these ECs with hemangioblastic potential are present only transiently in the aorta.

Primitive and definitive erythropoiesis in the yolk sac: a birds eye view

“…Chick/quail and chick/chick chimera studies (Beaupain et al, 1979;Dieterlen-Lievre, 1975;Dieterlen-Lievre et al, 1976;Lassila et al, 1978;Lassila et al, 1982;Martin et al, 1978) and chick/chick parabiotic studies (Metcalf and Moore, 1971;Moore and Owen, 1965; indicated that, at least during the first few days of definitive erythrocyte generation, the source of cells is mainly yolk sac-derived. At about the same time, if not earlier, intraembryonic hematopoietic cells have been observed in association with the dorsal aorta (DieterlenLievre and Martin, 1981;Jaffredo et al, 2003;Jaffredo et al, 2005;Jaffredo et al, 2000;Jaffredo et al, 1998;Jordan, 1916;Nagai and Sheng, 2008;Olah et al, 1988;Pardanaud et al, 1996). A number of questions can be asked, for instance, about whether dorsal aorta-derived hematopoietic cells undergo definitive erythropoiesis either in circulation or in the yolk sac niche; whether yolk sac-derived hematopoietic cells can find their way to the dorsal aorta; and what is the exact type of cells from the yolk sac that contribute to early definitive erythropoiesis.…”

A short history of hemogenic endothelium

“…The first in vivo experimental support came much later, from lineage tracing of endothelium in the chick embryo [25,26]. More recently, lineage-tracing studies in which Cre recombinase was expressed under the regulatory control of the endothelial marker VE-Cadherin (VECadh) lent further support to this model [27,28].…”

Erythropoiesis from acetyl LDL incorporating endothelial cells at the preliver stage

“…Alternatively, intraembryonic hematopoietic cell production is thought to occur through a completely different process involving the production of HCs by a mature, already differentiated endothelium. In vivo production of HCs by ECs has been demonstrated at the level of the aorta in the avian model 22,23 and is strongly assumed for the mouse AGM. 35 Consistent with data from gene inactivation in mice, several genes have been identified that affect one lineage but not the other.…”

“…Two different tracers were used: Ac-LDL, which is specifically and rapidly endocytosed by ECs, and nonreplicative retroviral vectors carrying a lacZ reporter gene. 22,23 We have applied the Ac-LDL-mediated tracing method to tag the mouse vascular tree in vivo when definitive hematopoiesis occurred. We have designed a system comprising a step of Ac-LDL intracardiac delivery followed by a whole-embryo culture period.…”