Fig. 7. Altered pattern of cranial NCC migration after surface ectoderm removal. The surface ectoderm overlying r3 was removed unilaterally (left side) and embryos were allowed to develop for a further 20 hours in ovo before Sox10 in situ hybridisation. (A) A dorsolateral view of the operated side after 20 hours. Sox10-expressing NCCs form a robust aberrant projection between the r2 and r4 NCC streams, similar to that seen in r3 removal experiments. (B-D) Serial, slightly oblique, transverse sections through this embryo (broken lines in A show the planes of section in B-D). Arrow in C shows the aberrant NCC projection, while the broken line delineates the dorsoventral extent of surface ectoderm removal. The progression of the phenotype was determined by DiO labelling of dorsal r2 and by DiI labelling of dorsal r4, prior to removal of the r3 surface ectoderm. (E) Combined phase/DiO/DiI dorsal view after 5 hours; (F) DiO/DiI only. Broken lines delineate rhombomere boundaries and outline the neural tube. Some r4 cells (red) deviate rostrally into r3 mesenchyme. (G) A different embryo after 5 hours, revealing some aberrant rostral migration of Sox10-expressing r4 NCCs (arrow). (H) Combined phase/DiO/DiI dorsal view after 10 hours; (I) DiO/DiI only. Predominantly, r4 cells (red) enter r3 mesenchyme. (J) A different embryo after 10 hours, showing a bridge of Sox10-expressing NCC traversing r3 mesenchyme. (K,L) DiI labelling of surface ectoderm prior to r3 surface ectoderm removal revealed that labelled ectodermal cells had only occasionally re-grown into r3 mesenchyme after 10 hours (L shows combined phase/DiI; broken lines delineate the rostrocaudal extent of surface ectoderm removal). (M-T) Separate r2 DiI labelling and r4 DiI labelling experiments were performed to examine cell migration after 20 hours. (M,N) Several r2 cells migrate aberrantly into r3 mesenchyme (arrow in N). (O) The same embryo processed for Sox10 in situ reveals a more sharply defined bridge of Sox10-expressing NCCs through r3 mesenchyme. (P,Q) Several r4 cells also migrate aberrantly into r3 mesenchyme after 20 hours (arrow in Q). (R) Same embryo processed for Sox10 in situ reveals a band of NCCs through r3 mesenchyme. (S,T) DiI labelling of surface ectoderm before r3 surface ectoderm removal revealed that several labelled ectodermal cells had re-grown into r3 mesenchyme after 20 hours (arrow), suggesting that the more disperse Sox10-negative cells within r3 mesenchyme may be of ectodermal origin (T shows combined phase/DiI). ba1 and ba2, branchial arches 1 and 2.