Arterio-Venous Remodeling in the Zebrafish Trunk Is Controlled by Genetic Programming and Flow-Mediated Fine-Tuning
How developing vascular networks acquire the right balance of arteries, veins and lymphatics to efficiently supply and drain tissues is poorly understood [1, 2]. In zebrafish embryos, the robust and regular 50:50 global balance of intersegmental veins and arteries that form along the trunk [3], prompts the intriguing question how the organism keeps \"count\". Previous studies suggest that the ultimate fate of an intersegmental vessel (ISV) is determined by the identity of the approaching secondary sprout emerging from the posterior cardinal vein (PCV) [1, 4-7]. Here, using high time-resolution imaging, advanced cell tracking and computational analysis, we show that the formation of a balanced trunk vasculature involves an early heterogeneity in endothelial cell (EC) behavior in the seemingly identical primary ISVs and an adaptive flow-mediated mechanism that fine-tunes the balance of arteries and veins along the trunk. Detailed examination of the trunk vasculature dynamics throughout development reveals the frequent formation of three-way vascular connections between primary ISVs, the dorsal aorta (DA) and the PCV. Differential resolution of these connections into arteries or veins is mediated by polarized cell movement of the ECs within the ISV. Quantitative analysis of the cellular organization, polarity and directional movement of ECs in primary ISVs identifies an early differential behavior between future arteries and veins that is largely specified in the ECs of the individual ISVs, is dependent on Dll4/Notch, and occurs even in the absence of secondary sprouting. Notch signaling is involved in a local patterning mechanism normally favoring the formation of alternating arteries and veins. The global artery-vein balance is however maintained through a flow-dependent mechanism that can overwrite the local patterning. We propose that this dual mechanism driving arterio-venous identity during developmental angiogenesis in the zebrafish trunk provides the adaptability required to establish a balanced network of arteries, veins and lymphatic vessels.