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Fig. 4 Expression of raldh2 in wild-type and neckless embryos. (A-O) In situ hybridisation to detect raldh2 mRNA in wild types. (A) Expression in marginal cells at 30% epiboly. (B) Dorsal view showing expression in the germ ring of the gastrula at 70% epiboly and absence of dorsal expression. (C) Lateral view at 85% epiboly, showing expression in deep, involuted cells of the hypoblast. (D) Dorsal view at tail bud stage showing expression in the presomitic mesoderm. (E) Dorsal view at the 12-somite stage showing expression in somites and lateral plate mesoderm (arrows). (F) Lateral view at 17 hpf showing expression in the anterior of each somite (arrows denote levels of sections in I,J). (G) Lateral view at 32 hpf, showing expression at somite boundaries, dorsal and ventral somite extremities, and pronephric mesoderm (pnm). (H) Dorsal view at 32 hpf, showing expression in the eyes and in mesenchyme flanking the otic vesicle (m1 and m2), pectoral fin buds (pec) and somites (arrows denote levels of sections in K,L). (I-M) Transverse sections at 17 hpf (I,J), 32 hpf (K,L) and 60 hpf (M), showing expression in the distal myotome but not adaxial cells (I, somite 14-level) (n, notochord), in the periphery of mature somites (J, somite 7 level), adjacent to the otic vesicle (ov) (K), and in pectoral fin and somitic mesoderm adjacent to the spinal cord (sc) (L, somite 3 level). (M) Expression in ventral motoneurones (mn) and dorsal spinal cord neurones at pectoral fin level. (N) Dorsolateral view at 30 hpf, showing expression in posterior pectoral fin buds (blue), double-stained for tbx5.1 (orange). (O) Lateral view at 33 hpf showing expression in the dorsal retina and anterior to the choroid fissure (arrowhead). (P-S) Colocalisation of krox20 and raldh2 at 19 hpf in wild types (P,Q) and in nls (R,S). (P,R) Lateral views showing upregulation of somitic expression in nls embryos, (Q,S) Dorsal views of same embryos showing upregulated raldh2 expression in lateral plate mesoderm in nls mutants (arrows); broadened krox20 expression in the hindbrain distinguishes nls from wild-type embryos.