Gene expression domains at the mhb share a common border that coincides with the lineage restriction boundary. (A,B) The posterior gene expression boundary of otx2 shifts from perpendicular to the neural axis to oblique between the 16-somite stage and 24 hpf, reflecting morphological changes during mhb formation (compare with Fig. 1). (B,C) From dorsal to ventral, the opening (broken line) in the otx2 expression domain becomes broader. (D,E) Similar to otx2, the anterior gbx2 and fgf8 expression domain borders become oblique with respect to the neural axis at 24 hpf. Further ventrally, the expression domains of gbx2 and fgf8 extend more anteriorly, probably filling the opening in the otx2 expression domain (D,E, compare with C). (F) Summary scheme of gene expression boundaries in the mhb area at 24 hpf. Dots indicate co-expression. The lineage restriction boundary between the otx2 and gbx2 territories is marked with broken black lines. All panels show dorsal views, anterior is towards the left. In situ probes were detected with fluorescent FastRed substrate and embryos optically sectioned on a Zeiss confocal microscope. Scale bar: 50 µm.