Fig. 6. Lineage analysis of the r4 cells confirms the lack of migrating ncc in the Hoxa1-/-/Hoxb1 3'RARE-/- mutants. Diagrammatic representation of the experimental procedure (a-c) and the results (d-i). Wild-type (a,d,g), Hoxa1-/- (b,e,h) and Hoxa1-/-/Hoxb1 3'RARE-/- (c,f,i) embryos are labelled with DiI at the presumptive r4 or rx territory (arrowheads in d-f) at the five-somite stage, left in culture for 30-32 hours and examined under fluorescence for ncc migratory patterns (arrows in d,e,g,h). The area of pa2 is shown in g-i under high-power magnification. Ncc migration is reduced in the Hoxa1-/- embryos and completely abolished in Hoxa1-/-/Hoxb1 3'RARE-/- embryos. The segmentation defects of Hoxa1-/- or Hoxa1-/-/Hoxb1 3'RARE-/- result in a less conspicuous preotic sulcus (pos in a-c) and an imperceptible otic sulcus (os in a-c). Brackets in b,e denote the area of the neuroepithelium that was labelled with DiI; brackets in d-f denote the pa2 neural crest migration domain. pa1 and pa2, pharyngeal arches 1 and 2, respectively.