Fig. 1. Activation of Wnt target genes and expression of Tcf3 during gastrulation. (A-E') Tcf/Lef-ß-catenin transcriptional activation revealed by X-gal staining of TOPGal transgenic embryo. The stage of gastrulation (Downs and Davies, 1993) is indicated in the top right-hand corner; the view displayed is indicated in the lower left-hand corner. The direction of posterior (P) to anterior (A) is indicated by an arrow. Boxed regions are those shown at higher magnification. TOPGal activity marks the posterior EEX border at the onset of gastrulation (A) and remains in the primitive streak region throughout gastrulation. (C-E') Neural-fold staged embryos express TOPGal in the primitive and the node. (E,E') Sagittal sections of TOPGal-stained embryos shows expression in both ectodermal and mesodermal cell types and in the posterior cells of the ventral node. (F-I) Indirect immunofluorescence microscopy with antibodies displayed in the lower right-hand corner (color coded according to the secondary antibody used for detection) and embryo sections of the age and plane of sectioning displayed in the upper right-hand corner. (F) Tcf3 (red) is expressed throughout the embryonic ectoderm before primitive streak induction, and (G,H) diminished posteriorly during gastrulation. (I) Close inspection of the node reveals a gradient of Tcf3 expression rising in concentration from the posterior to the anterior of the node. ES, early streak stage; LS, later streak stage; EEX, embryonic/extra-embryonic border; ENF/LNF, early/late neural-fold stage; end, endoderm; mes, mesoderm; ect, ectoderm; ps, primitive streak.