Fig. 2. Expression patterns of homeodomain (HD) protein genes in the hindbrain. (A-J) Adjacent sections of E12.5 wild-type (A-E) and Pax6 mutant (F-J) hindbrains at r7 level. Double immunostaining for Nkx2.2 and Pax6 (A,F), and in situ hybridisation for Nkx6.1 (B,G), Irx3 (C,H), Dbx2 (D,I), and Dbx1 (E,J). (A,F) In the wild type, expression of Nkx2.2 is observed in the ventricular zone, and the expression domain is adjacent ventrally to the floor plate and dorsal to Pax6 domain (white arrowhead in A). In the Pax6 homozygous embryo, Nkx2.2 domain expands dorsally (white arrowhead in F). In the wild type, Pax6 protein expression is at a very low level in most of the ventral area (white bracket in A) from which SM neurones develop. Immunoreactivity for Pax6 is not detected in the Pax6 mutant (F). (B,G) In the wild type, Nkx6.1 is localised in the ventral domain excluding the floor plate and adjacent to the Dbx2 domain (double black arrowhead). In the Pax6 mutant, the dorsal boundary of Nkx6.1 expression slightly became blurred (double green arrowhead). Nkx6.1 expression is also seen in BM neurones in the mantle zone of both the wild type and Pax6 mutant (arrowheads in B,G). (C,H) In the wild type, Irx3 is expressed in the regions that overlap with Pax6, except the SM neurone precursor domain (bracket in C). In the Pax6 mutant, Irx3 domain expands ventrally into Nkx2.2 domain (red arrowhead). In the wild type, Irx3 transcripts are also detected in SM neurones (arrows in C), while such population is not seen in the Pax6 mutant (H). (D,I) In the wild type, the expression domain of Dbx2 is ventral to the Nkx6.1 domain (black double arrowhead), and also covers a more dorsal region than Dbx1 domain. In the Pax6 mutant, The ventral boundary of Dbx2 expression also becomes blurred (green double arrowhead in I). (E,J) In the wild type, expression of Dbx1 is confined to a narrow region within the ventrolateral neural tube. In the Pax6 mutant, Dbx1-positive cells decrease in number and expand ventrally (red arrowhead in J). (K,L) Illustration of the expression of HD protein genes and subtypes of ventral neurones in the hindbrain of the wild type (K) and Pax6 mutant (L). In the Pax6 mutant rat, expression domains of HD protein genes are shifted, the boundaries become blurred, and ventral neurones are mis-specified. A small population of V1 interneurones and SM neurones seem to be generated from narrow V1 and SM progenitor domains, respectively. Note that mis-specification profiles are different depending on to anteroposterior levels.
