An Introduction to Planarians and Their Stem Cells


Of all animals, it is perhaps planarians that have the greatest ability to regenerate. These once classical classroom organisms have now become the subject of advanced molecular genetic studies that have the potential to inform us about our own biology. Regeneration relies upon a population of pluripotent adult stem cells, called neoblasts. These collectively pluripotent stem cells underpin the regenerative capacity of planarians. The underlying molecular mechanisms controlling their proliferation, self‐renewal and differentiation are now being described. In particular, progress has been made in describing the expression profile of these cells and identifying some of the molecular mechanisms that regulate their behaviour leading to a better understanding of how these cells power regeneration.

Key Concepts

  • Planarians are bilaterians and share many cell and tissue types with other animals including vertebrates.

  • Planarians are able to mount a profound regenerative response to wounding allowing even small fragments to regenerate whole new worms.

  • Planarian regeneration requires the proliferation and differentiation of a population of pluripotent stem cell population called neoblasts.

  • Expression profiling of the neoblast population shows they are a heterogeneous population and express gene regulatory networks in common with both somatic and germline stem cells

  • Many cell biological and molecular genetic approaches can be used to study gene function in planarians allowing novel biological insights to be made, relevant to our own biology.

  • Current research has uncovered many important regulatory processes that regulate stem cells, including those regulated by transcription factors, chromatin modification and RNA binding proteins.

  • Future work will uncover the detailed mechanisms that control stem cell proliferation, self‐renewal and differentiation as well as how missing structures are accurately restored.

Keywords: pluripotency; regeneration; stem cell; progeny; RNA interference; differentiation

Figure 1. (a) Examples of planarians from left to right Dugesia benazzi, Dugesia tigrina, Polycelis tenuis, Polycelis felina, and Schmidtea mediterranea. (b) Live imaging of planarian gut structures in purple showing primary, secondary and tertiary anterior and posterior branches using acetyl‐pentafluorobenzene sulphonyl fluorescein (which is converted to its fluorescent form when exposed to H2O2). (c) Monoclonal anti‐3C11 antibody in green shows the nervous system structures cephalic ganglia, ventral nerve cords and transverse fibres. (d) RNA probe for neoblast marker histone 2b (h2b) in yellow, nuclear stain DAPI in blue. (e) Monoclonal anti‐H3P antibody labels mitotic cells in green against red autofluorescence. The antibody recognises Histone H3 when phosphorylated at Serine 10 during mitosis. (f) Genes required for neoblast maintenance are often associated with chromatoid bodies. Cells are stained with anti‐tudor antibody labelling the peri‐nuclear CB structures of neoblasts in green and neoblasts are also labelled in red by marker h2b. (g) Anti‐a tubulin in red. The antibody recognizes acetylated tubulin and labels planarian protonephridia cilia as well as dorsal and ventral cilia. Scale bars represent 100 µm for (a)–(e) and (g) and 10 µm for (f).
Figure 2. (a) An overview of selected genes required for the maintenance/self‐renewal and differentiation of two discrete neoblast populations (σ and ζ). Boxed in orange are several genes known to be necessary for neoblast maintenance. Nanos is required for the formation of the germline. p53, CHD4, MBD 2/3 and RbAp48 are not initially required for stem cell maintenance, but are necessary for the proper production of progeny and ultimately for the production of new differentiated tissues as required during regeneration and homeostasis. All these genes are expressed in neoblasts and p53 is also expressed in early post‐mitotic progeny. Boxed in green are a selection of genes that are used as markers of different progenitor types. Each is associated with a specific lineage choice/tissue fate, except the category marker AGAT‐1 that labels a transitioning progenitor cell based on their expression dynamics after irradiation but are of unknown lineage. (b) Cartoon representation of neoblasts in yellow (h2b+), with their sub‐population (σ and ζ) illustrated with a red and blue outline, respectively. Early progenitors in green (NB21.11.e+) and late progenitors in purple (AGAT‐1+) are shown to have different spatial distributions during homeostasis and regeneration. The h2b+ cells are located within the mesenchyme with NB21.11.e+ cells radiating more peripherally and AGAT‐1+ cells expressing the most peripheral expression. During regeneration, the spatial distribution of h2b+, NB21.11.e+ and AGAT‐1+ breaks down and cell types can no longer be compartmentalised. Mitotic σ h2b+ accumulate at the post‐blastema region, with post‐mitotic NB21.11.e+ and AGAT‐1+ cells located with the blastema proper. (c) In situ showing from left to right merged image of h2b+, NB21.11.e and AGAT‐1 distribution during homeostasis, with subsequent single channels labelled. (d) Showing from left to right h2b+ cells accumulating at the post‐blastema region in a 5‐day regenerate and not within the blastema proper. In green is a separate image of NB21.11.e+ cells located throughout the animal, including within the blastema proper, a distribution mirrored by purple AGAT‐1+ and highlighted more clearly in the merged image of NB21.11.e+ and AGAT‐1+ at day 5 of regeneration. Scale bars represent 100 µm for all images.
Figure 3. Lines indicate cell types that express the adjacent gene and the overlapping of lines represents co‐expression overlaps between cell types. Models propose that a large σ neoblast population produces (1) optic cup, (2) protonephridia, (3) serotonergic progenitors and (4) intestinal phagocytes and that the production of these post‐mitotic progeny are all specified from the cycling neoblast population (H2b+ and smedwi+ mRNA). Different groups of neoblasts have been shown to co‐express (1) h2b+/smedwi+ and sp6‐9+/eya+ (optic cup progenitors), (2) h2b+/smedwi+ and six1/2‐2+/POU2/3+ (protonephridia progenitors), (3) h2b+/smedwi+ and lhx1‐5+/PITX (serotonergic progenitors), or (4) h2b+/smedwi+ and nkx 2.2 (intestinal phagocyte progenitors). Progenitors subsequently undergo changes in gene expression that include the loss of neoblast markers h2b‐/smedwi‐, the retention of existing tissue‐specific expression and the additional expression of differentiated markers. SMEDWI protein produced in neoblasts, but persistent in progenitors, has also been used to show lineage‐related differentiation: (1) Optic cup progenitors that are h2b−/smedwi− retain sp6‐9+/eya+ expression and additionally express tyrosine+ as they are incorporated into the eye. (2) Protonephridia progenitors that become h2b‐/smedwi‐ remain six1/2‐2+ (tubule‐associated cell fate) or POU2/3+ (tubule cell fate) and additionally express carbonic anhydrase+ or rootletin/cubilin+, respectively. (3) Serotonergic precursors that are h2b‐/smedwi‐ retain lhx1‐5+/PITX and co‐express differentiation marker tryptophan hydroxylase+ once differentiated. (4) Finally, intestinal phagocytic precursors that are h2b‐/smedwi‐ retain nkx 2.2 expression, and co‐express the terminal marker, ceramide synthase.


