Anaesthetized embryos at indicated stage were mounted in 1.0% low-melt agarose (Sigma Aldrich) for imaging19 (link). Images were acquired by using an Olympus FluoView FV1000 laser scanning confocal microscope. Z-stacks were acquired with a 3 μM z-step, and images were 3D rendered. Neurolucida software (MBF Bioscience) was used to analyze the length of ISVs and SIV.