Speaker 1: Hey, welcome to sign Stuff, a production of iHeartRadio. I'm
Speaker 1: hoore Chim and today we're asking why can't we regrow limbs?
Speaker 1: If you lose an arm or a leg, we know
Speaker 1: that that arm or leg is not going to grow back,
Speaker 1: but we know there are animals in nature that can
Speaker 1: do that. If you cut off a starfish arm or
Speaker 1: a salamander's leg, it'll regrow a new one. There's even
Speaker 1: a fish that can regrow its heart if you cut
Speaker 1: off a piece of it. So how do they do
Speaker 1: it and why can't we do it? We're going to
Speaker 1: talk to several regeneration and animal experts and we're gonna
Speaker 1: find out if it's possible to activate that ability in people.
Speaker 1: So get ready to go out on a limb with
Speaker 1: us as we explore the regeneration of body parts. I
Speaker 1: promise it won't cost you an arm and a leg.
Speaker 1: Enjoy Hey everyone. To answer this question, I started by
Speaker 1: looking up a list of animals that can regrow their
Speaker 1: body parts. Not all animals can do it, but a
Speaker 1: few if you cut off a part of their body
Speaker 1: doultes regrow it back. On this list are starfish, flatworms,
Speaker 1: salamanders like the axe lotto and some fishes like the
Speaker 1: zebrafish and the Mexican tetrafish. We're going to talk to
Speaker 1: experts in each of these animals, and while we talk
Speaker 1: to them, we're going to learn three things. Number one,
Speaker 1: how these animals regrow their limbs, like what's their secret?
Speaker 1: Number two why humans can't regrow our limbs or can we?
Speaker 1: And three why do some animals evolve this ability and
Speaker 1: others don't? It seems like a pretty handy skill to have,
Speaker 1: all right. The first expert I talked to was doctor
Speaker 1: Andrew Wolff. Doctor Wolf is a researcher at the University
Speaker 1: of Maryland, Baltimore County who studies regenerate in starfish and flatworms.
Speaker 1: The first thing I wanted to know is can these
Speaker 1: animals really regrow anything? Well, let's start with the starfish
Speaker 1: and then we'll do the flatworms.
Speaker 2: So starfish are what are called a kinoderm, so that's
Speaker 2: a phylum of animals. So it's things like sea urchins,
Speaker 2: sea cucumbers, things called sea lilies or crinoids, where they
Speaker 2: also have sort of called feather stars that can move,
Speaker 2: and also brittle stars, so they're somewhat closely related. To vertebrates.
Speaker 1: What do we know about tissue regeneration in sea stars
Speaker 1: and sea urchins?
Speaker 2: So my work was understanding regeneration in their larvae. So
Speaker 2: they're very small, probably like a millimeter or so, very clear.
Speaker 2: They started to swim around the ocean for about two
Speaker 2: months or so until they metamorphous into the adult. So
Speaker 2: you can cut them in half, but those halves will regenerate.
Speaker 1: Okay, so you're saying, if you cut at the bottom,
Speaker 1: half will grow a top and the top will grow
Speaker 1: by and you'll get too identical animals. Yes, huh. And
Speaker 1: what about the flatworm?
Speaker 2: That's what I'm interested now. And so it not only
Speaker 2: does it regrow what's lost, it sort of reshapes everything
Speaker 2: else such that it's not out of proportion. So let's
Speaker 2: say you cut a worm in the three pieces like
Speaker 2: a top, of a middle, on a bottom. They'll all
Speaker 2: regrow what was lost, but the actual ending size of
Speaker 2: that worm is going to be smaller than the original
Speaker 2: wholeworm because if the tail regrows to be really small,
Speaker 2: the brain is going to be too big for the
Speaker 2: new body size. So now it needs to reshape everything
Speaker 2: to be in proportion to the new size that it is.
Speaker 1: So these animals will pretty much regenerate anything. If you
Speaker 1: cut them in half or in three pieces, each piece
Speaker 1: will regrow what's missing, and then you'll end up with
Speaker 1: almost identical copies of the original. Next, I talked to
Speaker 1: doctor Nadia f. Frobish, who is an expert in salamanders.
