Figure 4

A schematic presentation of mural cell development in trunk vessels during early developmental stages. Potential pdgfrb^low mural cell precursors (light green) are detectable from 10 hpf and are distributed throughout the trunk by 24 hpf, preferentially along the somite boundary where ISV is formed (a). Then, specification from pdgfrb^lowprecursors located in the vicinity of arterial endothelial cells into mural cells (green) starts after approximately 36 hpf beneath the dorsal aorta and 48 hpf on the ISVs. Subsequently, the proliferation and migration of mural cells are both induced in a PDGFRβ signaling-dependent manner. The arterial ISV (aISV) (b), the dorsal aorta (c), and the dorsal portion of the venous ISV (vISV) are well covered by mural cells by 96 hpf. Mural cells beneath the dorsal aorta start to wrap around the dorsal aorta and express VSMC markers such as acta2 and tagln at approximately 72 hpf. Later, these VSMC markers become positive in some ISV mural cells. In most mural cells differentiating into VSMC, acta2 expression is induced earlier than that of tagln. Images obtained in the vicinity of the dorsal aorta of TgBAC(pdgfrb:mCherry) ^ncv23Tg;TgBAC(tagln:GFP)^ncv25Tg;Tg(kdrl:tagBFP)^mu293Tg at 96 hpf are shown (b). Interestingly, the expression of tagln;GFP in mural cells increases when these cells are combined with pdgfrb reporter lines. Mural cells that are tagln-positive but negative for pdgfrb become apparent on the dorsal side of the dorsal aorta, mostly at approximately 96 hpf. However, whether VSMCs on the dorsal aorta consist of both VSMC sources or are replaced by mural cells emerging from the dorsal side, as observed in the mouse, remains to be determined. Scale bars: 20 μm (a,b), 50 μm (c).