The FGFR pathway is required for the trunk-inducing functions of Spemann'sorganizer

Xenopus laevis embryogenesis is controlled by the inducing activities of Sp emann's organizer. These inducing activities are separated into two distinc t suborganizers: a trunk organizer and a head organizer. The trunk organize r induces the formation of posterior structures by emitting signals and dir ecting morphogenesis. Here, we report that the fibroblast growth factor rec eptor (FGFR) signaling pathway, also known to regulate posterior developmen t, performs critical functions within the cells of Spemann's organizer. Spe cifically, the FGFR pathway was required in the organizer cells in order fo r those cells to induce the formation of somitic muscle and the pronephros. Since the organizer influences the differentiation of these tissues by emi tting signals that pattern the mesodermal germ layer, our data indicate tha t the FGFR regulates the production of these signals. In addition, the FGFR pathway was required for the expression of chordin, an organizer-specific protein required for the trunk-inducing activities of Spemann's organizer. Significantly, the FGFR pathway had a minimal effect on the function of the head organizer. We propose that the FGFR pathway is a defining molecular c omponent that distinguishes the trunk organizer from the head organizer by controlling the expression of organizer-specific genes required to induce t he formation of posterior structures and somitic muscle in neighboring cell s. The implications of our findings for the evolutionarily conserved role o f the FGFR pathway in the functions of Spemann's organizer and other verteb rate-signaling centers are discussed. (C) 2001 Academic Press.