Fig. 4. Analysing rhombomere 1 (r1) and the ventral mesencephalon in ace mutants. The rostralization stops at caudal r1 in ace mutants (A-H). The ventral mesencephalon displays altered gene expression profiles in ace mutants (A,B,I-L). All views are rostral to the left. A,B,G-L are lateral views; C-F are dorsal views. (A-D) Expression of epha4a, labeling the caudal part of r1, is still detectable in ace mutants. However, this domain in ace embryos (B) shows a distorted, slanted orientation in comparison with wild-type siblings (A), where it is approximately perpendicular to the axis of the hindbrain. Small arrows (A-D) point to the epha4a-expressing caudal r1 compartment. Note that the expression of epha4a at the fore-midbrain junction is expanded toward the ventral mesencephalon in ace mutants. Arrow (A) points to the caudal limit of epha4a expression at the fore-midbrain region. Two arrowheads (B) point to the expanded epha4a expression domain in the mutant embryo. (E,F) In the hindbrain, a new interface is detectable between the otx2 (red) and epha4a (purple) expression domains in ace embryos. The arrowhead (E) points to the caudal limit of otx2; the small arrow labels the rostral end of epha4a in the caudal r1 of wild-type embryos. The arrowhead above the small arrow (F) marks the new otx2/epha4a interface in the ace mutant. (G,H) The Locus ceruleus (LC; arrowhead in G) is missing from r1 in ace mutants. (I,J) In ace mutant embryos, the diencephalic expression domain of zash1b expands toward the mesencephalic tegmentum and fuses to the hindbrain expression domain (J). Arrowheads (I) mark the gap between the rostral and caudal expression domains of zash1b. (K,L) Expression of twhh is severely compromised in the ventral mesencephalic region of pharyngula-stage mutant embryos in comparison with wild-type embryos. The arrow (L) points to the reduced ventral mesencephalic twhh expression domain.