Planarian Worms: Real World Wolverines
by Clover LaMorgese
When people think of various superheroes, one that may come to mind is Marvel’s Wolverine. Although it is true that his regeneration powers would be impossible for a human, some animals do have this ability. In fact, one such animal comes from the most unlikely place. The planarian worm has amazing regeneration properties, and scientists are studying them like human lives may depend on it, for a very good reason. The planarian worm has an incredible attribute: it can be cut into many pieces, and every last one of those pieces will regenerate into a brand new worm that retains all of its original cellular and chemical memories. But that's just barely the tip of the iceberg for these wonder worms, so let's dive in!
Before we get into the incredible feats of the planarian, we need to understand them on a simple level. Planarians are flatworms under the order Tricladida. They are classified by its three–branched intestine, including a single anterior (head) and two posterior, or rear, branches. Planarians can reproduce sexually or asexually, though some can reproduce both ways. Planarians reproduce asexually when adult stem cells proliferate and produce different cells. Planarians reproduce sexually when their gametes are combined. Eggs develop inside the body and are shed in capsules. Planarians can live on land, underground, and in fresh water or salt water, although most only reside in one environment throughout their lifespans.
However, it’s what is on the inside that counts, which is why planarian organs are our next main focus. Planarians don't have a circulatory system so they absorb oxygen through a body wall, which is kind of like breathing through your skin. They have a muscular pharynx that they use to move food to their gut. They have a three-branched intestine that runs along their entire body, which splits into three halfway through the body. The intestine is what is known as a “blind sac,” so the planarians consume food and release waste through the same orifice, located near the middle of the ventral body surface. Many species of planarians have two eyes, but others can have more distributed along the body, although most subterranean planarians are blind or even lack eyes entirely.
Now that you better understand basic facts of the planarian, now is a great time to explain some of the phenomenal things this worm can do. Planarians have always been considered "immortal under a knife” and there's a good reason: it's estimated that a planarian can be cut into up to 279 equal pieces, and each of those pieces will grow into a new worm within a couple of weeks. Now you’re probably wondering how they do that, but it’s not magic: neoblasts near the injury site proliferate to generate a structure of differentiating cells called blastema. Believe it or not, planarians regenerative powers can create TWO HEADS on the same body. This happens when a cut through the head separates it into two parts, so the halves heal separately. Some researchers have cut off planarian heads and sent them to space to see what would happen. They discovered that only one of the amputated fragments sent to space regenerated into a double-headed worm, though the majority of such amputated worms (95%) did not do so.
Planarian worms have a powerful ability to retain sensory information, capable of remembering things even after the head and body have been cut apart. This is because planarians have what's called a biochemical memory. A biochemical memory is the physical storage of learned information through chemical processes and molecular changes within a biological system, primarily involving synaptic plasticity between neurons.3 How did they discover this? It all started in 1955 when Robert Thompson and James V. McConnell were testing planarians by pairing a bright light with an electric shock. They would turn on the light and then shock the worms; after they repeated this several times, they took away the shock and simply turned on the light. The planarians continued reacting as they did when shocked. They repeated this process with another group of worms, but before removing the shock they cut the worm in half, and once again both halves acted as though they were being shocked. In 1963, McConnell repeated the experiment, but instead of cutting the trained flatworms in two, he ground them into small pieces and fed them to other flatworms. These worms then developed the light-shock reaction as well.
The planarian worm is a wonder of nature, and it certainly is an interesting subject of science. Our knowledge of the planarian worm is constantly expanding with new experiments and technology to better understand them. And perhaps one day, you, dear reader, will be the next one to make a new discovery about our little friends. Now you may be thinking, “That's cool, but how does this apply to my life?” and that's a great question. Studying the planarian worm's amazing regeneration gives us a better idea of stem cell biology and helps point us in the right direction for human tissue and organ healing and regeneration.
