Intraaortic hemopoietic cells are derived from endothelial cells during ontogeny

We have investigated the developmental relationship of the hemopoietic and endothelial lineages in the floor of the chicken aorta, a site of hemopoiet ic progenitor emergence in the embryo proper. We show that, prior to the on set of hemopoiesis, the aortic endothelium uniformly expresses the endothel ium-specific membrane receptor VEGF-R2. The onset of hemopoiesis can be det ermined by detecting the common leukocyte antigen CD45. VEGF-R2 and CD45 ar e expressed in complementary fashion, namely the hemopoietic cluster-bearin g floor of the aorta is CD45(+)/VEGF-R2(-), while the rest of the aortic en dothelium is CD45(-)/VEGF-R2(+). To determine if the hemopoietic clusters are derived from endothelial cells , we tagged the E2 endothelial tree from the inside with low-density lipopr oteins (LDL) coupled to DiI, 24 hours later, hemopoietic clusters were labe lled by LDL. Since no CD45(+) cells were inserted among endothelial cells a t the time of vascular labelling, hemopoietic clusters must be concluded to derive from precursors with an endothelial phenotype.