Speaker 3: My name is Nadia Flebish. I work at the Natural
Speaker 3: History Museum in Berlin and at Humboldt University. I'm a
Speaker 3: professor for evolutionary biology. Actually, everything that's alive can regenerate
Speaker 3: tissue to a certain degree, which is very convenient because
Speaker 3: otherwise I think we would all grow up with a
Speaker 3: lot of open scrapes and scratches as we go through childhood.
Speaker 1: That makes sense, Yeah.
Speaker 3: So of course we can all do wound healing to
Speaker 3: a certain degree, but some animals are even better at
Speaker 3: it than we are among vertebrates. So the animals that
Speaker 3: have a background, it's only salamanders that are the masters
Speaker 3: of regeneration. I think a lot of people are familiar
Speaker 3: with lizards regenerating tales and sort of throwing them off
Speaker 3: as a decoy for predators. But lizards, in contrast to salamanders,
Speaker 3: only get sort of like a fake tale back. It's
Speaker 3: more like a rotch of cartilage that's in there. And salamanders,
Speaker 3: if they lose their tail, they get a brand new
Speaker 3: tail with retever column and you know, spinal cord and musculature,
Speaker 3: so just like it was before. And they cannot only
Speaker 3: do that with their tails. They can also regenerate their limbs.
Speaker 3: They can regenerate the lenses of their eyes, a huge
Speaker 3: portion of their hearts, liver, so they're really really excellent regeneration.
Speaker 1: Pretty much any part of the body has been tested.
Speaker 3: Oh yes, this has been tested. Not any part, but
Speaker 3: a lot of the parts of the body. So they
Speaker 3: cannot lose their entire heart, for instance, and then regenerated.
Speaker 3: There still has to be like a critical amount left
Speaker 3: in order for regeneration to proceed.
Speaker 1: So the muscles, toenails, everything indeed.
Speaker 3: And sometimes it's really amazing because when the salamanders bite
Speaker 3: each other they can have quite gruesome injuries, so where like,
Speaker 3: for instance, the upper arm bone is sticking out and
Speaker 3: all the soft tissue is gone. Looking at it, you
Speaker 3: would think, oh my god, this is never going to
Speaker 3: grow back, but it does, and it's really quite amazing.
Speaker 1: And the last expert I talked to was Ezra Sengul,
Speaker 1: a graduate researcher at Oxford University who studies Mexican cavefish.
Speaker 4: I am Estra, and I use animal models, particularly Mexican
Speaker 4: cavefish and zebra fish.
Speaker 1: Okay, what do we know about tissue regeneration from zebrafish
Speaker 1: and cayfish?
Speaker 4: To start with zebra fish. Together with zebrafish, there are
Speaker 4: certain kinds of cavefish have regeneration catasity, and they have
Speaker 4: a remarkable precision. While doing that, they repair their sins,
Speaker 4: they replace it with an new functional one. They also
Speaker 4: regenerate their heart, spinal cord, retinas, and kidneys. They use
Speaker 4: them to study heart repay it in particular because unlike humans,
Speaker 4: the heart damage leads to starting in unspins, and zebra
Speaker 4: fish the heart cells replace that lost tissue.
Speaker 1: So all these animals basically have a superpower. If you're
Speaker 1: thinking of Deadpool or Wolverine from the Marvel movies, these
Speaker 1: animals can do what they do and they're real. Chop
Speaker 1: off leg and a new one grows back. Cut some
Speaker 1: of them in half, and both halves regrow into whole
Speaker 1: new organisms. I had a lot of questions. First of all,
Speaker 1: how do you even regrow a limb? And second of all,
Speaker 1: why can't we and other animals like cats and dogs
Speaker 1: and birds do this? To start, I asked our experts
Speaker 1: to give me a play by play of what actually
Speaker 1: happens when these animals are missing a body part. According
Speaker 1: to them, it's a step by step process, and it's
Speaker 1: pretty similar whether you're a starfish or a flatworm or
Speaker 1: a salamander. Step one is basically stop the bleeding.
