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Planarians and Regeneration: Facts and Possible Applications

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

What Is a Planarian?

For many students of biology, the word “planarian” conjures up an image of a strange flatworm with crossed eyes and an amazing ability to regenerate. Even small pieces of a planarian can regenerate missing body parts and form a complete individual. The animal is popular in school laboratories and in scientific research. Recent discoveries about its biology may help us in our quest to trigger the regeneration of human tissues, organs, and body parts.

The animals in the phylum Platyhelminthes and the class Turbellaria are referred to as planarians, even though most of them don’t belong to the genus Planaria. (That genus is also a member of the class Turbellaria.) The genus Dugesia is often used as an example of a planarian in school labs.

Planarians are freshwater creatures that have many characteristics in common, including most of their anatomical features and their ability to regenerate. They are small creatures that can be seen with the unaided eye but are best viewed under a microscope. Scientists are making some interesting discoveries about their cells and behaviour.

Size of typical lab planarians

Size of typical lab planarians

Planarians belong to the flatworm phylum, or the phylum Platyhelminthes. The name of the phylum comes from the Greek words platy, which means flat, and helminth, which means worm. Planarians are traditionally classified in the class Turbellaria within the phylum Platyhelminthes. Other classification schemes for the animals exist, however.

External Features of Planarians

As the name of their phylum implies, planarians have a flattened body. Their colour varies. They move by a gliding and undulating motion. Their "eyes" are actually eyespots (or ocelli) that can detect the intensity of light but can't form an image.

The animals often have an ear-like projection on each side of their body beside their eyes. These projections are called auricles. They don't play a role in hearing as their name might suggest but instead contain chemoreceptors to detect chemicals. They are also sensitive to touch. The auricles help a planarian to find food.

A planarian's mouth is located about half way down the underside of its body. In many individuals, a rod-like structure can be seen next to the mouth and under the surface of the animal. This is the pharynx, a tubular structure that leads to the rest of the digestive tract. A planarian extends its pharynx through its mouth in order to suck up food. All planarians have a pharynx and feed by this method, even if the structure isn't visible externally.

Digestive and Excretory Systems

A planarian has a digestive, excretory, and nervous system but no respiratory or circulatory one. Oxygen enters the body and travels to the animal's cells by diffusion. Carbon dioxide leaves the cells and travels to the body surface via the same process. The thinness of the animal's body makes gas exchange without special structures practical.


Planarians are carnivores and obtain their food by predation or scavenging. The muscular pharynx extends through the mouth to pick up food and then withdraws into the body. The pharynx leads to a branched digestive tract. Nutrients from the food diffuse through the wall of this tract and into the animal's cells. Indigestible food is released through the mouth. Planarians have no anus.


The body of a planarian contains tubular structures called protonephridia, which contain flame cells. The flame cells contain thread-like structures called flagella. The flagella beat, reminding observers of a flickering flame and giving the cells their name. The beating flagella move fluid containing waste substances out of the body through pores on the surface of the animal.

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The structure above is a mammalian neuron, not a planarian one, but it shows the relationship between the cell body and the axon in planarians. The axon is the extension from the cell body. Several types of neurons exist, even in us. All of them have a cell body and an axon, but they are slightly different in other ways.

Nervous System of a Planarian

The head of a planarian contains two connected ganglia, which are known as the cerebral ganglia. A ganglion is a mass of nervous tissue composed of the cell bodies of neurons. The cell body contains the nucleus and organelles of a neuron. An extension from the cell body called an axon transmits the nerve impulse to the next neuron. The nerves of a planarian contain a bundle of axons.

Nerves extend from the cerebral ganglia through the planarian's body, which contains other ganglia. Ganglia and nerves form a ladder-like nervous system, as shown in the illustration below.

The connected ganglia in the head of a planarian are sometimes referred to as a brain, though they form a much simpler structure than our brain. Nevertheless, the activity of the animal's "brain" is interesting. This activity is being explored in learning and pharmacology experiments involving the animal.

