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Dorsal and ventral positional cues required for the onset of planarian regeneration may reside in differentiated cells.

We previously showed by grafting experiments that the dorsoventral (DV) interaction evokes morphogenetic events similar to those that occur in regeneration. However, it is not yet understood whether the stem cells themselves or differentiated cells have the ability to induce regeneration. Here we demonstrated by a combination of X-ray irradiation and grafting experiments that the dorsal and ventral positional cues inducing morphogenetic events are retained in X-ray-irradiated tissues, suggesting that the differentiated cells may be responsible for the positional cues. We grafted a small piece of irradiated worm, in which the stem cells were certainly eliminated, to an intact one in DV-reversed orientation. We observed that projections were developed from the host-donor boundary, as in the previous experiments. Whole-mount in situ hybridization with several markers demonstrated that the projections had a newly established DV axis and also had anterior or posterior characteristics. Furthermore, chimeric analysis with a strain-specific marker showed that the projections consisted of nonirradiated cells and that IFb-expressing cells, which normally belonged to the ventral tissue, could be generated even from the stem cells located on the dorsal side. Taken together, the findings suggest that the stem cells may simply differentiate depending on the surroundings and that differentiated cells may present positional cues that induce morphogenesis.