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.