The role of the neural crest in patterning of avian cranial skeletal, connective, and muscle tissues.

The morphology of skeletal tissues formed in each of the branchial arches of higher vertebrates is unique. In addition to these structures, which are derived from the neural crest, the crest-derived connective tissues and mesodermal muscles also form different patterns in each of the branchial arches. The objective of this study was to examine how these patterns arise during avian embryonic development. Presumptive second or third arch neural crest cells were excised from chick hosts and replaced with presumptive first arch crest cells. Both quail and chick embryos were used as donors; orthotopic crest grafts were performed as controls. Following heterotopic transplantation, the hosts developed several unexpected anomalies. Externally they were characterized by the appearance of ectopic, beak-like projections from the ventrolateral surface of the neck and also by the formation of supernumerary external auditory depressions located immediately caudal to the normal external ear. Internally, the grafted cells migrated in accordance with normal, second arch pathways but then formed a complete, duplicate first arch skeletal system in their new location. Squamosal, quadrate, pterygoid, Meckel's, and angular elements were present in most cases. In addition, anomalous first arch-type muscles were found associated with the ectopic skeletal tissues in the second arch. These results indicate that the basis for patterning of branchial arch skeletal and connective tissues resides within the neural crest population prior to its emigration from the neural epithelium, and not within the pharynx or pharyngeal pouches as had previously been suggested. Furthermore, the patterns of myogenesis by mesenchymal populations derived from paraxial mesoderm is dependent upon properties inherent to the neural crest.