A DISTINCT DEVELOPMENTAL PROGRAM FOR THE CRANIAL PARAXIAL MESODERM INTHE CHICK-EMBRYO

Cells of the cranial paraxial mesoderm give rise to parts of the skull and muscles of the head. Some mesoderm cells migrate from locations c lose to the hindbrain into the branchial arches where they undergo mus cle differentiation. We have characterised these migratory pathways in chick embryos either by DiI-labelling cells before migration or by gr afting quail cranial paraxial mesoderm orthotopically, These experimen ts demonstrate that depending on their initial rostrocaudal position, cranial paraxial mesoderm cells migrate to fill the core of specific b ranchial arches. A survey of the expression of myogenic genes showed t hat the myogenic markers Myf5, MyoD and myogenin were expressed in bra nchial arch muscle, but at comparatively late stages compared with the ir expression in the somites, Pax3 was not expressed by myogenic cells that migrate into the branchial arches despite its expression in migr ating precursors of limb muscles. In order to test whether segmental p late or semitic mesoderm has the ability to migrate in a cranial locat ion, we grafted quail trunk mesoderm into the cranial paraxial mesoder m region. While segmental plate mesoderm cells did not migrate into th e branchial arches, semitic cells were capable of migrating and were i ncorporated into the branchial arch muscle mass. Grafted semitic cells in the vicinity of the neural tube maintained expression of the semit ic markers Pax3, MyoD and Pax1. By contrast, ectopic semitic cells loc ated distal to the neural tube and in the branchial arches did not exp ress Pax3, These data imply that signals in the vicinity of the hindbr ain and branchial arches act on migrating myogenic cells to influence their gene expression and developmental pathways.