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Further Reading

Aboobaker AA (2011) Planarian stem cells: a simple paradigm for regeneration. Trends in Cell Biology 21 (5): 304–11. DOI: 10.1016/j.tcb.2011.01.005.

Baguñà J (2012) The planarian neoblast: the rambling history of its origin and some current black boxes. The International Journal of Developmental Biology 56 (1–3): 19–37. DOI: 10.1387/ijdb.113463jb.

Elliott SA and Sánchez AA (2013) The history and enduring contributions of planarians to the study of animal regeneration. Wiley Interdisciplinary Reviews: Developmental Biology 2 (3): 301–26. DOI: 10.1002/wdev.82.

Gentile L, Cebrià F and Bartscherer K (2011) The planarian flatworm: an in vivo model for stem cell biology and nervous system regeneration. Disease Models & Mechanisms 4 (1): 12–9. DOI: 10.1242/dmm.006692.

Lobo D, Beane WS and Levin M (2012) Modeling planarian regeneration: a primer for reverse‐engineering the worm. PLOS Computational Biology 8 (4): e1002481. DOI: 10.1371/journal.pcbi.1002481.

Tanaka EM and Reddien PW (2011) The cellular basis for animal regeneration. Developmental Cell 21 (1): 172–85. DOI: 10.1016/j.devcel.2011.06.016.

Umesono Y and Agata K (2009) Evolution and regeneration of the planarian central nervous system. Development, Growth and Differentiation 51 (3): 185–95. DOI: 10.1111/j.1440-169X.2009.01099.x.

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Aboukhatwa, Ellen, and Aboobaker, Aziz(Jan 2015) An Introduction to Planarians and Their Stem Cells. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001097.pub2]