Fig. 4. Fgf9 prevents the mesenchymal fibroblast-myofibroblast transition by inhibition of Tgfβ signaling. Immunofluorescence-stained sections of E14.5, E16.5 and E18.5 small intestines from (A-C,G-I) control and (D-F,J-L) Fgf9^-/- embryos. Sections were stained with antisera directed against -SMA (A-F; green) and p-Smad2/3 (G-L; red). At all three time points, the mesenchyme from control embryos showed no reactivity to -SMA and only scattered p-Smad2/3 positive cells were present in this region. By contrast, -SMA in Fgf9^-/- embryos displayed numerous positive staining cells in the mesenchyme. Nearly all mesenchymal cells in Fgf9^-/- embryos were positive for pSmad2/3. The broken white lines indicate the epithelial-mesenchymal borders. The broken yellow lines indicate the mesenchyme-muscularis propria borders. Scale bar: 30 µm.