Effects of retinoic acid on Xenopus embryos.

Retinoic acid (RA) has powerful dose-dependent dysmorphogenic effects on Xenopus embryos. The defects produced are dependent upon the stage and duration of treatment. Characteristic dysmorphogenic effects occur in the visceral (branchial) arch region and the tail. These regions correspond to the domains of expression of the retinoid receptors RAR gamma and RXR beta. By expressing domain-swapped and dominant negative derivatives of RAR gamma in embryos, we have shown that the effects of RA are mediated transcriptionally. Furthermore, the expression of dominant negative receptors in the absence of exogenous ligand has no apparent harmful effect upon early development. Finally, we have identified a novel MAP kinase phosphatase whose expression pattern is localized to regions similar to RAR gamma, and which is upregulated by RA.