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Fig. 8 Inter-ray connections are formed by lateral sprouting and intussusceptive angiogenesis. (A) Maximum intensity projections of confocal z-stacks of Tg(-0.8flt1:RFP)hu5333; Tg(flt4: citrine)hu7135 double-transgenic fish labelling all arterial ECs (red) and venous ECs (white). Caudal fin of 1.5-month-old fish. Scale bar: 300 μm. (B) Distal end of caudal fin vasculature. Scale bar: 20 μm. (C) Blue arrowheads indicate venous ECs; red arrowheads indicate arterialized ECs. Scale bar: 5 μm. (C′) 3D reconstruction of boxed region in C shows pillars in veins (red dashed lines). (D) Connection between bone-enclosed artery and peripheral arterial cells (red arrowheads). Scale bar: 10 μm. Intussusceptive angiogenesis and looping occurred in 22 out of 25 fin rays (n=4 animals). (E) Lateral connections. Scale bar: 20 μm. (E′,E″) Sprouts originating from veins. Venous ECs occupy the leading edge (blue arrowheads); arterial cells trail behind (red arrowheads). Scale bars: 4 μm for E′; 5 μm for E″. (F,F′) Inter-ray connection between artery (red arrowheads) and vein (blue arrowheads). Scale bar: 20 μm for F; 5 μm for F′. (G) Stages in the formation of inter-ray connections.