Fig. 3

Prospective endothelial cells migrate from the yolk sac to form the dorsal aortae and endocardium. A = anterior, P = posterior, DA = dorsal aortae, YS = yolk sac. The subfigures all show Tal1 expression in frames of movies of live imaging time series, imaged by light sheet microscopy, together with the tracks of particular individual cells. (a1–3), (b1–3), (c1–5), and (d): stills from Additional File 17, Additional File 18, Additional File 19 and Additional File 20 respectively, showing that cells from within the distal border of the yolk sac migrate into the embryo to form the dorsal aortae (a1–3), that some endothelial cells return to or remain in the yolk sac (b1–3), and that cells at the border of the yolk sac migrate into the embryo to form the endocardium (c1–5). (a1): black arrows indicate single cells within the distal edge of the yolk sac that were tracked. (a2,3): green lines show the path along which the cells moved. Because the embryo also grows while the cells are migrating, the origin of the line no longer represents the origin of the cell migration inside the embryo shown, but the end of the line does indicate where the cells are at that timepoint. The lines represent the total displacement of the cells, which is the sum of morphogenetic movement and active migration. (a3): green tracking lines show that the migrating cells reached the forming dorsal aortae, on the distal side of the avascular zone forming between the yolk sac and the dorsal aortae. (b1): a white arrow on the left points to a cell within the embryo that was tracked, and a white arrow on the right points at a cell at the border of the yolk sac that was tracked (starting from an earlier time point) from within the yolk sac and moved along the green line. (b2–3): these show that the former cell divides and that its two daughter cells migrate to join the yolk sac, at different timepoints, and that the latter cell remained within the yolk sac, at its distal border. (c1–5): black and white arrows indicate cells within the distal border of the yolk sac, that migrate to contribute to the endocardium. The green lines represent the tracking lines as before. Cells that join the endocardium early come from the more anterior part of the yolk sac; cells that join later come from the lower down sideways edge of the yolk sac and join the venous poles. (d): this shows the direction of the cell movement of all the Tal1 positive cells in the embryo. This (d) tracking was automatic, while the (A-C) tracking was manual