The interstitial stem cells in Hydractinia and their role in regeneration

Hydractinia species have been animal models in developmental biology and comparative immunology for over a century, but are having a renaissance due to the establishment of modern genetic and genomic tools by the growing community of researchers utilizing them. Hydractinia has a predictable and accessible life cycle and its stem cell system, known as interstitial- or i-cells has been a paradigm for animal stem cells since the late 1800s. In adult Hydractinia, i-cells continuously provide progenitors to sustain clonal growth, tissue homeostasis, sexual reproduction and regeneration. We review recent developments in stem cell and regeneration research centered on this animal. Hydractinia joins an established team of cnidarian genetic models in times of rapid progress in these disciplines. While each animal is particularly suited to specific experimental settings, jointly they can provide an integrative insight into the diversity of animal stem cell systems, how they drive regeneration, and how they evolved.