Development of the zebrafish arterial valve follows conserved events. (A) Schematic of 30 hpf OFT. Two cell layers, the wall (black) and endocardium (green), are separated by ECM (grey). (A′) Representative haematoxylin and eosin–stained midline section through the long axis of the OFT at 30 hpf; arrow denotes direction of blood flow (n = 6). (B–C′) Between 38 and 46 hpf, the two cell layers remain distinct and separated by ECM (38 hpf, n = 7; 46 hpf, n = 10). (D and D′) At 54 hpf, a multi-layering of cells is present at the distal-most point of the OFT where no ECM is visible (D′, arrowhead), with cells of unknown fate in yellow (n = 8). (E and E′) ECM in the ventricle and OFT is largely absent at 62 hpf (n = 11). (F–G′). Between 70 and 78 hpf, any remaining ECM is lost and the OFT lengthens and tapers where it connects to the ventral aorta (70 hpf, n = 5; 78 hpf, n = 10). (H and H′) Sinuses are visible by 86 hpf, defining two leaflets in the OFT (arrowheads) (n = 5/8). (I–L) Live fluorescent imaging of the OFT between 62 and 86 hpf of Tg(kdrl:GFP) embryos, marking the endocardium. Two primordia are visible at 62 hpf (I) (n = 13) and 70 hpf (J) (n = 14). The beginning of sinus formation is detectable at 78 hpf (K) (n = 11/15), with tips of leaflets visible (arrowheads) and is mostly complete by 86 hpf (L) (n = 17). V, ventricle; BA, bulbus arteriosus. Scale bars: (A)–(H): 20 μm; (I)–(L): 5 μm.
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