The surface ectoderm is essential for nephric duct formation in intermediate mesoderm

The nephric duct is the first epithelial tubule to differentiate from inter mediate mesoderm that is essential for all further urogenital development. In this study we identify the domain of intermediate mesoderm that gives ri se to the nephric duct and demonstrate that the surface ectoderm is require d for its differentiation. Removal of the surface ectoderm resulted in decr eased levels of Sim-1 and Pax-2 mRNA expression in mesenchymal nephric duct progenitors, and caused inhibition of nephric duet formation and subsequen t kidney development. The surface ectoderm expresses BMP-4 and we show that it is required for the maintenance of high-level BMP-4 expression in later al plate mesoderm. Addition of a BMP- 4-coated bead to embryos lacking the surface ectoderm restored normal levels of Sim-1 and Pax-2 mRNA expression in nephric duct progenitors, nephric duct formation and the initiation of n ephrogenesis, Thus, BMP-4 signaling can substitute for the surface ectoderm in supporting nephric duct morphogenesis. Collectively, these data suggest that inductive interactions between the surface ectoderm, lateral mesoderm and intermediate mesoderm are essential for nephric duct formation and the initiation of urogenital development.