Planarian module

Planarians are free-living representatives of the phylum Platyhelminthes, a group of some 50,000 species of flatworms. Flatworms are among the simplest bilaterally symmetric animals: they are acoelomates, yet they possess derivatives of all three germ layers organized into complex organ systems. Thus, Platyhelminthes have been thought to occupy an important position in Metazoan evolution. Current models place the Platyhelminthes in a large assemblage of protostome invertebrates, known as the Lophotrochozoa, a sister group to the Ecdysozoa (to which insects and nematodes belong).

Planarians are best known for their capacity to regenerate complete individuals from minuscule body parts, as well as for their ability to “de-grow” when starved. Such extraordinary plasticity in the adult is in direct contrast to the rigidity displayed by currently used invertebrate models such as Caenorhabditis elegans and Drosophila melanogaster. The difference lies in a population of adult somatic stem cells, called neoblasts, that are distributed throughout the planarian body. Neoblasts are the only mitotically active cells in planarians, and their division progeny generate the 30-40 different cell types found in these organisms. In intact planarians these stem cells replace cells lost to normal physiological turnover; whereas, in amputated animals, they give rise to the regeneration blastema, the structure in which missing tissues are regenerated.

Until the mid-20th Century, planarians were a key model for studying development and regeneration. Yet, as attention shifted towards animals amenable to classical genetic analysis, the use of planarians declined. Recently, however, the successful introduction of cell, molecular, and RNAi techniques in planarians, along with heightened interest in stem-cell biology and the plasticity of the differentiated state, has re-kindled interest in these fascinating organisms. Part of this renaissance, includes an ongoing Genome sequencing project, being carried out by the University of Washington Genome Sequencing Center in St. Louis, MO ( The species Schmidtea mediterranea was selected for sequencing since it is a stable sexual diploid with a genome size of approximately ~8 x 108 bp. S. mediterranea provides a vital resource for the development of a unique model to study metazoan evolution, regeneration, and the regulation of pluripotentiality. Mechanistic insights into these basic biological problems will have deep and obvious implications to our understanding on biology and perhaps for the improvement of human health.

In this module students will learn many techniques and biological phenomenon used to study planarians, including amputating animals for regeneration analysis, injecting animals with dyes, performing tissue transplantation, observing wild type regenerating animals, watching neoblast deficiency phenotypes, observing RNAi-induced phenotypes in Schmidtea mediterranea, examining planarian embryos, performing antibody staining, observing other planarian species, and learning cell isolation.