Regenerating a new limb or, you know, entire body

Tiny Matters

Regeneration is a hot topic in the sciences, and for good reason. We humans are not known for being great at regenerating, but if we could understand how other organisms do it so easily we might be able to heal a spinal cord injury or damaged organs and limbs. Or we could even slow the aging process and not suffer from diseases like Alzheimer’s.

Transcript of this Episode

Sam Jones: We humans have a lot going for us. We have opposable thumbs that allow us to grasp onto stuff, we speak tons of different languages, our brains have allowed us to develop things like electricity and cell phones and a telescope that can show us galaxies 13 billion light years away.

But there are many things that we can’t do, or can’t do as well as some other organisms. One of those things is regeneration. There are salamanders that can regenerate many of their limbs and organs. Sea stars can regenerate their arms. And a little freshwater animal called a hydra can get cut in half and form two new hydra.

Deboki Chakravarti: We can regenerate some things, like skin cells and most of a liver, but we certainly can’t regenerate our arms and legs, and definitely not our heads! Why not?

Welcome to Tiny Matters. I’m Deboki Chakravarti and I’m joined by my co-host Sam Jones. Regeneration is a hot topic in the sciences, which makes sense. Think about it: if we really understood how things regenerate, the potential there is… kind of unimaginable. We’d not only be able to heal a spinal cord injury or damaged organs and limbs, we might be able to slow the aging process and not suffer from diseases like Alzheimer’s.

Sam: Today on the show we’re talking to a couple of scientists who are trying to understand how regeneration works by asking, “how do other animals regenerate?” Because maybe one day, we can take a page out of their books and do the same.

So. Let’s start from the top: what is regeneration?

Alejandro Sánchez Alvarado: For me, regeneration means the ability of an organism to restore a missing body part lost some sort of injury or catastrophic insult. So if you can regenerate a hand or you can regenerate an eye, that's what I refer to as regeneration.

Deboki: That’s developmental biologist Alejandro Sánchez Alvarado. He’s the Executive Director and Chief Scientific Officer of the Stowers Institute for Medical Research in Kansas City, Missouri.

Alejandro: The curious thing about regeneration is that it's broadly but unevenly distributed across different animal species. And not understanding why is probably preventing us from understanding the root cause of some of our own diseases and some of the maladies that affect our health. Because many of those are actually a consequence of an inability to sustain or restore the wear and tear of tissues in our bodies. And so if we could understand animals who do this easily, I think that will help us understand why we cannot really repair tissues in our body when they go bad due to disease or just degeneration.

Deboki: Sam, you know Alejandro, right?

Sam: Yeah I’ve actually known Alejandro for over a decade. I was a research technician in his lab right out of college. I knew very little about anything when I showed up, but what I did know was that this ‘regeneration thing’ was pretty cool. And Alejandro is an A+ story teller who knows a lot about the history of this field so it was fun chatting with him and taking a bit of a walk down memory lane…starting in the 18th century.

Alejandro: Historically, the discovery of regeneration took place in a microscopic organism known as Hydra.

Deboki: Hydra live in freshwater, typically streams and rivers. They’re these cute little opaque, tube-shaped animals with a crown of tentacles. They were first really studied by a Swiss Naturalist named Abraham Trembley. In 1740, Trembley discovered that he could cut the animal in half and both halves could regenerate, producing two fully functional hydra.

Sam: Alejandro told us that this was not only something people did not believe at first—a lot of people were actually upset by it, because a common belief at the time was that the soul of an animal was indivisible. So if you divided an animal’s body in half, it would of course die. But these hydra didn’t die, they were doing just fine. All they did was multiply.

Ultimately, most scientists got on board with this.

Alejandro: When Trembley showed that this organism could be split in half and each half could regenerate a complete animal, that really lit the intellectual worlds of late-18th century, mid-18th century Europe on fire.

Sam: And from there, people started looking at other organisms to see if they too could regenerate. Hydra and snails were popular to work with, but amphibians ruled the roost.

