Fig. 7. Retinoic acid (RA) is required and sufficient to induce formation of th-expressing NA neurons in the posterior hindbrain. (A-P) Analysis of th expression in 72 hpf embryos by whole-mount in situ hybridization. In the diencephalon (DC) no differences of th expression are detected in (B) RA treated wild-type embryos, (C) nls/raldh2 mutants and (D) tfap2a mutant embryos exposed to RA. (E-H) th expression is normal in the locus coeruleus (LC) of (F) RA-treated embryos, (G) nls/raldh2 and (H) remains absent in tfap2a mutant embryos exposed to RA. (J) In the medulla oblongata and area postrema (MO), RA treatment induces th expression. (K) nls/raldh2 mutant embryos express th in one or two cells of the MO only. (L) RA fails to induce th expression in the hindbrain of tfap2a mutant embryos. (M,N) A dramatic increase of enteric neurons expressing th in the gut region occurs after RA treatment. (O) Embryos mutant for nls/raldh2 have a reduced number of th-expressing cells in the sympathetic ganglia (Symp). (P) RA is not able to induce th expression in the gut region of tfap2a mutant embryos. (A-P) Dorsal views, anterior towards the left.