Fig. 7. An endogenous AP gradient of Wnt/ß-catenin signalling in the late Xenopus gastrula. (A) Eight-cell stage embryos were injected into the four animal blastomeres with 25 pg/blastomere TOP-Flash or FOP-Flash (firefly luciferase) and 5 pg/blastomere pRL (Renilla luciferase) plasmids, cultured until early neurula stage (stage 13) and cut into four AP slices. Slices from three embryos per measurement were pooled, extracted and a double luciferase assay was performed. Firefly luciferase activity was normalized to Renilla luciferase activity. Relative TOP/FOP-Flash reporter activation is shown on the right. Note increasing AP activation for TOP- but not for FOP-Flash. (B) Eight-cell stage embryos were injected into the four animal blastomeres with 25 pg/blastomere TOP-Flash and 25 pg/blastomere pRL plasmids, cultured until early neurula stage (stage 13) and four AP slices of neuroectoderm were explanted from each embryo. Explants were extracted and a double luciferase assay was performed. Mean reporter activations are shown for three independent experiments (every column represents 90 ectodermal explants).