Fig. 3.

Elevated Wnt signaling in the late primitive streak of Axin2^canp embryos. (A–H) Canonical Wnt activity assayed by TOPGAL staining is elevated in the late streak of Axin2^canp embryos. (A and B) TOPGAL is strong in the wild-type primitive streak of e7.5 embryos and decreases by e8.5. (C) TOPGAL expression in a section through an e9.0 wild-type embryo (14–16 somites) at the anterior limit of the presomitic mesoderm. (D) TOPGAL activity is detected in the primitive streak of e7.5 Axin2^canp embryos. (E) Elevated TOPGAL activity in the primitive streak of e8.5 Axin2^canp embryos. (F) Section through an e9.0 Axin2^canp embryo at the same level as in C shows that TOPGAL is elevated in the epiblast and presomitic mesoderm (arrows), but relatively low in the mutant notochord (arrowheads). (G and H) β-Galactosidase activity assays show that canonical Wnt activity is low in the anterior (G), but not posterior region (H) of mutant embryos. (I–L) Elevated Wnt activity leads to the formation of a duplicated primitive streak in the canp-doubletail embryos. (I and J) Expression of T. (I) T is expressed in the primitive streak of wild-type embryos. (J) In canp-double tail embryos, both a normal-appearing primitive streak and a smaller, dorsal protrusion shaped like a primitive streak express T (arrow) (J). Meox1, expressed in the wild-type somites and presomitic mesoderm (K), is expressed in the mutant double tail (L, arrow). (Scale bars, 100 mm.)