02162nas a2200265   4500008004100000020001400041245010600055210006900161260000800230300001200238490000800250520141300258100001801671700001601689700001501705700001401720700002001734700001601754700001501770700001601785700001501801700001701816700001701833856004601850       2009                        eng d  a0012-160600aStem cells are differentially regulated during development, regeneration and homeostasis in flatworms0 aStem cells are differentially regulated during development regen  cOct  a198-2120 v3343 aThe flatworm stem cell system is exceptional within the animal kingdom, as totipotent stem cells (neoblasts) are the only dividing cells within the organism. In contrast to most organisms, piwi-like gene expression in. flatworms is extended from germ cells to somatic stem cells. We describe the isolation and characterization of the piwi homologue macpiwi in the flatworm Macrostomum lignano. We use in situ hybridization, antibody staining and RNA interference to study macpiwi expression and function in adults, during postembryonic development, regeneration and upon starvation. We found novelties regarding piwi function and observed differences to current piwi functions in flatworms. First, macpiwi was essential for the maintenance of somatic stem cells in adult animals. A knock-down of macpiwi led to a complete elimination of stem cells and death of the animals. Second, the regulation of stem cells was different in adults and regenerates compared to postembryonic development. Third, sexual reproduction of M. lignano allowed to follow germline formation during postembryonic development, regeneration, and starvation. Fourth, piwi expression in hatchlings further supports an embryonic formation of the germline in M. lignano. Our findings address new questions in flatworm stem cell research and provide a basis for comparison with higher organisms. (C) 2009 Elsevier Inc. All rights reserved.1 aDe Mulder, K.1 aPfister, D.1 aKuales, G.1 aEgger, B.1 aSalvenmoser, W.1 aWillems, M.1 aSteger, J.1 aFauster, K.1 aMicura, R.1 aBorgonie, G.1 aLadurner, P.  uhttps://nematodes.myspecies.info/node/125