Expression of Xenopus snail in mesoderm and prospective neural fold ectoderm.

Expression of the Xsna gene during Xenopus laevis embryogenesis has been analysed by in situ hybridisation. Like its homologue snail in Drosophila, Xsna is expressed zygotically in all early mesoderm. Expression starts during stage 9 in the dorsal marginal zone and spreads to the ventral side by stage 10. During gastrulation, each cell begins to express as it involutes so that cells newly expressing Xsna are added to the forming mesoderm mantle in an anterior-to-posterior progression. Xsna expression is then down-regulated in a tissue-specific fashion that reveals the subdivision of the mesoderm before its derivatives are overtly differentiated; e.g., the appearance of the notochord, myotomes, and pronephroi are preceded by the disappearance of Xsna mRNA, while undifferentiated mesoderm remains labelled, even into tadpole stages. Xsna is expressed in the suprablastoporal endoderm during gastrulation and in its derivatives, the prechordal and sub-notochordal endoderm, during neurulation. Relationships between Xbra, Xtwi, and Xsna expression are examined. Xsna is also expressed in the prospective neural fold ectoderm from stage 11 in a low arc above the dorsal marginal zone, precisely identifying a distinct band of cells that surrounds the prospective neural plate that we designate the neural plate border. The anterior transverse neural fold, which becomes forebrain, ceases Xsna expression during neurulation. In the longitudinal neural folds, the deep and superficial ectoderm compartments labelled by Xsna expression are the prospective neural crest and prospective roof of the neural tube, respectively. Xsna expression persists in the neural crest during migration and in some derivatives at least until metamorphosis but ceases in the roof of the neural tube soon after neurulation.