Fig. 2. Irx1 expression is absent from tooth germs in activin ßA mutants. Radioactive in situ hybridisation was used to compare Irx1 expression in frontal sections of wild-type (+/+, A,E) and mutant (-/-, B,D) heads. Irx1 expression is lost specifically from the epithelia of all incisor (B) and molar tooth buds (F) in mutants. Expression in non-odontogenic tissues such as the superficial dermis of the skin is, however, normal (white arrows, A,B,E,F). The position of the incisor and molar tooth buds in the mutant tissue is indicated by broken orange lines on the darkfield photomicrographs (B,F), corresponding brightfield (bf) photomicrographs (C,G, black arrowheads) and mesenchymal Pax9 expression in consecutive serial sections (D,H), which underlies the oral epithelium and is unaffected in the activin ßA mutants. Note that bud stage tooth germs of wild-type (E13.5) and mutant heads (E14.5) are compared here.
