Fig. 3. Prdm1 acts downstream of retinoic acid and Wnt2b signaling during limb bud initiation. (A,B) Lateral views of the retinaldehyde dehydrogenase 2 (raldh2) expression pattern in 12-somite stage wild-type (A) or prdm1 MO-injected (B) embryos. Anterior is to the left. A' and B' depict close-up views of lateral plate mesoderm (arrows) and somite expression at the axial level of pectoral fin formation. (C,D) Expression of the retinoic acid degrading enzyme cyp26a1 in wild-type and prdm1 morphant embryos at the 12-somite stage. Arrow indicates cyp26a1 expression within somites 1 and 2. Note that there is no difference in raldh2 and cyp26a1 expression between prdm1 morphants and wild types. (E-F') prdm1 in situ hybridization of control DMSO-treated 17-somite stage embryos (E,E') and DEAB-treated embryos (F,F'). Note that DEAB treatment leads to a reduction of prdm1 expression within branchial arches (arrow in E) and anterior somites (compare straight and dotted lines in E' and F'). (G,H) prdm1 in situ hybridization on 24 hpf wild-type (G) and neckless mutant (H) pectoral fin bud regions. Arrows in G indicate fin buds; asterisks in F and H indicate a absence of prdm1 expression within the corresponding structure. (I-L) Expression pattern of wnt2b mRNA in wild-type (I), prdm1 morphant (J), 16 µM SU5402-treated (K) and neckless mutant (L) embryos at 24 hpf. Panels show dorsal views with anterior to the top. Note that wnt2b expression is normal in MOprdm1 and SU5402-treated embryos, but is lost in the raldh2 mutant nls (asterisks). DEAB, diethylaminobenzoic acid; DMSO, dimethylsulfoxide; nls, neckless; MOprdm1, prdm1 morphant; WT, wild type.