Fig. 1. Migration of facial branchiomotor neurons and development of the facial nucleus. (A,B) An ectopic mass of postmitotic neurons (arrow in B), revealed on coronal sections by class III ß-tubulin in situ hybridization at E11.5, accumulates medially at the level of r4 in the ventral hindbrain of Psen1 mutants (B). (C) At E12.75, Tag1 is expressed in FBM neurons migrating in r4/5/6 in wild-type embryos; (D) FBM fail to migrate in the mutant; bilateral Tag1 expression domains border the ectopic neuronal mass (arrow), visible medially on flat mount of the hindbrain (ventricular view) under Nomarski optics. (E,F) Progenitors of CVA neurons (arrowheads), identified by Gata3 expression at E11.5, are generated normally close to the floor plate in a ventral stripe in r4 in wild-type (E) and mutant (F). (G,H) Ventricular views of flat mounted hindbrains. Retrograde tracing of FBM (bm) and VM (vm) neurons by DiI injection into the root of the VIIth nerve at E11.5. In the wild type (G), the normal trajectory of FBM and VM, normal contralateral projections of the CVA neurons (G, inset) are shown; in the mutant (H), the FBM neurons are stalled. Focal injections of DiO into the ectopic mass (outlined in H after observation under Nomarski optics) only label axons extending unilaterally; CVA contralateral projections are missing from the mutant. Black and white asterisks indicate, respectively, the DiI and DiO injection sites. (I,J) The facial nucleus, detected by Isl1 expression on coronal sections at E16.5 is significantly smaller and fragmented in the mutant (J). Notice ectopic Isl1-expressing cells in the floor plate (arrowhead in J). (K,L) Pax6 expression is downregulated in the mutant hindbrain at E11.5 (L). Notice abnormal streams of Pax6-expressing cells in ventral r3 and r5/6 (arrowheads) and the neuronal mass (white arrow). Ventricular view; flat-mount preparations of the ventral hindbrain. r, rhombomere.