The body of a planarian contains muscles extending in several directions. Its undersurface produces mucus and has many hair-like structures called cilia. These features enable the animal to move over surfaces using a gliding motion as the cilia beat. Planarians also swim through water.

Reproductive System

Some species of planarians reproduce both sexually and asexually. Others reproduce only asexually. The species that can reproduce sexually contain both ovaries and testes and are therefore hermaphrodites. Sperm is exchanged between two animals during mating. The eggs are fertilized internally and are laid in capsules.

In asexual reproduction, the tail end of a planarian separates from the rest of its body. The tail develops a new head and the head end of the animal develops a new tail. As a result, two individuals are produced.

Planarian Stem Cells

Planarians can regenerate missing parts due to the widespread presence of stem cells. A stem cell is unspecialized but can produce specialized cells when stimulated correctly. Planarian stem cells are known as neoblasts. The nature of the neoblasts and the processes that occur as regeneration is activated and carried out are still being investigated.

Humans also have stem cells, but proportionally they are much less common than in planarians. The cells have a characteristic known as potency and are classified as follows.

  • Totipotent stem cells can produce every type of cell in the body plus the cells of the placenta.
  • Pluripotent cells can produce every type of cell in the body but not the cells of the placenta.
  • Multipotent cells can produce several types of specialized cells.
  • Unipotent cells can produce only one type of specialized cell.

The stem cells in planarians are pluripotent (or at least the ones that have been studied are). There are so many of them throughout the body that even a small piece of a planarian contains the cells.

The video below shows a planarian being cut in half and then growing into two organisms. Biologists assume that because a planarian's nervous system is so much simpler than ours, it doesn't feel pain in the way that we do. This is just an assumption, however. We don't know exactly what the animal feels.

Ability to Regenerate

New individuals produced by cutting a particular planarian into pieces are genetically identical to their "parent". Even when the body is cut into more than a hundred pieces, each piece will grow into a complete animal. In the nineteenth century, a scientist named Thomas Hunt Morgan claimed that 279 pieces of a planarian can regenerate new individuals.

It isn't necessary to completely separate a planarian into pieces in order to trigger regeneration. If the head is cut down the middle while the rest of the body is left intact, each half of the head regenerates the missing part. As a result, the animal ends up with two heads. Regeneration in a planarian takes about seven days or sometimes a little longer.

Facts About Planarian Regeneration

  • If its neoblasts are destroyed by radiation, a planarian that has been cut is unable to regenerate missing parts and dies within a few weeks.
  • If new neoblasts are transplanted into an irradiated animal, it regains its ability to regenerate.
  • When part of a planarian is amputated, neoblasts travel to the wound and form a structure called a blastema. The production and differentiation of new cells occurs in this structure.
  • Pieces obtained from two areas of a planarian's body are unable to regenerate an entire animal. These areas are the pharynx and the head in front of the eyespots.

Researchers are investigating the signaling processes that tell neoblasts to migrate to the injured area and then to produce a range of specialized cells. The research is important for understanding the behaviour of stem cells in planarians and perhaps in humans.

Cells release signaling molecules to influence other cells. The molecules are often proteins. They do their job by joining to receptors on the surface of other cells, which are also proteins. The union of a signaling molecule and its receptor triggers a particular response in the recipient cell.

The DNA in the nucleus of a cell contains encoded instructions for making the proteins needed by an organism, including those that act as signaling molecules. The code for making a specific protein is transcribed onto a molecule of messenger RNA, which travels to the ribosomes outside the nucleus. Here the relevant protein is made.

Each gene in a DNA molecule codes for a specific protein. Some planarian researchers are focusing their studies on genes and RNA transcripts (messenger RNA transcribed from a specific gene in a DNA molecule). These studies may offer new insights into the regeneration process in the animals.

One planarian stem cell gene that's believed to be involved in regeneration is called the piwi (pronounced pee-wee) gene. We have a closely-related gene in our sperm and eggs. It also plays a role in the activity of our stem cells. Some of the other genes involved in planarian regeneration resemble ones in humans. Perhaps we will one day learn how to use these genes in the regeneration of human body parts.

