Stem cell dynamics in Cnidaria: are there unifying principles?
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The study of stem cells in cnidarians has a history spanning hundreds of years, but it has primarily focused on the hydrozoan genus Hydra. While Hydra has a number of self-renewing cell types that act much like stem cells—in particular the interstitial cell line—finding cellular homologues outside of the Hydrozoa has been complicated by the morphological simplicity of stem cells and inconclusive gene expression data. In non-hydrozoan cnidarians, an enigmatic cell type known as the amoebocyte might play a similar role to interstitial cells, but there is little evidence that I-cells and amoebocytes are homologous. Instead, self-renewal and transdifferentiation of epithelial cells was probably more important to ancestral cnidarian development than any undifferentiated cell lineage, and only later in evolution did one or more cell types come under the regulation of a “stem” cell line. Ultimately, this hypothesis and competing ones will need to be tested by expanding genetic and developmental studies on a variety of cnidarian model systems.
KeywordsCnidaria Stem cells Interstitial cell Amoebocyte Hydra Aurelia
D.A.G. would like to thank Robert Steele and Volker Hartenstein for helpful discussion while preparing this manuscript, with additional thanks to Volker Hartenstein for help in translating Weismann’s text. We gratefully thank two anonymous reviewers for valuable advice, and acknowledge funding from an NIH Training Grant in Genomic Analysis and Interpretation T32HG002536 (D.A.G.) and the National Aeronautics and Space Administration Astrobiology Program (D.K.J.).
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