Fig. 2. Gradual loss of neural crest markers and reduction of the second arch mesenchymal population. Wild-type (a,d,g,j,m), Hoxa1^-/- (b,e,h,k,n) and Hoxa1^-/-/Hoxb1 3'RARE^-/- (c,f,i,l,o) embryos were processed for whole-mount in situ hybridisation with Hoxb2 (a-c), Hoxa2 (d-f), twist (g-i), Sox10 (j-l) antisense riboprobes and histological staining of paraffin sections (m-o). (a-f) Expression of Hoxb2 and Hoxa2 was significantly reduced in the second pharyngeal arch (pa2) of Hoxa1^-/- embryos (b,e) and completely lost in that of Hoxa1^-/-/Hoxb1 3'RARE^-/- embryos (c,f). Expression of Hoxb2 in the hindbrain of wild-type and Hoxa1^-/-embryos was continuous but discontinuous in the hindbrain of Hoxa1^-/-/Hoxb1 3'RARE^-/- embryos, suggesting the presence of a territory distinct from r3 and r5/r6 (noted with white arrowheads in c). (g-i) Expression of the mesenchymal ncc marker twist was reduced in the second pharyngeal arch of Hoxa1^-/- embryos and was lost in that of Hoxa1^-/-/Hoxb1 3'RARE^-/-embryos. (j-l) The neurogenic ncc marker Sox10 was expressed in the proximal ganglia of wild-type embryos (arrows in j). Its expression was reduced in the facioacoustic ganglion of Hoxa1^-/- embryos (arrow in k) and was lost in the more posterior ncc-derived ganglia of Hoxa1^-/- embryos (asterisks in k) and in all the ncc-derived ganglia of Hoxa1^-/-/Hoxb1 3'RARE^-/- embryos (asterisks in l). (m-o) The second pharyngeal arch mesenchymal population was reduced in the Hoxa1^-/- embryos and further diminished but not extinguished in the Hoxa1^-/-/Hoxb1 3'RARE^-/- embryos. ov, otic vesicle; Vg, trigeminal ganglion.