Fig. 1. Mis-specification of ventral neurones in the Pax6 mutant hindbrain. (A,B) Summary of detailed time-course analyses on expression of ventral neurone markers in the wild-type (A) and Pax6 homozygous (B) embryos. Five types of neurones develop in the ventral hindbrain: from ventral to dorsal, branchiomotor (BM) and somatic (SM) motoneurones, and V2, V1 and V0 interneurones, with distinct marker expressions. Pax6 is already expressed in the hindbrain at E10.5 (not shown here). During normal development, SM neurones expressing Islet2 and HB9/MNR2 emerge by E11.5, and expression of HB9/MNR2 and Islet2 is maintained at E13.5. In the Pax6 mutant, a small number of cells expressing Islet2 and HB9/MNR2 are transiently seen during E12.75-13.0 at r7 level. In the wild type, En1 expression is first detected at E12.0. In the Pax6 mutant, En1 is transiently expressed in a small number of cells at r3-6 levels of E13.0-13.25 embryo. (C-J) Expression patterns of ventral interneurone markers. Adjacent sections of E13.0 the wild-type (C-F) and Pax6 mutant (G-J) hindbrain at r5 level hybridised with Pax6 (C,G), Chx10 (D,H), En1 (E,I) and Evx1 (F,J) antisense probes. (C,G) Pax6 is expressed in the ventricular zone of the ventrolateral domain in both the wild-type and mutant neural tubes. (D,H) In the Pax6 mutant, Chx10-positive cells are located in a slightly wider region than those seen in the wild type, spreading more dorsally (see bracket in H). (E,I) In the wild type, En1-positive cells are detected in the region dorsal to Chx10 expression (arrow). (I) In the Pax6 mutant, En1-positive cells are undetectable at E12.5 (not shown here) (Ericson et al., 1997), a small number of En1-positive cells are observed in a similar dorsoventral position (arrow). (F,J) In the wild type, Evx1-positive cells are detected in the dorsally adjacent region (arrowhead in F). In the mutant, Evx1 expression seems to expand ventrally at r5 (arrowheads in J). (K-R) Expression of motoneurone markers in the hindbrain. Adjacent sections of E12.75 wild-type (K-N) and Pax6 mutant (O-R) hindbrain at r7 level. Distribution of Pax6 (K,O), Islet1/2 (L,P) and HB9/MNR2 (N,R) proteins, and Islet2 transcripts (M,Q). (K,O) In the wild type, Pax6 is observed in the ventricular zone of the ventrolateral domain of the neural tube (K). In the Pax6 mutant, immunoreaction for Pax6 is not detected with the antibody that recognises the C-terminal region of normal Pax6 protein (O). (L-N) During normal development, SM neurones are positive for Islet1, Islet2 and HB9/MNR2, while BM neurones (bracket in L) are negative for Islet2 and HB9/MNR2. Some of BM neurones are located dorsolaterally, as they migrate dorsally at this stage (arrowheads). (P-R) In the Pax6 mutant, Islet2- and HB9/MNR2-positive cells are missing at E12.5 (not shown here) (Osumi et al., 1997), but a small number of Islet2-positive and MNR2/HB9-positive cells are observed at E12.75 (arrows in Q,R). Inset in (R) shows higher magnification of HB9/MNR2-positive cells indicated by white arrows.