3041 – ENDOTHELIAL STRUTS, A NOVEL MECHANISM OF BLOOD VESSEL FORMATION THROUGH ORGANIZATION OF ARTERIAL, VENOUS AND HEMATOPOIETIC STEM CELL PRECURSORS.

By using zebrafish, we identified a novel model of vasculogenesis that explains how arterial (including hemogenic endothelial cells (HEC)) and venous endothelial cells (ECs) form the dorsal aorta and posterior cardinal vein without the prerequisite of a cord-like structure or a cell or cord hollowing step, two hallmarks of our current models of vasculogenesis. In the here presented model, ECs coalesce into a network of struts in the future lumen of the vessel, a process depending on BMP signaling. The vessel wall is initially non-existing and consists only out of a few patches of ECs. Through pruning of struts and unmixing of arterial and venous ECs, venous ECs migrate into and form the caudal vein wall, while arterial ECs and HECs form the floor of the dorsal aorta. Arterial and hemogenic ECs in struts have initially a low arterial identity, which allows them to participate in strut formation. Through a gradual increase of arterial specific genes, like the downstream Notch target gene ephrinb2a, arterial and hemogenic ECs are unmixed from the venous ECs. Finally, we experimentally severed struts and found that this resulted in the collapse of the vessel, disturbed blood flow and remodeling defects, demonstrating that struts maintain the patency and integrity of the vessel during its formation. Together, we show that the formation of the hemogenic endothelium from which the HSCs are specified occurs through the formation of an intricate network of arterial, hemogenic and venous ECs which subsequently unmixes into the dorsal aorta and cardinal vein, with the hemogenic endothelium integrated into the floor of the dorsal aorta.