Speaker 3: So the very first thing that's happening and that has
Speaker 3: to happen in order for regeneration to proceed is the
Speaker 3: wound healing. So the wound has to close over just
Speaker 3: like you know, we would have a wound if you.
Speaker 2: Think you're in the water. You don't want a bunch
Speaker 2: of stuff coming in and bacteria and all that stuff.
Speaker 2: So you have to heal the wound. There's different mechanisms
Speaker 2: that they do. Think like a purse string it sort
Speaker 2: of steals it, or you just have cells that spread
Speaker 2: that just close it up. Obviously, if you don't heal,
Speaker 2: then you're gonna fall apart. So there's a structural component.
Speaker 1: But that's the first thing, okay that makes sense. You
Speaker 1: first have to close off the gaping wound. That happens
Speaker 1: when you get an arm or a leg cut off,
Speaker 1: but that also happens when we get cut Our bodies
Speaker 1: also close the wounds. So why do we stop there
Speaker 1: and those animals don't.
Speaker 2: And it's really interesting is that the wound itself can
Speaker 2: signal to the rest of the body that regeneration needs
Speaker 2: to happen, big major thing that needs to happen for
Speaker 2: regeneration to proceed. So it's like a general response to
Speaker 2: a wound. But then from there it either says now
Speaker 2: I need to regrow or not.
Speaker 1: It says different signals.
Speaker 2: There's some sort of mechanism that they can detect that
Speaker 2: tissue was lost versus I just sort of made a cut.
Speaker 2: Something from the wound itself is able to recognize now
Speaker 2: I need to regrow ahead.
Speaker 1: So this is a key part of the process. Somehow
Speaker 1: these animals as bodies are able to tell it's missing
Speaker 1: a limb or a body part. There's some kind of
Speaker 1: signaling that happens that says, hey, this isn't just a
Speaker 1: flesh wound. We're missing an arm or a leg, and
Speaker 1: that calls in the cavalry. So once this signal goes out.
Speaker 1: You said it mobilizes things. What does that mean?
Speaker 2: So for regeneration, you need cells to make stuff right.
Speaker 2: Anything that you need that was lost needs to regrow,
Speaker 2: so you need to have cells for that. They have
Speaker 2: some sort of stem cells that mobilizes, stay, divide, then
Speaker 2: move to whatever they need to be, and they become
Speaker 2: the tissues that need to become.
Speaker 3: In salamanders, you have a few stem cells that are
Speaker 3: still in the system, but you also have cells that
Speaker 3: are sort of losing their identity that are sort of
Speaker 3: re entering the cell cycle to become something you're everywhere
Speaker 3: in the body, and then are recruited to that side.
Speaker 1: They actually move.
Speaker 3: Yeah, cells move a lot all the time. Actually, really.
Speaker 1: Okay. The next step is for the body to recruit
Speaker 1: a bunch of cells where you want the missing body
Speaker 1: part to grow. But these aren't just any cells. They
Speaker 1: have to be stem cells. If you haven't heard what
Speaker 1: these are. Stem cells are cells that can become any
Speaker 1: other kind of cell. If you think about it, the
Speaker 1: cells in your skin are very different than your eye cells,
Speaker 1: or your muscle cells, or your brain cells. That's because
Speaker 1: these are all cells that have become specialized parts of
Speaker 1: their DNA have been switched on or off so that
Speaker 1: they grow a stertin way. That's good because you wouldn't
Speaker 1: want your skin cells to suddenly turn into an eyeball cell,
Speaker 1: or you wouldn't want your brain cells to suddenly become
Speaker 1: muscle cells. Stem cells are cells that still have all
Speaker 1: their options open, and these are the cells that get
Speaker 1: called up to regrow a missing body part. Now, what's
Speaker 1: interesting is that some animals just have stem cells floating
Speaker 1: around their bodies, and some animals are also able to
Speaker 1: roll back their specialized cells to become stem cells. This
Speaker 1: part is going to be important later, but for now,
Speaker 1: just imagine a whole bunch of stem cells congregating at
Speaker 1: the wound site where the animal lost a body part.
