"Hemogenic endothelium" of the embryonic aorta: Does it exist?

Aggregates of intravascular, presumably endothelial-derived, undifferentiated cell were found in the aorta of 14-15 day Mongolian gerbil embryos, in several 10 mm pig embryos, in one 10 day mouse embryo and in one 9.5 mm human embryo. As in other species in which they have been identified, these "aortic cell clusters" generally occurred in groups of 25-100 cells and were typically observed adherent to the ventral luminal wall of the abdominal aorta during a brief period in gestation. The present electron microscopic study shows that these cells possess many characteristics in common with undifferentiated primitive blood cells of the yolk sac blood island. Their in situ origin is supported by ultrastructural similarity to underlying endothelium as well as the presence of numerous intercellular junctions among themselves and with subjacent endothelium. It is suggested that they may be morphologically undifferentiated hemopoietic precursor cells. Hypotheses are proposed for the mechanism of their origin from aortic endothelium.