Schmidtea mediterranea is a popular planarian in scientific research. It's native to Europe and has an excellent ability to regenerate.

Facts About Nb2 Stem Cells

A team of researchers from the United States has made some interesting discoveries about planarian stem cells. The researchers have developed a new method of identifying and classifying planarian neoblasts. As a result, they've discovered twelve types of neoblasts, including a type that they call subtype 2 or Nb2.

Nb2 is pluripotent and has a protein on its surface called tetraspanin. The protein is encoded in a gene called tetraspanin-1. Tetraspanin is actually the name of a family of proteins. Our bodies contain some members of the family. In humans, the proteins are involved in cell development and growth.

The scientists have discovered the following facts about Nb2 cell behaviour.

  • When the researchers cut planarians, they found that the population of Nb2 cells in each half increased rapidly.
  • Cells that were isolated in lab equipment survived a sublethal radiation treatment.
  • When planarians were exposed to a radiation dose that would normally have been lethal, a single injected Nb2 cell multiplied and then spread through the animals, rescuing them.
  • The transcriptome of a cell is the sum of all its RNA transcripts. The transcriptome of the Nb2 cells is different during normal life, after exposure to sublethal radiation, and during regeneration. This suggests that a different set of proteins is being made in each situation.

Possible Relevance to Human Biology

It may seem strange than a creature that appears to be so different from humans may hold information relevant to our biology. At the cellular level, however, planarians have much in common with humans. Even their organs and systems have some similarities to those of humans.

One researcher calls planarians an in-vivo Petri dish for pluripotent stem cells. An in-vivo experiment is done in living things. An in-vitro experiment in done in laboratory equipment, such as Petri dishes. Experiments done in glassware can be useful. They have limited value, however, because interactions found in living bodies are missing. In the planarian body, these interactions are present. Studying the animals might lead to breakthroughs in our understanding of human biology, which would be a great outcome of the research.


This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2018 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on June 29, 2018:

Thank you very much, Devika.

DDE on June 29, 2018:

You have entertained me with many facts and fascinating titles. This is no exception!

Linda Crampton (author) from British Columbia, Canada on June 21, 2018:

Thank you very much for the visit and the kind comment, RTalloni. I hate to see earthworms in trouble, too!

RTalloni on June 21, 2018:

So interesting, and shivery! :) Your writing is always insightful and entertaining, making the subjects you address even more captivating.

Thinking of how planarians are serving research efforts to understand how amazing cells are is fascinating stuff. This will serve students well and it gives the rest of us a lot to ponder.

So, as I work in the garden it seems inevitable that an earthworm gets in trouble with a tool. I always hope that what I've heard is true as I hate to see them in trouble!

Linda Crampton (author) from British Columbia, Canada on June 21, 2018:

Hi, Adrienne. Thank you for the visit. Planarians do have interesting features. It will be interesting to see whether they help us in the future.

Adrienne Farricelli on June 21, 2018:

With such interesting features, it's no wonder why planarians are being used in research. Hopefully, they can provide us with many answers.

Linda Crampton (author) from British Columbia, Canada on June 20, 2018:

Thank you very much for the comment, Liz.

Liz Westwood from UK on June 20, 2018:

This is a very informative and well-researched article. I always feel like I'm learning a lot through reading your articles.

Linda Crampton (author) from British Columbia, Canada on June 19, 2018:

I like both etymology and entomology too, Bede! Thank you very much for the comment. Planarians are definitely amazing creatures.

Bede from Minnesota on June 19, 2018:

I always learn new stuff from your articles, Linda. Planarians are simply amazing creatures and to think that human biology might benefit from studying their regenerative abilities is a cause for wonder. Thanks also for explaining the meaning of Greek words. Etymology or entomology- I like them both.

