Ontogeny of raldh2 expression in the vertebrate dorsal spinal cord. (A-C) In the lamprey (Petromyzon marinus) embryo, raldh2 is expressed in dIs (C, arrow), but not in the RP. (D-F) raldh2 expression in zebrafish embryos. (D) No raldh2 expression is detected in the zebrafish neural tube 24 hours post-fertilization (hpf), but at 31 hpf raldh2 expression is detected in a small domain in the hindbrain/SC transition at the pectoral fin bud level (E, asterisk; E,F, arrowheads). (G-I) raldh2 ISH in stage-matched medaka (Oryzias latipes) embryos indicates onset of expression in the same hindbrain/SC domain as in zebrafish (H,I). (J-R) Frog (J-L), chicken (M-O) and mouse (P-R) embryos express raldh2 in the RP (arrowheads) and in dIs (arrows). (M-O) SC Raldh2 expression starts in chicken interneurons at HH18, at the forelimb bud level (M, arrows). Raldh2 expression is restricted to the RP in HH32 chicken SC (M-O). In frogs (J-L) and mice (P-R), raldh2 is initially expressed in RP and in dIs, but is subsequently restricted to the RP. Insets show whole embryos, with plane of section indicated (dashed line). Inset in M shows Raldh2 staining (arrows) in chicken brachial sections. Expression patterns are represented schematically on the right.