Fig. 3. Fgf8 is required for mesenchymal Ptx1 expression. (A-C) Ptx1 expression was analyzed by in situ hybridization at E10.5 in wild-type (A) and Fgf8;Foxg1-cre mutant (B,C) embryos. (A,B) A whole-mount staining; (C) a section through the first branchial arch of a stained embryo. Although the ectodermal expression in the rostral first arch epithelium (arrows A-C) was conserved in the mutant, no mesenchymal expression (asterisks, A-C) was observed. (D) Dlx2 is expressed in the branchial arches and frontonasal mass of E10.5 wild-type embryos. Asterisk indicates the area where Ptx1 is also expressed. (E) In E10.5 Fgf8;Foxg1-cre mutant embryos, Dlx2 expression is still detected (asterisk), regardless of the smaller size of the first branchial arch (I). FN, frontonasal mass.