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Fig. 2 nos2b is expressed in two distinct primordial by 36 hours postfertilization (hpf). A: By 36 hpf, nos2b expression was found in two domains, one in the hypothalamus posterior to the hypophysis (arrow) and the second domain (arrowhead), at the base of the primordium of the lower jaw ventral of the anterior hindbrain. B: By 48 hpf, nos2b expression was maintained in the hypothalamus posterior to the hypophysis (arrow), but the expression level decreased. At the same time, the expression level in the domain (arrowhead) projecting onto the primordium of the lower jaw increased. C-H: This centrally localized domain of nos2b expression, flanked by tyrosine hydroxylase (TH) -positive arch associated cells (aac, C) and closely associated with the thyroid primordium marker tg (D), is absent in both smu-/- and noi-/- mutants (E-H). The arrowheads in E and G point to the localization of the second nos2b domain in noi-/+ and smu-/+ siblings respectively that is otherwise missing in the homozygous mutants. I,J: At 72 hpf, this domain of intense expression was localized at position of basihyal cartilage (bh, I,J). J shows the cross section; the section is shown by a straight line on I. The arrowhead in J indicates the nos2b-positive cells in position of the basihyal cartilage, whereas the arrows point to areas of lower level of expression in oral roof ectoderm. On all whole-mounts anterior is to the left. bh, basihyal; lj, lower jaw; in, infundibulum; hyp, hypophysis; tg, tegmentum.