Tracing the progeny of the aortic hemangioblast in the avian embryo

A population of hematopoietic progenitors becomes committed within the embr yo 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 a nd E8, diffuse hematopoiesis then occurs as "para-aortic foci" located in t he dorsal mesentery ventral to the aorta. These foci are not associated wit h endothelium. Whether these two hematopoietic cell populations arise from distinct or common progenitors is not known. We could recently trace back t he origin of intra-aortic clusters in the avian embryo by labeling aortic e ndothelial cells (EC) in vivo with acetylated low-density lipoproteins. Thi s approach established the derivation of early intraembryonic hemopoietic c ells from the endothelium, but did not indicate how long during ontogeny su ch 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 nonreplicat ive LacZ-bearing retroviral vector, numerous cells were labeled in the para -aortic foci at E6. In contrast, when the retroviral vector was inoculated at ES rather than E2, that is, after the disappearance of intra-aortic clus ters, no cells in the para-aortic foci were labeled. Taken together, our re sults demonstrate that ECs from the aortic floor seed the two aspects of ao rta-associated hemopoiesis and that these ECs with hemangioblastic potentia l are present only transiently in the aorta, (C) 2000 Academic Press.