title: Characterization of the stem cell system of the acoel Isodiametra pulchra.
abstract: BACKGROUND
  Tissue plasticity and a substantial regeneration capacity based on stem cells are the hallmark of several invertebrate groups such as sponges, cnidarians and Platyhelminthes. Traditionally, Acoela were seen as an early branching clade within the Platyhelminthes, but became recently positioned at the base of the Bilateria. However, little is known on how the stem cell system in this new phylum is organized. In this study, we wanted to examine if Acoela possess a neoblast-like stem cell system that is responsible for development, growth, homeostasis and regeneration.
  
  
  RESULTS
  We established enduring laboratory cultures of the acoel Isodiametra pulchra (Acoela, Acoelomorpha) and implemented in situ hybridization and RNA interference (RNAi) for this species. We used BrdU labelling, morphology, ultrastructure and molecular tools to illuminate the morphology, distribution and plasticity of acoel stem cells under different developmental conditions. We demonstrate that neoblasts are the only proliferating cells which are solely mesodermally located within the organism. By means of in situ hybridisation and protein localisation we could demonstrate that the piwi-like gene ipiwi1 is expressed in testes, ovaries as well as in a subpopulation of somatic stem cells. In addition, we show that germ cell progenitors are present in freshly hatched worms, suggesting an embryonic formation of the germline. We identified a potent stem cell system that is responsible for development, homeostasis, regeneration and regrowth upon starvation.
  
  
  CONCLUSIONS
  We introduce the acoel Isodiametra pulchra as potential new model organism, suitable to address developmental questions in this understudied phylum. We show that neoblasts in I. pulchra are crucial for tissue homeostasis, development and regeneration. Notably, epidermal cells were found to be renewed exclusively from parenchymally located stem cells, a situation known only from rhabditophoran flatworms so far. For further comparison, it will be important to analyse the stem cell systems of other key-positioned understudied taxa.
authors.affiliation: University of Innsbruck, Institute of Zoology, Technikerstrasse 25, A-6020 Innsbruck, Austria. K.mulder@Hubrecht.eu
authors.forename: Katrien
authors.initials: K
authors.lastname: De Mulder
authors.forename: Georg
authors.initials: G
authors.lastname: Kuales
authors.forename: Daniela
authors.initials: D
authors.lastname: Pfister
authors.forename: Maxime
authors.initials: M
authors.lastname: Willems
authors.forename: Bernhard
authors.initials: B
authors.lastname: Egger
authors.forename: Willi
authors.initials: W
authors.lastname: Salvenmoser
authors.forename: Marlene
authors.initials: M
authors.lastname: Thaler
authors.forename: Anne-Kathrin
authors.initials: AK
authors.lastname: Gorny
authors.forename: Martina
authors.initials: M
authors.lastname: Hrouda
authors.forename: Gaëtan
authors.initials: G
authors.lastname: Borgonie
authors.forename: Peter
authors.initials: P
authors.lastname: Ladurner
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002454:Cell Differentiation; D015801:Helminth Proteins / Q000235:genetics / Q000502:physiology; D017403:In Situ Hybridization; D010802:Phylogeny; D013234:Stem Cells / Q000166:cytology*; D014419:Turbellaria / Q000166:cytology* / Q000502:physiology
chemical_list: D015801:Helminth Proteins