Linda Crampton (author) from British Columbia, Canada on June 19, 2018:

Hi, Dora. I appreciate your comment. Planarians are unusual creatures. It's interesting to explore their abilities.

Linda Crampton (author) from British Columbia, Canada on June 19, 2018:

Thank you very much, Manatita. I hope planarian research proves useful to humans. We might discover some important facts about our biology by studying the animals.

Dora Weithers from The Caribbean on June 19, 2018:

You do a great in holding our interest in these mysterious creatures. Sure, planarians matter. The regeneration process is very interesting and the external features are so unlike what we guess they would be. As always, thanks for the lesson.

manatita44 from london on June 19, 2018:

Fascinating stuff!

Especially the videos. I was thinking that planarian research can possibly help humans, but you touched on this towards the end, especially the stem cells.

We are so advanced now intellectually, scientifically and electronically. So some answers are purely a matter of time.

Another briĺiant piece, Alicia.

Linda Crampton (author) from British Columbia, Canada on June 18, 2018:

That's a very interesting point, Heidi. I appreciate your comment and hope you have a wonderful week, too.

Heidi Thorne from Chicago Area on June 18, 2018:

Always educational! I think the value in studying these mini creatures will be super valuable when we finally discover life on other worlds. What's predicted is that simple organisms will probably be discovered. So comparing it to what's on our planet will definitely increase our understanding of what "life" is. Thanks for sharing and have a wonderful week!

Linda Crampton (author) from British Columbia, Canada on June 18, 2018:

That's so true, Bill. I hope new discoveries do appear soon.

Bill Holland from Olympia, WA on June 18, 2018:

There is so much we still don't know, and in a way that is very exciting. It just means new discoveries are coming soon, hopefully discoveries which will greatly benefit mankind.

Linda Crampton (author) from British Columbia, Canada on June 18, 2018:

Thanks, Mary. I hope the research is helpful for humans. I think the action of stem cells is a fascinating topic.

Mary Norton from Ontario, Canada on June 18, 2018:

This is interesting and I do hope this research continues as it looks very promising to humans especially in cell regeneration. There is so much in the cells both in animals and humans that we still don't know about. Thank you for writing on this.

Linda Crampton (author) from British Columbia, Canada on June 18, 2018:

Hi, Ann. Thanks for the visit. Planarians do look rather sweet, especially when they're magnified. They are interesting and potentially very helpful animals.

Ann Carr from SW England on June 18, 2018:

Your title drew me in. 'What on earth is a planarian?' I thought.

What fascinating creatures these are! They look rather sweet. Your article was fascinating too.

I'm always amazed at what nature can do; the complications are extensive. We think we are the superior race but this does make me wonder.

It would be great if research into the planarian would prove to help humans with stem cell treatment and the like.

Thanks for the education, Linda.


Linda Crampton (author) from British Columbia, Canada on June 17, 2018:

Thank you very much, Flourish. The possibilities are exciting. I hope the research eventually leads to some wonderful outcomes for humans. Have a great week!

FlourishAnyway from USA on June 17, 2018:

This was thoroughly fascinating and really gives credence to the notion that stem cells are of enormous significance to us and other living beings. Imagine the possibilities! Great article!

Linda Crampton (author) from British Columbia, Canada on June 17, 2018:

Hi, Jackie. I always appreciate your visits, especially so soon after I publish an article. Earthworm regeneration is often impressive, though the ability depends on the species and where the worm is cut. Hopefully the studies of both planarians and earthworms will help us one day.

I hope you have a great week, Jackie. You're definitely not a dummie!

Jackie Lynnley from the beautiful south on June 17, 2018:

No respiratory system is amazing. Surely the study and facts brought from that will have great value to us one day, or I would certainly think so.

The reproduction sounds much like the earthworm too, doesn't it? I mean, being a country girl is the only way I would have knowledge of this and not of any sexual nature but simply because I was taught take all the pinches of them to fish with you want to and they will just become another worm!

Hard to forget that lesson.

Great fun article as usual, to help educate us dummies, Linda!

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