EXPRESSION OF XENOPUS SNAIL IN MESODERM AND PROSPECTIVE NEURAL FOLD ECTODERM

Expression of the Xsna gene during Xenopus laevis embryogenesis has be en analysed by in situ hybridisation. Like its homologue snail in Dros ophila, Xsna is expressed zygotically in all early mesoderm. Expressio n starts during stage 9 in the dorsal marginal zone and spreads to the ventral side by stage 10. During gastrulation, each cell begins to ex press 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 th at reveals the subdivision of the mesoderm before its derivatives are overtly differentiated; e.g., the appearance of the notochord, myotome s, and pronephroi are preceded by the disappearance of Xsna mRNA, whil e undifferentiated mesoderm remains labelled, even into tadpole stages . Xsna is expressed in the suprablastoporal endoderm during gastrulati on and in its derivatives, the prechordal and sub-notochordal endoderm , during neurulation. Relationships between Xbra, Xtwi, and Xsna expre ssion are examined. Xsna is also expressed in the prospective neural f old ectoderm from stage 11 in a low arc above the dorsal marginal zone , precisely identifying a distinct band of cells that surrounds the pr ospective 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 d eep and superficial ectoderm compartments labelled by Xsna expression are the prospective neural crest and prospective roof of the neural tu be, respectively. Xsna expression persists in the neural crest during migration and in some derivatives at least until metamorphosis but cea ses in the roof of the neural tube soon after neurulation. (C) 1993 Wi ley-Liss, Inc.