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) int eraction evokes morphogenetic events similar to those that occur in regener ation. However, it is not yet understood whether the stem tells themselves or differentiated cells have the ability to induce regeneration. Here we de monstrated by a combination of X-ray irradiation and grafting experiments t hat the dorsal and ventral positional cues inducing morphogenetic events ar e 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 a n intact one in DV-reversed orientation. We observed that projections were developed from the host-donor boundary, as in the previous experiments. Who le-mount in situ hybridization with several markers demonstrated that the p rojections had a newly established DV axis and also had anterior or posteri or characteristics. furthermore, chimeric analysis with a strain-specific m arker 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 to gether, the findings suggest that the stem cells may simply differentiate d epending on the surroundings and that differentiated cells may present posi tional cues that induce morphogenesis. (C) 2001 Academic Press.