Speaker 2: And they form this sort of massive cells at the
Speaker 2: wound site. Oh okay, last, it's a term for sort
Speaker 2: of they haven't fully met their fate. Yeah, they sort
Speaker 2: of accumulate at the wound site, which.
Speaker 3: Is sort of like a nose if you want to,
Speaker 3: like a cone on the end of the limb, which
Speaker 3: is the gross thoughe where all the cells are proliferating.
Speaker 1: Okay, so where you cut off a limb or a
Speaker 1: piece of heart or a fin A blob of stem
Speaker 1: cells forms, and from that blob grows the new part.
Speaker 1: We're gonna talk about how that blob knows what shape
Speaker 1: to grow into, which is super fascinating, and we'll talk
Speaker 1: about why us humans can't do any of this. But
Speaker 1: first let's take a quick break. You're listening to sign stuff. Okay,
Speaker 1: all right, So now we've got this massive of cells
Speaker 1: who are there to regrow whatever's missing. How do they
Speaker 1: know what to regrow?
Speaker 2: It's a great question, a very complicated question that we
Speaker 2: don't quite one hundred percent know the answer to. It
Speaker 2: depends on sort of what the tissue is. There's some
Speaker 2: sort of way that the animal is able to detect
Speaker 2: this is what's missing, and I think it's related to
Speaker 2: the idea of physitional information.
Speaker 1: Okay, we talked about how when you cut off a
Speaker 1: limb or a body part, these animals with this superpower
Speaker 1: start by closing the wound and then amassing a lump
Speaker 1: of stem cells where the body part used to be.
Speaker 1: The next question is how do these cells know what
Speaker 1: to grow into? I mean, you wouldn't want a tail
Speaker 1: or a brain to grow where your arm used to
Speaker 1: be and The answer is that every cell in your
Speaker 1: body gets signals about its positional information. That is, somehow
Speaker 1: cells seem to know where they are or where they
Speaker 1: should be in the body. And this is something that
Speaker 1: scientists don't fully understand yet, but it involves basically every
Speaker 1: cell talking to every other cell through special chemicals and molecules.
Speaker 1: And in this case, scientists have noticed an interesting pattern
Speaker 1: about how this works.
Speaker 3: The way it works is that the tip of what
Speaker 3: has been lost is sort of specified first genetically, and
Speaker 3: then you have sort of a dissonance between that very
Speaker 3: tip and whatever the stump area is, and that dissonance
Speaker 3: or that disconnect between those two genetic identities leads to
Speaker 3: that growth and to the exact replacement of everything that's.
Speaker 1: In the middle betweening. Wow, how does the tip not
Speaker 1: to turn into the tip?
Speaker 3: It's a genetic marker sort of their genes expressed there
Speaker 3: that are saying you are the tip now and like
Speaker 3: giving it that identity.
Speaker 1: Oh I see. It's like the distensios that are at
Speaker 1: the tip, they're like, okay, I don't see anyone else
Speaker 1: around us. We must be the tip exactly. That's fascinating.
Speaker 1: But then how do the cells in the middle know
Speaker 1: how to fill in the middle.
Speaker 3: Well, we haven't actually sort of researched all the details
Speaker 3: of that entire process. It's really it's sort of an
Speaker 3: interplay between cells dividing and genetic markers giving those cells
Speaker 3: positional identity, like saying this is where you are now
Speaker 3: you are a wrist bone, or you will be part
Speaker 3: of the wrist area, or you will be part of
Speaker 3: the musculature that is in the lower arm, kind of
Speaker 3: like that. It's an incredibly complex process actually, so it's
Speaker 3: quite amazing that it works so well.