Alejandro: And salamanders really reigned supreme for most of the 19th century and the first half of the 20th century as a regeneration organism that people wanted to study to extract the secrets of missing body part regeneration from them.

Deboki: But Alejandro doesn’t work with hydra or snails or amphibians. He works with planarians—small flatworms that are around 5 mm long, which is about the size of a pencil eraser, maybe a little longer. They’re actually quite cute.
Sam: They are! Their eye spots kind of make them look like anime characters or the big sad-eyes emoji.

Deboki: Yes, that is very accurate. Definitely look it up. So about these cute worms: their ability to regenerate was discovered by a guy named Thomas Hunt Morgan—or T.H. Morgan—back in the early 1900s. If you’ve heard of T.H. Morgan before it’s likely because of his work with genetics and heredity. But he was also interested in regeneration and, once he realized planarians might be something special he decided to test their limits. He began cutting the worms into very small pieces to see how small they could be and still regenerate into a full animal. He actually got to 1/279th of an animal, which we now know is something like 10,000 cells. Which is actually very few cells, but still it was enough for the worm to regenerate itself.

Alejandro: The equivalent of me cutting my little finger and watching my little finger regenerate a complete Alejandro, which is a terrifying thought.

Sam: So after T.H. Morgan there were some scientists who continued to work with planarians, particularly in Europe and Japan, but they kind of fell out of favor in the states.

Alejandro: I have to be frank with you, when I set out to study regeneration I did not really have a specific animal in mind. I had already began to work with amphibian tail regeneration but I realized very quickly, at least for me that it was gonna be a dead end. And so I asked myself the following question: Is there an animal out there that does regeneration really well, that I could try to exploit to illuminate my understanding of the mechanistic underpinnings that make regeneration possible?

Sam: Then in 1995 Alejandro went to take a developmental biology course—specifically the embryology course at the Marine Biological Laboratory—in Woods Hole, Massachusetts. For those of you not familiar with this lab, it’s on Cape Cod and has been around since 1888 and this embryology course in particular has happened every summer since 1893.

There’s an incredible library at the Marine Biological Laboratory that has a rare books collection. So Alejandro is in this library and comes across a book that was written by T.H. Morgan in 1901. It’s titled Regeneration.

Alejandro: The first three or four chapters described all of these magical things, almost seemingly magical things, that planarians could do. And so I thought, well, somebody must be working on them. So I went to the library again, looked for publications, found very few. And I realized that there may be an opportunity here to take this remarkable exaggeration of a biological process in these animals and take it apart, dissect it, and analyze it molecularly and cellularly. And I thought, maybe this is the system.

Deboki: So once Alejandro decided to work on planarians, he needed to create a planarian colony to do experiments with in his lab at the Carnegie Institution for Science in Baltimore. It’s pretty straightforward starting a planaria colony because as long as you have a few of those planarians you can cut them into pieces and then those pieces regenerate into full animals within a couple of weeks. But then there was a mishap with some rerouted water that killed off almost every aquatic organism at the institute, including the planarians.

Alejandro and his colleague Phil Newmark needed to get things up and running again stat, so they traveled to where they knew they could find the worms: an abandoned fountain in Barcelona, Spain.

Alejandro: And it was cracked and it wasn't working, but when it rained, it would fill up with water and the worms would come from underground, where there is an aquifer, and then you could collect them that way.

Sam: And today almost every lab in the US is working with descendants of one of the worms that was collected from the fountain.

OK, so now that we have all that background settled Deboki, I say we talk a bit about what kinds of questions you can ask with planarians.

Deboki: Yeah let’s dive into it. So Alejandro says that one of the things that fascinates him with regeneration is this concept of ‘how do you teach an old dog to do new tricks?’

In this case, the old dog is a cell that’s in its final differentiated state, meaning that it’s got what seems like a pretty set identity and a pretty set way of life. Except that during regeneration, those old dogs are learning some new tricks that turn them into a completely different type of cell.  

