v-erbA and citral reduce the teratogenic effects of all-trans retinoic acid and retinol, respectively, in Xenopus embryogenesis.

Treatment of late blastula/early gastrula stage Xenopus embryos with all-trans retinoic acid results in disruption of the primary body axis through effects on both mesoderm and neuroectoderm. This effect of retinoic acid, coupled with the known presence of retinoic acid in Xenopus embryos has led to the proposal that retinoic acid may be an endogenous morphogen providing positional information in early development. To further elucidate the role of retinoic acid in early Xenopus development, we have attempted to interfere with the retinoic acid signalling pathway both at the level of retinoic acid formation, by treatment with citral (3,7-dimethy-2,6-octadienal), and at the level of nuclear retinoic acid receptor function, by microinjection of v-erbA mRNA. The feasibility of this approach was demonstrated by the ability of citral treatment and v-erbA mRNA injection to reduce the teratogenic effects of exogenous retinol and retinoic acid, respectively, in early Xenopus development. Interestingly, v-erbA mRNA injection and citral treatment of gastrula stage embryos resulted in tadpoles with a similar set of developmental defects. The defects were chiefly found in tissues that received a contribution of cells from the neural crest, suggesting that at least a subset of neural crest cells may be sensitive to the endogenous level of retinoic acid. In accord with this proposal, it was found that the expression patterns of two early markers of cranial neural crest cells, Xtwi and XAP-2, were altered in embryos injected with v-erbA mRNA. These results indicate that structures in addition to the primary axis are regulated by retinoic acid signalling during early Xenopus development.