Expression patterns of Fgf9, Fgfr2 and Fgfr3. In situ hybridizations were done on sections of heads from nontransgenic (NT) and FGF9 transgenic mice using [35S]-labeled riboprobes for Fgf9 (A-D'), Fgfr2 (E-H') and Fgfr3 (I-L'). Panels A-L are bright field images and panels A'-L' are the corresponding dark-field images. Expression of Fgf9 is not seen in the cranial mesenchymal (cm) cells of the nontransgenic mice at E11 (A') or E13 (C') in contrast to age-matched sections of FGF9 transgenic heads (B' and D'). Modest Fgfr2 expression is detected in the cranial mesenchymal cells at E11 in both the transgenic and nontransgenic mice (E' and F') and persists in the expanded cranial mesenchyme at E13 (H'). Fgfr3 expression is initially not seen in the cranial mesenchymal (cm) cells at E11 (J') but low level expression can be detected in the transgenic mesenchyme at E13 (L'). Scale bar: 100 μm in A-L'.