Manipulation of the angiopoietic/hemangiopoietic commitment in the avian embryo

The hypothesis that the endothelial and hemopoietic lineages have a common ontogenic origin is currently being revived. We have shown previously by means of quail/chick transplantations that two subsets of the mesoderm give rise to endothelial precursors: a dorsal one, the somite, produces pure angioblasts (angiopoietic potential), while a ventral one, the splanchnopleural mesoderm, gives rise to progenitors with a dual endothelial and hemopoietic potential (hemangiopoietic potential). To investigate the cellular and molecular controls of the angiopoietic/hemangiopoietic potential, we devised an in vivo assay based on the polarized homing of hemopoietic cell precursors to the floor of the aorta detectable in the quail/chick model. In the present work, quail mesoderm was grafted, after various pretreatments, onto the splanchnopleure of a chick host; the homing pattern and nature of graft-derived QH1(+) cells were analyzed thereafter. We report that transient contact with endoderm or ectoderm could change the behavior of cells derived from treated mesoderm, and that the effect of these germ layers could be mimicked by treatment with several growth factors VEGF, bFGF, TGFbeta1, EGF and TGF(&agr;), known to be involved in endothelial commitment and proliferation, and/or hemopoietic processes. The endoderm induced a hemangiopoietic potential in the associated mesoderm. Indeed, the association of somatopleural mesoderm with endoderm promoted the 'ventral homing' and the production of hemopoietic cells from mesoderm not normally endowed with this potential. The hemangiopoietic induction by endoderm could be mimicked by VEGF, bFGF and TGFbeta1. In contrast, contact with ectoderm or EGF/TGF(&agr;) treatments totally abrogated the hemangiopoietic capacity of the splanchnopleural mesoderm, which produced pure angioblasts with no 'ventral homing' behaviour. We postulate that two gradients, one positive and one negative, modulate the angiopoietic/hemangiopoietic potential of the mesoderm.