So when you have a fully formed animal you have muscle cells, skin cells, cells specific to your lungs and brain, right?

Sam: Right.

Deboki: In the case of planaria regeneration those cells somehow shift to become different types of cells. Maybe a tail cell becomes a head cell. The question is ‘how?’ How does a differentiated cell become something new?

One way Alejandro and other researchers are trying to tease apart the different genetic pathways and players involved is by using something called RNA interference or RNAi, a method that led to researchers Craig Mello and Andrew Fire winning a Nobel Prize in 2006.

So with RNAi you can generate double stranded RNA to target a specific gene, blocking it from being translated into protein. In simple terms you’re shutting down the function of that gene.

Alejandro: And now we can ask: in the absence of this gene, if I cut the planarian, will we have any effect on regeneration? And low and behold, we've been finding genes that actually are very, very important in the ability of these animals to regenerate. We have one gene, for example, that when we targeted with RNAi that no matter where we cut the animal, it only regenerates heads, not tails, but heads. And so we get these multi-headed animals from just perturbing one gene, and the list goes on and on and on. So we've been able to use this tool to give us an idea of what molecules are most likely necessary for allowing these animal to do what they do.

What I like the most about these animals is their ability to force us to question what we think we understand about cell biology, about genetics, and about how these things come together to produce form and function. Every time that we push these animals to yield some of their secrets, they push back and surprise us with how ignorant we truly are about how these things actually work.

Deboki: I love that.

Sam: Me too. We humans can be very confident. And I think sometimes what we really need is a cute little worm that can regenerate its head to put us in our place.

I think it’s safe to say that, by now, Alejandro has convinced you the listener that planarians are very cool. So now let’s talk about another animal that can regenerate better than us humans. The zebrafish.

Zebrafish are freshwater fish that are black and white striped…

Deboki: That makes a lot of sense, that’s some great naming.

Sam: I guess in certain lights they look more blue and white striped but whatever they’re striped, they’re zebrafish. These fish are native to South Asia but they are used in labs all around the world for tons of different research, including regeneration research.

Dana Klatt Shaw is a postdoctoral researcher in the Mokhalled lab at Washington University in St. Louis and she studies zebrafish spinal cord regeneration. Zebrafish can fully regenerate a functional spinal cord when theirs is severed. Something that is of course not possible in humans, for a variety of reasons.

Dana: When a neuron is injured in humans, even if it survives that initial injury, it actually degenerates. You lose additional neurons even after the initial injury.

Sam: Dana tells us another issue for humans is that we don’t really have the ability to generate new neurons once they’re lost. And there are broader issues beyond just what’s going on in the neuron itself.

Dana: We have all of these cells that recognize there's an injury and they infiltrate in and they actually participate in this scarring response. In the spinal cord, this results in what we call this kind of heterogeneous dense fibrotic scar, it's this really dense tissue that packs in the lesion and prevents axons from regrowing across it. So in the fish, this response is entirely different. Neurons, once they're damaged, we don't see this degenerative response in zebrafish and zebrafish are capable of adult neurogenesis, which humans are not.

We actually don't see this scarring response and we don't see the same kind of hyper-inflammation that we would see in the human spinal cord. The zebrafish gives us a really great opportunity where we can go in piece by piece and pick apart the system and say, okay, so the fish regenerates spontaneously, what are the conditions where it doesn't regenerate? And so we can, kind of like Jenga actually pull out piece by piece and say, what can actually cause the system to collapse to not regenerate?

One of the first projects that Dana worked on when she started in the Mokhalled lab was with specialized cells that zebrafish have called bridging glia. Glia are cells in the nervous system, but they aren’t neurons. Their job is to patch things up after an injury, but the way they do this is different depending on the animal.

In mammals, they’re involved in the scarring that happens after spinal cord injury. In zebrafish, the glia cells actually change shape and form a kind of bridge across the lesion, connecting the two halves of the severed spinal cord and providing a scaffold for new neurons. So in zebrafish there isn’t scarring, there’s regeneration.

