MCCAULEY, D.W.; BRONNER-FRASER, M.; California Institute of Technology; California Institute of Technology: Sox9 expression in neural crest cells of the branchial arches affects cartilage development in the lamprey, Petromyzon marinus

Cranial neural crest cells form numerous vertebrate structures, including craniofacial cartilage. In the lamprey, the basal-most extant vertebrate, ablation experiments provide indirect evidence that neural crest cells contribute to the cartilages of the branchial arches. In the present study, we use a combination of cell and molecular markers to directly characterize neural crest migration and branchial arch cartilage formation in the lamprey, Petromyzon marinus. Lamprey cartilage forms only in the rostrolateral quadrant of each arch whereas they form medially in gnathostomes. By DiI-labeling the neural folds, we show that neural crest cells migrate into both medial and lateral positions of the lamprey pharyngeal arches, even though cartilages form in the lateral position only. Later, DiI-labelled cells form condensations in the rostrolateral quadrant of the arches. By cloning the lamprey homologue of Sox9, we find that lampSox9 is expressed in premigratory neural crest, is down-regulated during migration and subsequently re-expressed in chondrogenic cells of the branchial arches. The later expression of lamprey Sox9 mirrors that of the DiI-labelled cells in the arches. To test whether Sox9 is functionally important in branchial arch development, we injected Sox9 antisense morpholino oligonucleotides into the lamprey fertilized egg. The embryos developed abnormally, with marked truncation of the pharynx, suggesting that Sox9 is required for proper pharyngeal development. These results suggest that pharyngeal cartilage development in the lamprey employs similar, probably homologous, developmental mechanisms as those described in gnathostomes.