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Stem Cell Dynamics in Cnidaria: Are There Unifying Principles?

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Stem Cell Dynamics in Cnidaria: Are There Unifying Principles? Dev Genes Evol (2013) 223:53–66 DOI 10.1007/s00427-012-0429-1 REVIEW Stem cell dynamics in Cnidaria: are there unifying principles? David A. Gold & David K. Jacobs Received: 2 July 2012 /Accepted: 26 October 2012 /Published online: 21 November 2012 # Springer-Verlag Berlin Heidelberg 2012 Abstract The study of stem cells in cnidarians has a history division, and then each produced one of the described spanning hundreds of years, but it has primarily focused on clusters of germ cells … The more I learn about the the hydrozoan genus Hydra.WhileHydra has a number of hydroid organism, the more unlikely it seems to me self-renewing cell types that act much like stem cells—in that specifically differentiated cells, such as the endo- particular the interstitial cell line—finding cellular homo- derm cells, directly transform into germ cells. logues outside of the Hydrozoa has been complicated by the - August Weismann (1883) pg. 46 (trans. by D.A.G) morphological simplicity of stem cells and inconclusive gene In his book Die Entstehung Der Sexualzellen Bei Den expression data. In non-hydrozoan cnidarians, an enigmatic Hydromedusen (The Origin of the Sex Cells in the cell type known as the amoebocyte might play a similar role to Hydromedusa) August Weismann not only provides the interstitial cells, but there is little evidence that I-cells and background to his famous germ plasm theory, but also amoebocytes are homologous. Instead, self-renewal and trans- produces one of the first writings regarding the nature of differentiation of epithelial cells was probably more important the stammzellen, or stem cells. Over the last few decades, to ancestral cnidarian development than any undifferentiated both stem cells and the “hydroids” Weismann studied—now cell lineage, and only later in evolution did one or more cell termed hydrozoans—have been the focus of resurgent bio- types come under the regulation of a “stem” cell line. logical inquiry. Ultimately, this hypothesis and competing ones will need to Hydrozoans are part of the phylum Cnidaria, along with be tested by expanding genetic and developmental studies on the anthozoans (sea pens, corals, and sea anemones), scypho- a variety of cnidarian model systems. zoans (“true” jellyfish), and cubozoans (box jellies) (Fig. 1a). Together, the cnidarians are thought to represent some of the Keywords Cnidaria . Stem cells . Interstitial cell . simplest living animals; they are classically considered to Amoebocyte . Hydra . Aurelia have “tissue-grade” complexity (lacking true organs like most animals), with radial (as opposed to bilateral) symmetry, and One could interpret [my observations] as meaning that two embryological germ layers (as opposed to the typical certain interstitial cells of the ectoderm are the stem three) (Brusca and Brusca 2003). Interest in cnidarians as cells of the germline, and that they started to differen- models of developmental processes has grown substantially tiate shortly before they penetrated into the endoderm. in recent years, partially because of this morphological sim- Once in the endoderm, they began to multiply by plicity, but also because of their ontogenic plasticity, remark- able regenerative capabilities, and the unique position they hold in the evolutionary tree (reviewed in Technau and Steele Communicated by: V. Hartenstein 2011). The majority of phylogenetic studies have placed the D. A. Gold : D. K. Jacobs (*) cnidarians as the major sister clade to the Bilateria (proto- Department of Ecology and Evolutionary Biology, stomes + deuterostomes), which encompasses 99 % of living University of California, Los Angeles, animal species (Glenner et al. 2004;Dunnetal.2008; 2154 Terasaki Life Science Building, Los Angeles, CA 90095, USA Wheeler et al. 2009; Philippe et al. 2009;Picketal.2010; e-mail: [email protected] Erwin et al. 2011). This suggests that, as their simplicity 54 Dev Genes Evol (2013) 223:53–66 Fig. 1 a A simple animal phylogeny, with special attention to the classes of cnidarians, based on Collins et al. (2006), Dunn et al. (2008), Philippe et al. (2009), and Pick et al. (2010). b Life cycles of several cnidarian genera. For Aurelia, derivations from the “canonical” life cycle are highlighted in red (based on Vagelli (2007) and personal observation) indicates, cnidarians can provide valuable insight into the working to address these questions, but drawing broad con- origins of animal complexity. clusions has remained elusive. However, as cnidarians have been subjected to molecular- Several factors complicate the development of unifying level inquiry, study after study has found substantial cryptic principles regarding cnidarian stem