Planarians and Regeneration: Facts and Recent Discoveries
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.
Multiple species are referred to as planarians, even though many of them don’t belong to the genus Planaria. Dugesia is often used as a planarian in school labs, for example. 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.
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. Tapeworms and flukes are also flatworms.
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. Planarians 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 planarians, 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 pharynx isn't visible externally.
Digestive and Excretory Systems
A planarian has a digestive, excretory, and nervous system but no respiratory or circulatory system. 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 out of the body through pores. In this way wastes are removed from the planarian.
Nervous Systems and Muscles
Ganglia, Brain, and Nerves
The head of a planarian contains two connected ganglia. A ganglion is a mass of nervous tissue composed of the cell bodies of neurons. The cell body of a neuron, or nerve cell, is the location of the nucleus and organelles. Extensions from the cell body help to transmit the nerve impulse.
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 planarian's brain is interesting. This activity is being explored in learning and pharmacology experiments involving the animal.
Nerves extend from the brain through the planarian's body, which contains other ganglia. Ganglia and nerves form a ladder-like nervous system, as shown in the illustration below.
Muscles and Movement
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.
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.
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 to a more limited extent than 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.
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 will 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.
New Trends in Research: Genes and RNA
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 and in some of our stem cells. Some of the other genes involved in planarian regeneration also 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.
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.
Flatworm information from Rice University
Platyhelminthes facts from the State University of New York
Facts about planarian regeneration from the Max Planck Institute for Molecular Medicine
Information about a newly discovered neoblast from Science magazine
A summary of new Nb2 research from the Cell journal
Questions & Answers
© 2018 Linda Crampton