Expression pattern of the transgene. In situ hybridizations were done on sections of heads from nontransgenic (NT) and FGF9 transgenic mice using a [35S]-labeled SV40 riboprobe. Panels A-F show bright-field images and panels A'-F' show the corresponding dark field images. Transgene expression can be seen in the cranial mesenchymal cells at E11 (B'), E13 (D') and E15 (F'). At later stages, expression of the transgene in the cranium was not detected (data not shown). Transgene expression was restricted to the mesenchymal cells overlying the mid (m) to hind brain regions (h) but not the forebrain (f) (G, arrows). Scale bar: 100 μm in A-F'. Whole mount in situ hybridizations on E12.5 heads using a digoxygenin-labelled SV40 riboprobe show expression of the transgene (purple color) in the cranial regions (H-K). Panels H and I are rear views and panels J and K are top views. Scale bar: 1 mm in H, I and K.