Speaker 1: In other words, scientists don't quite know how the cells
Speaker 1: in animals that regrow body parts know what's missing or
Speaker 1: where they are or what shape they're supposed to. Mank,
Speaker 1: they just do, Like all of that is in our
Speaker 1: DNA to recognize, Oh, I'm between the tip, I'm about
Speaker 1: three quarts of the way between the tip and the shoulder.
Speaker 1: I must be you know, for our bicyle cell.
Speaker 3: Wow, Yes, it's all in the genetic markers.
Speaker 1: Like I said, it's a superpower. But as our experts
Speaker 1: point out, it's basically what happens when you grow in
Speaker 1: the first place, when you go from a fertilized egg
Speaker 1: to a fetus and eventually to a grown person. Somehow
Speaker 1: your cells know how to grow every part of you.
Speaker 1: Animals that regrow their body parts basically just reactivate that
Speaker 1: growing ability we all have in our DNA. Okay, the
Speaker 1: next big question is why don't all animals have this ability?
Speaker 1: How's a starfish, a flatworm, or a salamander different than
Speaker 1: us or dogs or cats or monkeys. Why can't every
Speaker 1: animal regrow limbs? What do you think it is that
Speaker 1: flatworms and starfish have the humans don't have?
Speaker 2: Yeah, the million dollar questions people always like, if you
Speaker 2: find something in the lab, can I just grind up
Speaker 2: the worms and inject it into my arm?
Speaker 1: You could be a starfish man.
Speaker 2: Yeah, I mean, hey, that wouldn't be too bad, I guess.
Speaker 1: So.
Speaker 2: One other thing that I haven't mentioned so far is scarring.
Speaker 2: It stops regeneration and a lot of context. So if
Speaker 2: you cut off your hand, God forbid, you have obviously
Speaker 2: a wound response, and it's going to heal and it's
Speaker 2: going to scar. When a lot of these other things regenerate.
Speaker 2: The starfish, the worms, even things like a salamander, an
Speaker 2: axle auto, they don't scar. It's a major sort of
Speaker 2: block of regeneration, and also that inflammation prevents a lot
Speaker 2: of these other steps that could be happening. There's little
Speaker 2: to no inflammation and a lot of these animals that
Speaker 2: can regenerate. So it's a relationship between scar and inflammation
Speaker 2: and ability to regrow.
Speaker 1: Ah, here's the first piece of the puzzle. Animals with
Speaker 1: the ability to regrow body parts don't scar and they
Speaker 1: don't get inflammation when they get cut. Making a scar
Speaker 1: is basically like patching a hole in your wall with
Speaker 1: a giant pile of bricks. Scar tissue is thick and
Speaker 1: tough and dense, and it basically gets in the way
Speaker 1: of regrowing a new body part.
Speaker 2: But these simpler animals, they just seal it up and
Speaker 2: that seems to be enough.
Speaker 1: They don't make a scar, basically, they do not, So
Speaker 1: that's one factor. The other fact that you said was inflammation.
Speaker 1: So when we get cut, it becomes inflamed, and how
Speaker 1: does that make things hard to regenerate?
Speaker 2: So it prevents any of the later steps from happening.
Speaker 2: It just prevents the ability of the tissue to regrow.
Speaker 1: It's not a happy state for cells to regenerate. No,
Speaker 1: that is not. What doctor Wolf is saying is that
Speaker 1: when we get an arm or a leg cut off,
Speaker 1: our body basically overreacts. It rushes to patch up the wound,
Speaker 1: building thick SCRs that block any regrowth, and it deploys
Speaker 1: the immune system, which makes everything so inflame that it
Speaker 1: prevents all the steps needed to regrow the body part.
Speaker 1: Cells don't revert back to stem cells, they don't divide well,
Speaker 1: so they don't form that block of generic cells that
Speaker 1: can then become the missing part. So now the question
Speaker 1: is why do we do that? Why do our bodies
Speaker 1: react that way, especially because at some point in our
Speaker 1: evolutionary history it seems we did something different. Okay, now
Speaker 1: the question is why can't humans regrow our lips or
Speaker 1: can we? Is that possible?
