Light and electron microscopic studies of the intestinal epithelium in Notoplana humilis (Platyhelminthes, Polycladida): the contribution of mesodermal/gastrodermal neoblasts to intestinal regeneration

Abstract

Some free-living flatworms in the phylum Platyhelminthes possess strong regenerative capability that depends on putative pluripotent stem cells known as neoblasts. These neoblasts are defined based on several criteria, including their proliferative capacity and the presence of cellular components known as chromatoid bodies. Polyclads, which are marine flatworms, have the potential to be a good model system for stem cell research, yet little information is available regarding neoblasts and regeneration. In this study, transmission electron microscopy and immunostaining analyses, using antibodies against phospho-histone H3 and BrdU, were used to identify two populations of neoblasts in the polyclad Notoplana humilis: mesodermal neoblasts (located in the mesenchymal space) and gastrodermal neoblasts (located within the intestine, where granular club cells and phagocytic cells are also located). Light and electron microscopic analyses also suggested that phagocytic cells and mesodermal/gastrodermal neoblasts, but not granular club cells, migrated into blastemas and remodeled the intestine during regeneration. Therefore, we suggest that, in polyclads, intestinal regeneration is accomplished by mechanisms underlying both morphallaxis (remodeling of pre-existing tissues) and epimorphosis (de novo tissue formation derived from mesodermal/gastrodermal neoblasts). Based on the assumption that gastrodermal neoblasts, which are derived from mesodermal neoblasts, are intestinal stem cells, we propose a model to study intestinal regeneration.

No head regeneration here: regeneration capacity and stem cell dynamics of Theama mediterranea (Polycladida, Platyhelminthes)

“…In the polyclad Notocomplana humilis, the posterior blastema is more pronounced. In a recent study, proliferation in the posterior blastema was shown in histological sections, consistent with the results shown here (Okano et al 2015). In T. brocchi, histological sections in 7 to 15 dpa posterior blastemas also showed neoblast stem cells within the blastema, although no proliferation marker like BrdU or EdU was available at the time (von Levetzow 1939).…”

“…Scale bars a, j-l: 200 μm; d, m-n: 100 μm; c, e-h: 50 μm; i: 5 μm Over 160 years ago, Dalyell was the first to report regeneration phenomena in polyclads (Dalyell 1853). The first dedicated work on polyclad regeneration was published more than a century ago (Monti 1900), but only two studies dealing with polyclad regeneration were published within the last 20 years (Lapraz et al 2013;Okano et al 2015). In a nutshell, in most cotylean and acotylean polyclads, the ability to regenerate all parts posterior to the brain was observed.…”

“…While most studies on polyclad regeneration were focussed on determining the regeneration capacity, only two studies provided some insights into stem cell dynamics during regeneration (von Levetzow 1939;Okano et al 2015). In flatworms, missing tissues are replenished by pluripotent stem cells, the so-called neoblasts (Wagner et al 2011).…”

“…In flatworms, missing tissues are replenished by pluripotent stem cells, the so-called neoblasts (Wagner et al 2011). They are the only proliferating cells in juveniles and adults, are responsible for tissue maintenance, growth, and regeneration (Baguñà et al 1989), and are conspicuously lacking in the epidermis of rhabditophoran flatworms, including polyclads (Egger et al 2009b;Lapraz et al 2013;Okano et al 2015). In triclads, proliferating cells were reported to be lacking also in the regeneration blastema (Saló and Baguñà 1984;Morita and Best 1984), although newer studies indicate that mitoses are occasionally found inside the blastema (Wenemoser and Reddien 2010).…”

Cell-type diversity and regionalized gene expression in the planarian intestine revealed by laser-capture microdissection transcriptome profiling

“…Development of a robust method for isolating intestinal tissue, and characterization of the intestinal transcriptome, will facilitate mechanistic studies in planarians. The methods and resources presented here will also support comparative analyses, complementing current and future efforts to understand digestive tract regeneration in platyhelminths, sea cucumbers, annelids, ascidians, amphibians, and mammals (147–156). In addition, studies in planarians and other regeneration models are likely to generate new insights into cellular processes (e.g., proliferation, differentiation, metabolism, and stress responses) whose dysregulation underlies human gastrointestinal pathologies associated with aging and disease.…”