Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells.

Fate maps based on quail-chick grafting of avian cephalic neural crest precursors and paraxial mesoderm cells have identified the majority of derivatives from each population but have not unequivocally resolved the precise locations of and population dynamics at the interface between them. The relation between these two mesenchymal tissues is especially critical for the development of skeletal muscles, because crest cells play an essential role in their differentiation and subsequent spatial organization. It is not known whether myogenic mesoderm and skeletogenic neural crest cells establish permanent relations while en route to their final destinations, or later at the sites where musculoskeletal morphogenesis is completed. We applied beta-galactosidase-encoding, replication-incompetent retroviruses to paraxial mesoderm, to crest progenitors, or at the interface between mesodermal and overlying neural crest as both were en route to branchial or periocular regions in chick embryos. With respect to skeletal structures, the results identify the avian neural crest:mesoderm boundary at the junction of the supraorbital and calvarial regions of the frontal bone, lateral to the hypophyseal foramen, and rostral to laryngeal cartilages. Therefore, in the chick embryo, most of the frontal and the entire parietal bone are of mesodermal, not neural crest, origin. Within paraxial mesoderm, the progenitors of each lineage display different behaviors. Chondrogenic cells are relatively stationary and intramembranous osteogenic cells move only in transverse planes around the brain. Angioblasts migrate invasively in all directions. Extraocular muscle precursors form tightly aggregated masses that en masse cross the crest:mesoderm interface to enter periocular territories, while branchial myogenic lineages shift ventrally coincidental with the movements of corresponding neural crest cells. En route to the branchial arches, myogenic mesoderm cells do not maintain constant, nearest-neighbor relations with adjacent, overlying neural crest cells. Thus, progenitors of individual muscles do not establish stable, permanent relations with their connective tissues until both populations reach the sites of their morphogenesis within branchial arches or orbital regions.