Speaker 3: Well, that's a question that always comes to all people
Speaker 3: who who work the regeneration. We always get that question,
Speaker 3: and that's not an easy question to answer because it's
Speaker 3: probably a combination of very complex factors that play the
Speaker 3: role in this.
Speaker 1: But we know from the.
Speaker 3: Fossil record that a lot of the ancient amphibians could
Speaker 3: regenerate too, and lungfish are the closest living relatives of
Speaker 3: os fourlimd vertebrates can also regenerate very similar to salamanders.
Speaker 3: So it is likely that regeneration as we see it
Speaker 3: in salamanders now is actually not something that is special
Speaker 3: to modern salamanders that evolved. But salamanders probably are the
Speaker 3: only living vertebrates today that can still regenerate. Back in
Speaker 3: evolutionary history, that was probably a widespread feature that then
Speaker 3: got lost in the course of evolution.
Speaker 1: Oh so interesting. It's not like they developed the special ability,
Speaker 1: they're the only ones who kept it exactly. Oh my goodness.
Speaker 1: What doctor Frobish is saying is that at some point
Speaker 1: basic all animals were able to regrow their body parts. Salamanders, starfish, flatworms.
Speaker 1: They don't have a superpower. It used to be a
Speaker 1: standard feature of all animals. But evolution at some point said, eh,
Speaker 1: you know what, let's not do that. It's not a
Speaker 1: skill some species evolved, it's a skill some of us lost. Okay.
Speaker 1: The obvious next two questions are why did evolution choose
Speaker 1: not to regrow limbs in species like ours? And if
Speaker 1: it's something we had but lost, could we bring it back.
Speaker 1: We'll answer both those questions after the break. We'll be
Speaker 1: right back, and we're back all right. We just learned
Speaker 1: a shocking piece of information, which is that scientists believe
Speaker 1: it's likely that basically all animals, these ambiotes, which are
Speaker 1: all for legged animals, used to be able to regrow
Speaker 1: missing body parts, but at some point in our evolution
Speaker 1: this ability was selected as not being the best for
Speaker 1: our survival, which is strange because you think that being
Speaker 1: able to regrow limbs would be a good thing. I mean,
Speaker 1: if you use a leg you can't really run away
Speaker 1: from a predator that's trying to eat you. So I
Speaker 1: asked our experts, why would we lose this ability?
Speaker 3: It seems like something so useful to have, right, that
Speaker 3: capacity to regenerate, but it also comes at the cost.
Speaker 3: So during the course of evolution, it probably was a
Speaker 3: matter of a trade off, and it's probably for that
Speaker 3: reason that regeneration was selected against during evolution at some
Speaker 3: point in amniote evolution. And it could have happened once,
Speaker 3: it probably had happened multiple times.
Speaker 1: Oh well, what's the trade of.
Speaker 3: Salamander's a giant cells, So they have like huge cells
Speaker 3: because they do have a lot of DNA that they accumulate.
Speaker 3: They just can afford to have all that DNA, But
Speaker 3: all that DNA has to be the transcribed, right, it
Speaker 3: has to be done for a second copy, and that
Speaker 3: caused a lot of energy and resources.
Speaker 1: Okay, the first reason we might have lost our ability
Speaker 1: to regrow body parts is that there's a cost to
Speaker 1: having that ability. First of all, it requires extra DNA.
Speaker 1: The program to regrow LIMB takes extra instructions and extra
Speaker 1: genes that can come at a cost, especially if you
Speaker 1: want to move faster and have a faster metabolism. I
Speaker 1: wonder if what you're saying is that at some point
Speaker 1: in our lineage, the evolution pressure was to like, hey,
Speaker 1: let's make things more efficient. We don't need all this DNA,
Speaker 1: and maybe one of the cuts was our ability to regenerate.
Speaker 3: Yes, that's it. So it's not just that we lost DNA.
