Molecular Regulation of Decision Making in the Interstitial Stem Cell Lineage of Hydra Revisited

Multipotent interstitial stem cells in the freshwater polyp Hydra define one of the best-studied pre-bilaterian adult cell lineages. They represent a population of small, fast-cycling cells that give rise to three somatic differentiation products (neurons, nematocytes, and gland cells) under conditions of indefinite asexual growth and reproduction, and they also form the gametes when sexual reproduction is initiated. Interstitial stem cells in Hydra and other marine hydrozoans have been studied intensively using sophisticated cellular and molecular methods over several decades. Here, we discuss the properties of interstitial stem cells in Hydra and the known feedback control mechanisms maintaining tissue homeostasis and spatial distribution of interstitial cells along the polyp’s major body axis. We summarize the current state of knowledge about molecular regulation of self-renewal and somatic differentiation, and put particular emphasis on those molecular factors that have been shown to affect decision-making by using methods of functional interference.