Fig. 2. Conditional inactivation of ß-catenin in Pax2⁺ cells promotes epidermal fates at the expense of the otic placode. (A) Cre-loxP reporter analysis of Pax2-Cre mice (left) and whole-mount images of the ß-catenin CKO embryo at E10.5 from lateral (top) and dorsal (bottom) aspects. Cre-loxP reporter signal (green) is detected in the Pax2⁺ ectoderm from E8.5. By E8.75, reporter signal is detected both in the thickened otic placode and in the ectoderm (brackets) lateral to the otic placode. DAPI staining is shown in magenta. By E10.5, reporter signal (blue) is detected in virtually the entire otic vesicle (arrowheads). In E10.5 ß-catenin CKO embryos, the mid-hindbrain region is entirely missing (asterisk) and the size of the otic vesicle (broken line) is significantly smaller than controls. (B) Whole-mount in situ hybridization of otic, epidermal and hindbrain markers in ß-catenin CKO embryos. Pax2 and Pax8 are downregulated in presumptive otic ectoderm (arrowheads) and in the mid-hindbrain boundary of CKO embryos (asterisks) at E8.5. The epidermal marker Foxi2 indicates the size of the otic placode as a white patch (dashed oval). In E8.5 CKO embryos the Pax2/Pax8-expressing domain is smaller and the Foxi2⁺ ectodermal domain is expanded. At E8.75, Krox20 marks rhombomeres (r) 3 and 5 in the CKO and control. A small otic cup (dashed oval) is formed adjacent to r5. The posterior end of the otic cup in CKO is the same level as that in control. Dlx5 expression is greatly diminished in CKO embryos, but remains strongly expressed in the rim of the otic cup in control embryos. Scale bars: 100 µm in fluorescent images.