Stem Cells in Immortal Hydra

Abstract

Hydra’s potential immortality and extensive capacity to regenerate and self-renew is due to the presence of three distinct stem cell lineages: ectodermal and endodermal epithelial stem cells, and interstitial stem cells. Over the last few years, stem cells in Hydra became well-defined in cellular terms of their biology. More recently, efforts using the nearly unlimited potential for tissue manipulation combined with functional transgenesis have shed light on the molecular control mechanisms involved. Here I review those efforts in an attempt to give both a historical perspective and an update on the recent experimental highlights. In particular, I will focus on six aspects of stem cells in Hydra: (i) their continuous transition through the proliferation/differentiation switch; (ii) their rapid responses to signals from the cellular environment; (iii) the emerging importance of Wnt and Notch signaling in controlling stem cell behavior; (iv) the role of chromatin modification in terminal differentiation; (v) the observation of transdifferentiation in some of the stem cell progeny; and (vi) the implications for the evolution of germ cells, ageing and cancer. Together, these findings seem to indicate that Hydra not only provides insights into signalling pathways involved in stem cell differentiation in the Bilaterian ancestor; they also demonstrate that despite morphological and functional differences, and more than 500 million years of phylogenic separation between Hydra and human, common signaling pathways are responsible for stem cell maintenance, lineage determination, and differentiation.