Analysis of cranial neural crest migratory pathways in axolotl using cell markers and transplantation

We have examined the ability of normal and heterotopically transplanted neu ral crest cells to migrate along cranial neural crest pathways in the axolo tl using focal DiI injections and in situ hybridization with the neural cre st marker, AP-2, DiI labeling demonstrates that cranial neural crest cells migrate as distinct streams along prescribed pathways to populate the maxil lary and mandibular processes of the first branchial arch, the hyoid arch a nd gill arches 1-4, following migratory pathways similar to those observed in other vertebrates. Another neural crest marker, the transcription factor AP-2, is expressed by premigratory neural crest cells within the neural fo lds and migrating neural crest cells en route to and within the branchial a rches. Rotations of the cranial neural folds suggest that premigratory neur al crest cells are not committed to a specific branchial arch fate, but can compensate when displaced short distances from their targets by migrating to a new target arch. In contrast, when cells are displaced far from their original location, they appear unable to respond appropriately to their new milieu such that they fail to migrate or appear to migrate randomly. When trunk neural folds are grafted heterotopically into the head, trunk neural crest cells migrate in a highly disorganized fashion and fail to follow nor mal cranial neural crest pathways. Importantly, we find incorporation of so me trunk cells into branchial arch cartilage despite the random nature of t heir migration. This is the first demonstration that trunk neural crest cel ls can form cartilage when transplanted to the head. Our results indicate t hat, although cranial and trunk neural crest cells have inherent difference s in ability to recognize migratory pathways, trunk neural crest can differ entiate into cranial cartilage when given proper instructive cues.