ENDODERM INDUCTION BY THE ORGANIZER-SECRETED FACTORS CHORDIN AND NOGGIN IN XENOPUS ANIMAL CAPS

Spemann's organizer has potent neural inducing and mesoderm dorsalizin g activities in the Xenopus gastrula. A third activity, the organizer' s ability to induce a secondary gut, has been difficult to analyze exp erimentally due to the lack of early gene markers, Here we introduce e ndodermin, a pan-endodermal gene marker, and use it to demonstrate tha t chordin (Chd), a protein secreted by the organizer region, is able t o induce endodermal differentiation in Xenopus. The ability of chd, as well as that of noggin, to induce endoderm in animal cap explants is repressed by the ventralizing factor BMP-4. When FGF signaling is bloc ked by a dominant-negative FGF receptor in chd-injected animal caps, n eural induction is inhibited and most of the explant is induced to bec ome endoderm. The results suggest that proteins secreted by the organi zer, acting together with known peptide growth factors, regulate diffe rentiation of the endodermal germ layer.