Mesoderm formation in Xenopus ectodermal explants overexpressing Xwnt8: evidence for a cooperating signal reaching the animal pole by gastrulation.

It is demonstrated here that the ability of injected Xwnt8 RNA to trigger mesoderm formation in Xenopus presumptive ectoderm (animal caps) depends on the time of explantation. Animal caps isolated from Xwnt8 injected embryos at the late blastula/early gastrula stages differentiate mesodermal tissues whereas caps isolated from early blastula do not. This finding suggests that an endogenous signal reaches the animal cap by the late blastula stage and cooperates with Xwnt8 to induce mesoderm. Similarly, late animal caps isolated at st. 10 from lithium-treated embryos, but not those from control embryos, elongate and express muscle-specific actin transcripts. In addition, the data presented suggests that the cooperating signal is distributed homogeneously with respect to the future dorsoventral axis and may require FGF- and activin-dependent signal transduction pathways. These observations support a model in which mesoderm is induced in vivo by a combined action of several different signals.