Fig. 6. Comparison of the patterns of Osr2, Osr1 and Pax9 mRNA expression during palate development. mRNA signals are shown in red in all panels. (A-C) Osr2 mRNA is abundantly expressed throughout the developing palatal shelves from E12.5 to E13.5. Osr2 mRNA is also highly expressed in the developing tooth bud mesenchyme at E13.5 (arrows in C). (D-F) Osr1 mRNA expression is weak in the developing palatal shelves at E12.5 (D) and E13.0 (E) but it is highly abundant in the developing tongue and several regions of the mandible. By E13.5, the lateral halves of the palatal shelves express moderate levels of Osr1 mRNA, while the medial halves of the palatal shelves completely lack Osr1 mRNA expression (F). Arrowheads indicate the sharp boundaries between the lateral Osr1-expressing and the medial Osr1-nonexpressing palatal mesenchyme cells. In contrast to Osr2, Osr1 is not expressed in the developing tooth bud mesenchyme (arrows in F). (G-I) Pax9 mRNA exhibits a lateral to medial expression gradient in the downward growing palatal shelves at E12.5 (G) and E13.0 (H), with higher levels in the lateral regions. By E13.5, Pax9 expression is upregulated in the medial regions of the palatal shelves and the Pax9 mRNA gradient is reversed, with higher levels in the medial regions of the palatal shelves (I). Pax9 mRNA is also expressed in the tooth bud mesenchyme at E13.5 (arrows in I). p, palatal shelf; t, tongue.