Fig. 6. Wnt signaling regulates Sp8 expression. (A) Implantation of a control cell pellet had no effect on Sp8 expression in the chick limb bud. (B) Implantation of a Wnt3a-expressing cell pellet induced Sp8 expression in the ectoderm (arrowheads). Asterisks indicate pellets. (C) Injection of RCAS dominant-active ß-catenin induced Sp8 expression in the wide region of ectoderm. (D) Electron microscopic analysis of RCAS dominant-active ß-catenin-injected limb. ß-Catenin activity induced ectopic ridge-like spikes. The inset in D is a closer view of the ectoderm. (E,F) Sp8 expression in the forelimb bud in wild-type (E) and Lrp6^–/– (F) embryos at E10.5. The Sp8 expression domain in E and F are labeled with black and red arrowheads, respectively. (G,H) Sp8 expression in the forelimb bud in wild-type (G) and Dkk1^–/– (H) embryos at E12.5. In Dkk1^–/– embryos, the Sp8 expression is stronger in the AER (blue arrow) and ectopically induced outside the AER (blue arrowhead).