Migration of multipotent interstitial stem cells in Hydra.

Stem cells in Hydra represent one of the phylogenetically most ancient stem cell systems and, therefore, provide information for reconstructing the early history of stem cell control mechanisms. Hydra's interstitial stem cells are multipotent and differentiate into both somatic cell types and germ line cells. Although it is well accepted that cells of the interstitial cell lineage are migratory, the in vivo migratory potential of multipotent interstitial stem cells has never been explored. Combining in vivo tracing of genetically labeled interstitial stem cells and tissue transplantation, we show that in contrast to precursor cells, multipotent interstitial stem cells are stationary. Only when exposed to tissue depleted of the interstitial cell lineage, interstitial stem cells start to migrate and to repopulate emptied stem cell niches. We conclude that multipotent interstitial stem cells in Hydra are static and that microenvironmental cues including signals derived from the interstitial cell lineage or from niche cells can trigger a shift in collective stem cell behavior to start migration.