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 me ans of quail/chick transplantations that two subsets of the mesoderm give r ise to endothelial precursors: a dorsal one, the somite, produces pure angi oblasts (angiopoietic potential), while a ventral one, the splanchnopleural mesoderm, gives rise to progenitors with a dual endothelial and hemopoieti c potential (hemangiopoietic potential), To investigate the cellular and molecular controls of the angiopoietic/hema ngiopoietic potential, we devised an in vivo assay based on the polarized h oming of hemopoietic cell precursors to the floor of the aorta detectable i n the quail/chick model, In the present work, quail mesoderm was grafted, a fter various pretreatments, onto the splanchnopleure of a chick host; the h oming pattern and nature of graft-derived QH1(+) cells were analyzed therea fter. We report that transient contact with endoderm or ectoderm could chan ge the behavior of cells derived from treated mesoderm, and that the effect of these germ layers could be mimicked by treatment with several growth fa ctors VEGF, bFGF, TGF beta 1, EGF and TGF alpha, known to be involved in en dothelial 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 TGF beta 1, In contrast, conta ct with ectoderm or EGF/TGF alpha treatments totally abrogated the hemangio poietic capacity of the splanchnopleural mesoderm, which produced pure angi oblasts with no 'ventral homing' behaviour. We postulate that two gradients , one positive and one negative, modulate the angiopoietic/hemangiopoietic potential of the mesoderm.