During development, extensive remodelling of the embryonic vasculature, the first organ to develop, ensures that the embryo's cells are constantly supplied with oxygen and nutrients. The first major vascular remodelling event in mammalian and avian embryos is fusion of the bilateral dorsal aortae at the midline to form the dorsal aorta. Takashi Mikawa and co-workers now show that a developmental switch in notochord activity signals this fusion in chick and quail embryos (see ). Prior to fusion, the researchers report, the notochord secretes positive and negative factors that together block the initiation of aortae fusion. Notably, whereas the expression of positive vascular regulators is maintained during fusion, the expression of negative regulators such as chordin, an antagonist of the positive regulator BMP, is downregulated along the anteroposterior axis. The discovery of this novel mechanism for modifying vascular pattern – modulation of vascular inhibitors without changes in positive vascular regulator levels – could aid the development of treatments for vascular injury.
