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Fig. 4 Migration of Endocardial Precursors in wt Embryos (A) Embryos transgenic for vegfr4:gfp were subjected to time-lapse confocal microscopy, revealing rapid endocardial migration prior to heart tube formation. A movie demonstrating this process can be viewed in the Video S1. Twelve individual frames from this movie at indicated stages and time-points are shown seen in (A), with white dashed lines indicating the position of (pre-) endocardial cells. Frames 1–5 show the fusion of the bilateral endocardial precursors between the 14- and 16-somite stages. Frames 5–8 indicate the posterior migration of endocardial cells to cover the lateral and posterior regions of the cardiac disc; note the posterior migration of the paired lateral dorsal aortas between the 18- and 22-somite stages (white arrows). The apex of the endocardial disc (pink dashed line) appears to be constricted below the aortic arches between the 18- and 22-somite stages (frames 7–10). A leftward movement of the endocardium is visible between the 20- and 26-somite stages (frames 9–12), and is coinciding with the appearance of single vegfr4:gfp-positive cells lateral to the remaining endocardium (red dashed line). Also note the migration of the venous posterior hindbrain channels (red arrows) between the 22- and 26-somite stage (frames 10–12). (B) Relative locations of endocardial and myocardial precursors during fusion of endocardial precursor populations, revealed by two-color in situ hybridization showing cdh5 (blue, endocardium) and nkx2.5 (red, myocardium) expression. The bilateral populations of endocardial precursors (arrows) are located anterior to the myocardial precursors until the 14-somite stage, then migrate medially and posteriorly to assume a position in between the myocardial precursors at the 18-somite stage. (C) Embryos transgenic for fli1a:gfp were subjected to time-lapse confocal microscopy, revealing slow medial movement of gfp-positive cells between the six- and 12-somite stage (frames 1–4) and rapid migration starting at the 14-somite stage. A movie demonstrating this process can be viewed in the Video S2.