Fig. 6. Ectopic expression of cell-surface receptors in facial branchio-motoneurons of Nkx6.1 mutants. Whole-mount in situ hybridization with the indicated probes on flat-mounted hindbrains from wild-type (left columns) and Nkx6.1 mutant (right columns, except J) embryos at the indicated ages. Phox2b marks facial branchio-motor (fbm) neurons (nVII, A-D). At E10.75, few fbm neurons have migrated into r5 in wild type (arrowhead in A) and Nkx6.1 mutants (arrowhead in B). At E12.5, some fbm neurons have completed migration into r6 in wild-type embryos (C), but no neurons are found caudal to upper r5 in Nkx6.1 mutants (D). Tag1 is normally expressed in fbm neurons in r4 in both wild-type (E) and Nkx6.1 mutant embryos (F). Ret, which is normally restricted to migrating fbm neurons in r5 and r6 (G), is ectopically expressed in r4 in Nkx6.1 mutants (H). In wild-type embryos, Unc5h3 is first detected at E12.0 (I) in facial and trigeminal (nV) motoneurons, and motoneurons of the superior (nSS) and inferior salivatory nucleus (nIS), as well as the nucleus ambiguous (nA). The expression of Unc5h3 in these neurons is maintained at E13.5 (J). Neogenin is expressed in dorsolaterally migrating facial and trigeminal motoneurons (O). In Nkx6.1 mutants, fbm neurons ectopically express Unc5h3 (L) and do not express neogenin (P). In situ hybridization for Unc5h3 on coronal sections through r4 verifies that the ectopic expression is specific to fbm neurons (N). Abducens nucleus, nVI.