RESUMO
Treatment of early gastrula- and neurula-staged Xenopus embryos with all-trans retinoic acid (RA) results in truncation of the anterior structures of the forebrain and head. The extent of truncation is dependent upon both the stage of immersion and the RA concentration used. As a method to investigate genes important during early forebrain regionalization, late gastrula and neurula embryos were immersed for 2h within low (1x10(-9)M to 5x10(-8)M) concentrations of RA. Embryos were allowed to develop to tadpole stages and forebrain marker genes were assessed for any alteration in patterns of expression. Comparisons of controls to experimental groups indicated that the greatest sensitivity to low levels of RA occurred just before the initial expression of the forebrain-specific markers investigated. We concluded that forebrain regionalization and gene expression occurred in the following order: Xotx2-->Xsix3-->Xrx (&Xfez1)-->Xbf1-->Xemx1. Xsix3 seems to be very important for the initial parcellation of telencephalon, retinal and diencephalon areas.