Dana: So this was characterized over 10 years ago, but we didn't really have a good understanding of the signals that told these glia to have this dramatic transformation after injury. So we were able to identify kind of a list of genes that's important for instructing these glia to respond to injury in the way that they do ultimately with, with the goal to say, how can we actually kind of trigger human glia to perform a similar function?

Deboki: Now Dana is also working on understanding how the immune system interacts with neurons after injury.

Dana: For the longest time we thought that the human immune system was actually pretty anti-regenerative. But we're learning that it's really not that simple. So if we deplete the immune response in mammals, like mice, after injury they actually have worse regenerative outcomes. So it's not an all or nothing thing. It seems that there are pro-regenerative aspects of the immune system as well as anti-regenerative.

The zebrafish immune system is really well conserved with mammals—very similar between the two species. So it's giving us this great opportunity where we can go in and say, well, what aspects of the immune response post injury are actually pro-regenerative? So again, picking apart piece by piece to look at different immune cell types and different molecular pathways.

Sam: All of these findings are allowing researchers to understand more about these complications when it comes to spinal cord injury.

Dana: I think, you know, we're understanding so much more about these complications. So for instance, 10 years ago, if you came into a clinic with the spinal cord injury they would treat you with a drug called dexamethasone. So this is a broad anti-inflammatory drug, and it was the idea that, oh, this immune response is generally detrimental. So if we can dampen that immune response, we can try and save these axons from degenerating and prevent this secondary damage. And, as I've mentioned, we now know that the immune response is actually pretty important after injury, even in humans.

I do generally feel optimistic. We know so much more now than we knew 10 years ago. And I think we're going to continue on in that direction.

Dana: I think every model organism has its strengths and weaknesses. So I think collaboration across species is incredibly important for this field. Planaria, they don't have a spinal cord, but you can still use them to ask super important research questions that we can't ask in the fish community and people in the mouse community can't ask. So I think it's leaning on each organism's strengths to really make progress in this very complex field.

Sam: OK. It is Tiny Show & Tell time.

Deboki: Sure is.

Sam: I'm just pretending there's some sort of music to amp us up right now.

Deboki: Yeah. Yeah.

Sam: Okay. I'm trying to remember. I think last time-

Deboki: I think I'm first.

Yes, you are first. Okay. Whenever you are ready, go for it.

Deboki: So, today for my Tiny Show & Tell, I'm going to talk about something that I don't think I talk about that much on this podcast, physics, because physics, it's hard. Anyways, the thing about physics is I feel like it's a field that exists to make even the simplest things feel so much harder than they need to be. And I know it's valuable, but it's a lot. And this is evidence number, I don't know, a billion for me in this thing, but it's part of also what makes physics so cool. And this is based on an article that I was reading in Quanta about whether or not hot water freezes faster than cold water, which is not something I had really thought about because to me, just immediately it's like, "Well, duh, the cold water should freeze faster," but apparently this is a thing that people have been talking about for ages, because actually it's not always the case.

Apparently in the fourth century BCE, Aristotle was talking about how sometimes people who wanted to cool down water would actually put it outside in the sun to kind of get it warm first, I guess, before they started cooling it down. But there's actually a name for this phenomenon, this potential phenomenon, and it's called the Mpemba effect. It's named for Erasto Mpemba who in the 1960s was a teenager. And he was in school and he was making ice cream. And for some reason, his boiled mixture that he was making for his ice cream cooled a lot faster compared to the mixtures that his classmates put into the fridge, which had already been previously cooled. And he was really curious about this, so he actually asked a physicist named Denis Osborne about this. So they were working on some experiments to basically document does hot water in particular cool down faster than cold water. And they published it in a journal in 1969.

