EXPRESSION OF RET IN THE CHICKEN-EMBRYO SUGGESTS ROLES IN REGIONALIZATION OF THE VAGAL NEURAL-TUBE AND SOMITES AND IN DEVELOPMENT OF MULTIPLE NEURAL CREST AND PLACODAL LINEAGES

In a screen for receptor tyrosine kinase genes regionally expressed in the developing hindbrain, we cloned and characterised the chicken ret gene. Data derived from studies of congenital human disease and from disruption of murine ret have demonstrated roles for ret in developmen t of the kidney and enteric nervous system; the latter has been most w ell-studied in the avian embryo. In agreement with studies of the mous e embryo, we find expression of ret in both the intermediate mesoderm and the enteric nervous system. However, we additionally detect transc ripts specifically in the vagal neural tube prior to the migration of enteric crest precursors, suggesting a possible earlier function in re gionalisation of the neural tube and vagal neural crest. This spatial restriction in the neural tube is modulated by retinoic acid, but it i s not coordinately regulated with RAR-beta which shares a common anter ior limit of expression with ret in normal embryos. Widespread express ion of ret in placodal and neural crest derivatives raises the possibi lity of other roles in the development of the peripheral nervous syste m. In addition, ret may function in the spatial organisation of the ep ithelial somite, where it might play a role in the specification of an terior dermamyotome.