Temporal alteration of dose-dependent response to activin in newt animal-cap explants

In this study, we examined the dose-dependent responses of animal-pole cells of the Japanese newt, Cynops pyrrhogaster, to activin. Cynops has a slower developmental rate and a simpler animal cap structure than Xenopus. These features enable temporal differences in animal-cap competence to be identified more easily and relatively sharp dose-response profiles can be obtained without cell dissociation. When Cynops caps were excised at the mid-blastula stage and transcript levels of marker genes were examined at the early gastrula stage, the strongest induction of brachyury occurred at a low activin dose, suggesting that cells can recognize changing concentrations of an inducing signal in the embryo. Furthermore, the time course of brachyury expression revealed that caps from the mid-blastula stage exposed to a high dose of activin maintained a low expression level after induction. This suggests that Cynops animal-pole cells can assess activin concentration in a simple and direct manner. In addition, we found that animal-cap competence significantly changes during the blastula stage. The data presented here suggest that this change does not autonomously occur within animal-pole cells but requires signals that emanate from other germ layers.