Regional Specification of the Head and Trunk-Tail Organizers of a Urodele (Cynops pyrrhogaster) Embryo Is Patterned during Gastrulation

The dorsal marginal zone (DMZ) of an amphibian early gastrula is thought to consist of at least two distinct domains: the future head and trunk–tail organizers. We studied the mechanism by which the organizing activities of the lower half of the DMZ (LDMZ) of the urodelean (Cynops pyrrhogaster) embryo are changed. The uninvoluted LDMZ induces the notochord and then organizes the trunk–tail structures, whereas after cultivation in vitro or suramin treatment, the same LDMZ loses the notochord-inducing ability and organizes the head structures. A cell-lineage experiment indicated that the change in the organizing activity of the LDMZ was reflected in the transformation of the inductive ability: from notochord-inducing to neural-inducing activity. Using RT-PCR, we showed that the LDMZ expressed gsc, lim-1, chordin, and noggin, but not the mesoderm marker bra. In the sandwich assay, the LDMZ induced bra expression in the animal cap ectoderm, but the inductive activity was inhibited by cultivation or suramin treatment. The present study indicates that the change in the organizing activity of the LDMZ from trunk–tail to head is coupled with the loss of notochord-inducing activity. Based on these results, we suggest that this change is essential for the specification of the head and trunk–tail organizers during gastrulation.