(A–C) Frontal sections through the first molar tooth germs (arrows) of E18.5 wildtype (A), Msx−/− (B), and Msx1−/−Osr2−/− (C) embryos. (D–I) Bmp4 mRNA expression in the first molar tooth mesenchyme (arrows) in wildtype (D, G), Msx1−/− (E, H), and Msx1−/−Osr2−/− (F, I) embryos at E13.5 (D–F) and E14.5 (G-I). (J–L) Lef1 mRNA expression in the first molar tooth mesenchyme (arrows) in E15 wildtype (J), Msx1−/− (K), and Msx1−/−Osr2−/− (L) littermates. ps, palatal shelf; to, tongue. Scale bar, 200 µm.