raldh2 morpholino induced phenocopies and rescue of mesodermal and pectoral fin development in nls through RA application. (A,B) Expression of krox20 and myoD, dorsal views. (A) Injection of a raldh2-morpholino into wild-type phenocopies the mesodermal defects in nls. (B) 10-6 M RA rescues mesoderm development in nls (12 hpf); brackets indicate the postotic head. (C-L) Wild-type (middle panels) and nls (right panels) embryos in dorsal (E-J) or lateral (C,D,K,L) view, anterior towards the left. (C-F) In situ hybridisation reveals an absence of tbx5.1 expression, which marks forelimb mesoderm, in nls at the 12-somite stage (C,D), as well as later during fin outgrowth at 28 hpf (E,F). (G,H) At 32 hpf, shh expression, a marker for posterior fin mesenchyme, is absent in nls embryos. (J,L) 36 hpf, 10-7 M RA rescues tbx5.1 expression in nls pectoral fin buds (arrow); rescued fin buds often develop apical folds (L, arrow), although never as progressed in growth as in wildtype siblings.