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Not your father's planarian: a classic model enters the era of functional genomics

Key Points

  • The planarian was established as a system for the study of regenerative development more than 200 years ago. As attention shifted towards organisms with more rigid patterns of development, and molecular and genetic techniques gained popularity, planarians fell out of favour.

  • Planarians show remarkable developmental plasticity. For example, a planarian can regenerate from a piece of tissue that represents less than 1/279 of the adult organism.

  • They also de-grow in nutritionally limiting conditions and can shrink to a size that is smaller than their size at hatching. De-growth is accompanied by loss of reproductive structures, which re-form when nutrition becomes plentiful again.

  • Developmental plasticity is dependent on a population of pluripotent cells called neoblasts. Neoblasts are thought to represent stem cells and are stimulated to migrate, grow and divide by discontinuities in the adult structure. Planarians therefore have great potential as a model system for studying stem-cell biology.

  • Genetic markers have recently become available that will help to study neoblasts, and to determine, for example, whether the cells are a homogeneous population or consist of a collection of lineage-restricted cells.

  • Further technological improvements in this system have been the development of methods for in situ hybridization, the successful application of RNA interference and an accumulation of genomic resources.

  • The current fascination with stem-cell biology, along with the technological advances in planarians, has set the stage for a resurgence of interest in these organisms. Planarians could provide important insights into the mechanisms that underlie regeneration and development.

Abstract

Freshwater planarians were a classic model for studying the problems of development and regeneration. However, as attention shifted towards animals with more rigid developmental processes, the planarians, with their notoriously plastic ontogeny, declined in significance as a model system. This trend was exacerbated with the introduction of genetic and molecular approaches, which did not work well in planarians. More recently, the heightened interest in stem-cell biology, along with the successful application of molecular, cellular and genomic approaches in planarians, is re-establishing these fascinating organisms as models for studying regeneration and developmental plasticity.

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Figure 1: Diagrams of the major organ systems in freshwater planarians.
Figure 2: The planarian's regenerative and remodelling abilities.
Figure 3: Growth and de-growth in response to food availability.
Figure 4: Regeneration in Schmidtea mediterranea.
Figure 5: Distribution of proliferating cells in the intact planarian.
Figure 6: The planarian Schmidtea mediterranea.
Figure 7: Representative whole-mount in situ hybridizations.
Figure 8: Scheme for using RNA interference to identify genes that are required for regenerative processes.

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Acknowledgements

We thank S. Robb and R. Juste for their outstanding contributions to the S. mediterranea EST project; M. Sepanski for electron microscopy; M. Pala for her generous gift of the sexual strain of S. mediterranea; K. Agata and the members of his lab for sharing unpublished results; N. Oviedo for sharing in situ results before publication; anonymous referees for their helpful suggestions; and the National Institutes of Health, National Institute of General Medical Science for supporting our work on planarians.

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Planarian papers published in the 18th–19th centuries in Europe

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Glossary

PLURIPOTENCY

The ability of a cell to contribute to multiple tissues in a developing organism. If a cell is able to contribute to all tissues, it is said to be totipotent.

COELOM

A fluid-filled body cavity that is lined by mesodermal cells.

BILATERIA

Animals in which the right and left halves are mirror images. Bilaterians include most animals: chordates, arthropods, worms, molluscs and others.

PROTOSTOME

Bilaterian animals, including arthropods, molluscs and worms, whose mouth develops before the anus during embryogenesis.

DEUTEROSTOME

Animals, including chordates and echinoderms, whose mouth develops after the anus during embryogenesis.

FLAME CELL

A cell that is distinguished by a tuft of beating cilia (resembling a flame) and that filters waste materials into the excretory system.

NERVOUS PLEXUS

A bundle or collection of nerves.

POLYP

A sessile form of an animal, such as a Hydra, that is attached to a substrate.

MORPHOGENETIC GRADIENT

A progressive increase or decrease in the concentration of molecules that cause cells to adopt different developmental fates at different concentrations.

POLAR GRANULE

A cytoplasmic organelle that is associated with the germ plasm (germ-line material) in Drosophila.

ANNELID

The phylum of segmented worms.

PARENCHYMA

Mesodermal tissue that fills the space between the epidermis and the gut in acoelomates (animals that lack a coelom).

BLASTEMA

A specialized structure that is composed of an epithelial layer and mesodermally derived, undifferentiated cells.

MIXOPLOID

Containing cells that are of different ploidy — for example, diploid and polyploid.

MARINER

A transposable element that was originally discovered in Drosophila and has since been shown to be present in the genomes of diverse species.

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Newmark, P., Alvarado, A. Not your father's planarian: a classic model enters the era of functional genomics. Nat Rev Genet 3, 210–219 (2002). https://doi.org/10.1038/nrg759

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