Fig. 8. Model of reciprocal FGF signaling in cecum development. During early cecal development, epithelial expression of FGF9 signals to mesenchymal FGFRs (FGFR1c and FGFR2c) to stimulate local mesenchymal proliferation and gene expression (Fgf10, Bmp4). Mesenchymal FGF10 signals to the epithelial FGFR2b to stimulate epithelial proliferation and extension of the epithelial bud into the expanded cecal mesenchyme. FGF9 is only able to induce mesenchymal proliferation and Fgf10 expression in the permissive junctional mesenchyme.