ANTEROPOSTERIOR NEURAL TISSUE SPECIFICATION BY ACTIVIN-INDUCED MESODERM

The transforming growth factor beta superfamily member, activin, is ab le to induce mesodermal tissues in animal cap explants from Xenopus la evis blastula stage embryos. Activin can act like a morphogen of the d orsoventral axis in that lower doses induce more ventral, and higher d oses more dorsal, tissue types, Activin has also previously been repor ted to induce neural tissues in animal caps. From cell mixing experime nts it was inferred that this might he an indirect effect of induced m esoderm signaling to uninduced ectoderm. Here we demonstrate directly that neural tissues do indeed arise by the action of induced mesoderm on uninduced ectoderm. Dorsal mesoderm is itself subdivided into poste rior and anterior domains in viva, but this had not been demonstrated for induced mesoderm, We therefore tested whether different concentrat ions of activin recreate these different anteroposterior properties as well. We show that the anteroposterior positional value of induced me soderm, including its neuroinductive properties, depends on the dose o f activin applied to the mesoderm, with lower doses inducing more post erior and higher doses giving more anterior markers. We discuss the im plications of these results for patterning signals and the relationshi p between anteroposterior and dorsoventral axes.