And so, since then this phenomenon of hot water cooling faster than cold water has been called the Mpemba effect. It's really interesting, but it's also really difficult to replicate these experiments because it turns out that how you measure the temperature of water again, because physics is so good at making simple things complicated, there's all of these confounding variables that come up. And so, there actually been scientists who've been trying to replicate this effect. And in 2016 there were scientists who are testing stuff out to see if they could replicate these experiments, and they found that where they put the thermometer in the water changed what temperature they've actually measured. And then there were basically ways that they could almost fake this effect in their experiment, even though their samples didn't actually show this result overall.

So, it was interesting because then they look back in the literature and they found that Mpemba and Osborne's results were the only ones where the effect was so strong that it couldn't be due to this kind of measurement issue. So, there's still clearly some kind of effect that was measured, but replicating it just turns out to be really hard. But it's really hard to pin down in part because this kind of cooling thing that physicists are trying to understand, apparently what makes it so hard is that this is a system that is out of equilibrium and is trying to approach equilibrium, and I guess that's just difficult to study. I really love this article because I thought it did a really great job of explaining the story behind how people have been trying to study this effect for so long and why it's so hard. So, I definitely really enjoyed this article.

Sam: I am proud of you for bringing something physics-related.

Deboki: Thank you. I'm going out of my comfort zone.

Sam: Yes, absolutely. It's kind of cool also, when you think about the really basic stuff in science that seem just intuitively "oh, of course this is what would happen. This is what makes sense." And then all of a sudden experimentation tells you otherwise. Even these things that just seem so obvious, maybe can't take everything at face value.

Deboki: Yeah. Is your intuition wrong? We don't know because it turns out measuring things is hard.

Sam: All right, I'm ready to share my Tiny Show & Tell.

Deboki: Yes. Go for it.

Sam: So, unlike you, I did not go outside my comfort zone in the very least because mine is fully biomedical science-related, which is what I did my graduate work in, so kudos to you. Let's start with a tiny bit of backstory. You might remember that at the beginning of 2022, so end of January of this year, there was the first pig to human heart transplant that happened in a living patient, this guy, 57 years old, unfortunately, he survived for a couple of months with a pig heart, but then he did pass away. That brings me to something that happened more recently, just happened. Two patients, they were considered brain dead and they were actually about to be taken off of life support. They were given pig heart transplants.

One of the patients, I guess the pig heart was a little bit small for them, and so they actually had to then adjust the blood vessels to account for the size differences and different mismatches. And so, the blood flow wasn't perfect, but it's giving them a real life opportunity to try and make this work in a person who's going to be taken off of life support. One of the surgeons had a news conference following this and said, "We learned so much.” If surgeons were able to learn so much with just a couple of these surgeries or a few of these surgeries, imagine maybe a year from now, a couple years from now, where we might be.

Deboki: Yeah, that's incredible. It reminds me so much of the stuff we were talking about in the body farms episode, in terms of both the ethics, but also the respect, the people who are working and these body farms have for the people who've donated their bodies in that case, because it is such a tremendous gift for people just to make that choice, which is not trivial. And in this case for the families, it's a huge decision. And that's incredible on a personal level, and then also the science and the potential medical impact is. That's crazy. That's so cool.

Sam: Thanks for tuning in to this week’s episode of Tiny Matters, a production of the American Chemical Society. I’m your exec producer and I’m joined by my co-host Deboki Chakravarti.

Deboki: This week’s script was written by Sam, edited by me and by Matt Radcliff who’s the Executive Producer of ACS Productions. As always, it was fact-checked by Michelle Boucher. The Tiny Matters theme and episode sound design are by Michael Simonelli and our artwork was created by Derek Bressler.

Sam: Thanks so much to Alejandro Sánchez Alvarado and Dana Klatt Shaw for chatting with us.

Deboki: Sam and I are planning to do a Q&A episode in the next couple of months so please send us questions–science questions, questions about the podcast, questions about us. We want to hear from you! Send them to  

You can find me on Twitter at okidoki_boki

Sam: And you can find me on Twitter at samjscience. See you next time.


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