cells. Firstly, cnidarians complexity, challenging most of the “simple” characteristics have complex life cycles that include different combinations of cnidarians mentioned previously (Kussarow et al. 2005; of planula larvae, benthic polyps, and free-swimming me- Lee et al. 2006;Jacobsetal.2007, 2010; Ryan and Baxevanis dusae (Fig. 1b). These life history stages are so disparate, 2007; Putnam et al. 2007; Matus et al. 2008). Even without that before the mid-1800 s different stages were classified as these molecular insights, scientists have long recognized that distinct taxa (recounted in Calder 1982). Cnidarians are the cnidarians evolved a dizzying array of diversity despite often chosen for study based on the unique morphological their “simple” morphology: from the clonal hydroids that aspects of their life cycle, which further complicates efforts build the superorganism, the Portuguese Man o' War to homologize developmental dynamics between the differ- (Physalia physalis), to box jellies guided by complex eyes ent model systems. Secondly, cnidarians can generate adult (Tripedalia cystophora), from the 2-mm-long cnidarian forms using a wide variety of developmental mechanisms, “worm” (Buddenbrockia plumatellae) to the 37-m-long lion’s including sexual reproduction, asexual reproduction (via mane jellyfish (Cyanea capillata), not to mention the corals of pinching, parthenogenesis, polarity reversal, strobilation, the great barrier reef that span over 26,000 km. Cnidarians are or the production of planuloids or propagules) and regener- present in the Cambrian fossil record (Hou et al. 2005; ation following damage (Lesh-Laurie and Suchy 1991;Van Cartwright et al. 2007), and molecular clock analyses suggests Lieshout and Martin 1992; Vagelli 2007). Even in relatively that the origin of crown-group cnidarians—which are as ge- closely related species, development and sexual maturity netically divergent as protostomes and deuterostomes (Erwin can be fundamentally different in extremes that are uncom- 2008)—might extend back 700 million years ago (Erwin et al. mon to the Bilateria. Consequently, attempts to classify taxa 2011). Thus, no single clade of cnidarians (much less any based on developmental mechanisms have proven problem- single species) can represent the cnidarians as a whole. atic (e.g., Fautin 1991). However, when it comes to the study of cnidarian stem To explore cnidarian stem cells and identify unifying cells, most research has focused on a single genus, the principles or important generalizations, we start by review- hydrozoan Hydra. While this research has generated signif- ing what is known about the self-renewing cells of hydro- icant knowledge regarding the role of stem cells in Hydra zoans—particularly the epithelial and interstitial cells—and development and regeneration, fundamental questions relat- then present the problems faced when extending these ing to the cnidarians as a whole have yet to be clarified. Are observations to other cnidarians. While non-hydrozoans there common principles to cnidarian stem cells? What is appear to lack interstitial cells, they have other undifferen- the relationship between cnidarian stem cells and the stem tiated cell types, collectively called amoebocytes, which cells of other animals? A growing group of researchers are might play a stem cell role. But there are significant Dev Genes Evol (2013) 223:53–66 55 differences between the morphology and cellular dynamics ectodermal epithelial cells, and endodermal epithelial cells of interstitial cells and amoebocytes, and the later most (Campbell and Bode 1983; Galliot et al. 2009; Watanabe et likely do not represent stem cells. Current molecular studies al. 2009; Bosch 2009; Technau and Steele 2011, reviewed in of vertebrate “stem cell markers” in cnidarians suggest that Fig. 2). However, it might be inappropriate to call the the genetic control of cell pluripotency might have deep ectodermal and endodermal epithelial cells true stem cells. ancestry, but these studies have been largely unsuccessful Stem cells are by most definitions undifferentiated, while in teasing out the homologies between cnidarian cell types Hydra’s epithelial cells are best described as differentiated, and their vertebrate counterparts. However, comparative epitheliomuscular cells (see Frank et al. 2009 for a similar studies in regeneration and gametogenesis suggest that cni- opinion). Hydra’s ectodermal epithelial cells are highly vac- darian cells are highly plastic, and that the transdifferentia- uolated, possess basal extensions composed of contractile tion of epithelial cells and dedifferentiation of “committed” fibers, and are connected together by gap and septate inter- cells is probably more ancestral than any particular stem cell cellular junctions (Wood 1959; West 1978; Campbell 1987). niche. Ultimately, this review suggests that
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