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Abstract
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.