Fig. 4. Formation of the parachordal vessels. Multiphoton confocal images (B,C) show lateral (top) and ventrolateral (bottom, image tilted upwards -40 to -50 degrees relative to the top images) views of vessels on one side of the mid-trunk of fli-egfp transgenic zebrafish embryos at 1.8-2.2 dpf. (A) Parachordal vessels form from sprouts derived from both the posterior cardinal vein (small arrow) and from future intersegmental veins (large arrow). (B) Parachordal segment connected to an intersegmental vein at one end (small arrow), and connected to the posterior cardinal vein at the other end via a ventral root (large arrow) adjacent to an intersegmental artery (arrowheads). A vestigial connection to the dorsal aorta persists on the intersegmental vein (asterisk). (C) Intersegmental vein connected to the adjacent parachordal vessel (arrow). Anterior is towards the left in all panels. Scale bar: 25 µm. 3D reconstructions of these images are available at http://dir.nichd.nih.gov/lmg/uvo/ISV3_D.html