Planarians: The Masters of Regeneration

By Neil Pilgrim

Imagine being in an accident and finding yourself decapitated. Then a relatively short time later your body grew a new head and even more astoundingly, your head grew a new body. Well, that is exactly what a Planarian can do (Morgan. 1898). They can be cut into at least 279 pieces and each piece can regenerate into a complete new organism (Morgan. 1898).

Our fascinating little Planarian friends are members of the family Planariidae, in the phylum Platyhelminthes (Wagner et al. 2011). They can be found all over the world, living mainly in freshwater ecosystems, but also in terrestrial and marine areas (Exploratorium 2012). Most species range in size from 3 mm – 2.5 cm, although some giant tropical species can grow up to 60 cm (Exploratorium 2012). Different species come in a variety of colors, such as white, black, striped or streaked, to even transparent (Exploratorium 2012). They are free-living, mainly carnivorous flatworms that prey predominantly upon insects, insect larvae, and other invertebrates (Wagner et al. 2011).

The planarian Polycelis felina. Photo by Eduard Solà. Used under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

But lets get down to the nitty-gritty, what everybody really wants to know. How do they regrow their heads?

The first thing you need to know is that scientists have described approximately 20 – 30 planarian cells, of which 20 – 30% are stem cells called neoblasts (Wagner et al. 2011). Neoblasts have the ability to differentiate into any other planarian cell and are essential to the planarians ability to regenerate (Wagner et al. 2011).

After decapitation or other injury, a chain of signals in the neoblasts begins, causing the stem cells to divide and multiply (Reddien et al. 2011). They then make their way to the wound site to form a protective mass of cells called a blastema (Reddien et al. 2011). At the same time this is happening there is also a carefully selected elimination of certain old cells, which is called apoptosis (Lobo et al. 2012). Essentially what is happening is a reworking of the new and old cells to recreate the damaged or lost parts of the body. The remaining parts of the body are then modified to the now smaller planarian size, at the same time keeping the same planarian body plan. It takes approximately two weeks for all of this to happen. (Lobo et al. 2012)

Amazing eh? But you might be thinking, smaller size? How does the head digest food with no digestive system? Well, what actually happens is the Planarian feeds off itself! In the case of a decapitation some cells in the body will begin to die, the apoptosis we talked about, and these cells provide the Planarian with substance, energy, to survive (Lobo et al. 2012). Then, as time passes the body will get smaller, until eventually the body size will match the new head that has regenerated (Lobo et al. 2012). Now there is a smaller version of the original body plan. After this our little friend will be able to eat and then return to its normal size.

Can you imagine the implications for humans if we can harness and replicate this ability? We actually have similar stem cells in our bodies. Our embryonic stem cells have the ability to become whatever tissue the body needs. Having an organism, such as the planarian, with few tissues and a simple make-up allows us to study them, with relative ease, letting us experiment with regeneration. Who knows, maybe in the future we could replace a severed head or even a severed body for that matter.

References

Wagner DE, Wang IE, Reddien PW (2011) Clonogenic neoblasts are pluripotent adult stem cells that underlie planarian regeneration. Science 332: 811–816.

Morgan T (1898) Experimental studies of the regeneration of Planaria maculata. Dev Genes Evol 7: 364–397.

Lobo D, Beane WS, Levin M (2012) Modeling planarian regeneration: a primer for reverse- engineering the worm. PLoS Comp. Biol 4: e1002481

Reddien PW, Sanchez Alvarado A (2004) Fundamentals of planarian regeneration. Annu. Rev. Cell Dev. Biol. 2004. 20:725–57 doi: 10.1146/annurev.cellbio.20.010403.095114

Exploratorium (2012) Planarian: A window on regeneration. http://www.exploratorium.edu/imaging-station/research/planaria/story_planaria.pdf. Accessed March 6, 2015