Induction of the mesendoderm in the zebrafish germ ring by yolk cell-derived TGF-beta family signals and discrimination of mesoderm and endoderm by FGF

The endoderm forms the gut and associated organs, and develops from a layer of cells which emerges during gastrula stages in the vertebrate embryo. In comparison to mesoderm and ectoderm, little is known about the signals whi ch induce the endoderm, The origin of the endoderm is intimately linked wit h that of mesoderm, broth by their position in the embryo, and by the molec ules that can induce them. We characterised a gene, zebrafish gata5, which is expressed in the endoderm from blastula stages and show that its transcr iption is induced by signals originating from the yolk cell. These signals also induce the mesoderm-expressed transcription factor no tail (ntl) whose initial expression coincides with gata5 in the cells closest to the blasto derm margin, then spreads to encompass the germ ring, We have characterised the induction of these genes and show that ectopic expression of activin i nduces gata5 and ntl in a pattern which mimics the endogenous expression, w hile expression of a dominant negative activin receptor abolishes ntl and g ata5 expression. Injection of RNA encoding a constitutively active activin receptor leads to ectopic expression of gata5 and ntl, gata5 is activated c ell-autonomously, whereas ntl is induced in cells distant from those which have received the RNA, showing that although expression of both genes is in duced by a TGF-beta signal, expression of ntl then spreads by a relay mecha nism, Expression of a fibroblast growth factor (eFGF) or a dominant negativ ely acting FGF receptor shows that ntl but not gata5 is regulated by FGF si gnalling, implying that this mag be the relay signal leading to the spread of ntl expression. In embryos lacking both squint and cyclops, members of t he nodal group of TGF-beta related molecules, gata5 expression in the blast oderm is abolished, making these factors primary candidates for the endogen ous TGF-beta signal inducing gafa5.