Fig. 2
Vascular structure of the CAM during interlacing. a, left panel: At low resolution, all arterioles exhibit a terminal segment which is distinctly different from the proximal segment (arrows). The arterial pattern shows the presence of vertical “chimneys” or “springs” of flow identified by the presence of “hotspots” along the arterioles. The hotspots provide flow sources located proximally with respect to the arteriolar hierarchy. The veins do not “navigate” towards the tip of arteries where the flat distal areas are found. a, right panel: Magnification shows that the flat distal areas are wider than the more proximal part of the arteries. The distal segment often has a “goose leg” widening (arrowheads). Scale bar 250 μm. b Scale bar 100 μm. One can in many cases identify the exact point of contact between the 3D arteriole and the ectoderm (Supplementary Movie 2). There, the vessel spreads, and an increase of diameter of order 15–30% is observed (b top panel is a day 11 embryo, bottom is a day 13). The flattened segments of the arteries tend to have regularly spaced sawtooth collaterals. The interspace distance between flat collaterals is the same as the interspace distance of the vertical chimneys as seen in (b) (bottom), and also in (a). c Scale bar 150 μm. When plotting the optical profile one finds that the terminal parts of the arterioles appear flat (top graphs for each arteriole), while more proximal vessels appear round (bottom graphs). The white lines correspond to the locations where the profiles were extracted, D stands for distal and P for proximal (see also Supplementary Information). This shows that the terminal part of the arteriole is flattened on the underneath surface of the ectoderm on which it creeps (N = 30 arterioles were processed giving similar results)