Speaker 3: It's also that we have to go through cell cycles
Speaker 3: at a certain speed in order to keep our very
Speaker 3: high metabolism that allows us to run really fast, to
Speaker 3: sustain a constant body temperature, and do really powerful energy
Speaker 3: consuming things like run and fly and do these kind
Speaker 3: of things. If you have a very high metabolism, you
Speaker 3: have to go through these processes of making new cells
Speaker 3: quicker than when you're an organism with a very slow metabolism.
Speaker 3: We're constantly building new cells all the time, regenerate your skin,
Speaker 3: your hair is growing, You're making new blood cells all
Speaker 3: the time. Salamander's day can regenerate because they don't have
Speaker 3: to sustain such a high metabolism. We cannot regenerate, but
Speaker 3: we can run fast and keep our body temperature or
Speaker 3: fly in these kind of things.
Speaker 1: What doctor Frobish is saying is that any extra ability
Speaker 1: meets extra DNA, and that can come at a cost
Speaker 1: if you want to be more active, because being more
Speaker 1: active means you're cycling through cells more. And each time
Speaker 1: you make new cells, you have to copy all that DNA,
Speaker 1: which takes a lot of energy. And the other cost
Speaker 1: is that it's just a lot to grow a new
Speaker 1: arm and a leg. I mean, you have to make
Speaker 1: a whole new limb from nothing.
Speaker 2: Can you imagine having a stump for how long to
Speaker 2: regrow all of this complex tissue. It would take a
Speaker 2: lot of energy and it'd take a lot of time.
Speaker 2: If the best thing to do is just to close
Speaker 2: it and seal it and make sure the structure there.
Speaker 2: With that scar is sound. Then maybe that's for the best,
Speaker 2: maybe more advantageous energetically and also just structurally to do that.
Speaker 1: Oh interesting, Maybe we don't need that extra arm. Maybe huh,
Speaker 1: I guess you don't. Technically, you don't need an extra
Speaker 1: arm to have more babies.
Speaker 2: Yes, that is all true.
Speaker 1: So that's the reason we can't regrow limbs. We, meaning
Speaker 1: four legged animals, used to be able to, but evolution
Speaker 1: at some point decided it's not worth it. Losing an
Speaker 1: arm or a leg is a huge deal if it
Speaker 1: happens to one of us, but as a species, maybe
Speaker 1: it's okay. Maybe it's better to just cut our losses,
Speaker 1: patch things up quickly with scar tissue, clean it up,
Speaker 1: and move on if it means everyone collectively being more
Speaker 1: efficient and having a faster metabolism. Now you're probably thinking,
Speaker 1: if the ability to regrow body parts was in the
Speaker 1: DNA of our animal ancestors, could we revive it somehow
Speaker 1: so that we can all regrow limbs again. I'll get
Speaker 1: to that in a minute, but first I wanted to
Speaker 1: tell you about something kind of cool about the Mexican
Speaker 1: tetrafish that sort of proves the whole hypothesis about why
Speaker 1: we can't regrow limbs. Here's how Ezra describe these fish.
Speaker 4: It is such a unique and perfectly slitted model organism
Speaker 4: to study it because it has two different sometimes. So
Speaker 4: the surface dewelling version has this silver colored functional eyes
Speaker 4: and dailian streams. They look very much like what we
Speaker 4: know f fish. But the cave dwelling version is tail pink.
Speaker 4: It is almost translucent. They don't have functional eyes.
Speaker 1: Okay, the Mexican tetarfish is fascinating because it's at a
Speaker 1: point where it's about to possibly evolve into two different species.
Speaker 1: There are currently two kinds of Mexican tetarfish. The ones
Speaker 1: that live normally in the rivers above ground. Those are
Speaker 1: called the surface dwelling kind, and there are the ones
Speaker 1: that live inside of caves. And the ones that live
Speaker 1: in the rivers on the surface can regenerate their hearts,
Speaker 1: but the ones that live in caves can't, even though
Speaker 1: the still technically the same species, So some of them
Speaker 1: can and some of them cannot. You said, it's related
Speaker 1: to where they live. What do you mean it's related
Speaker 1: to where they live.
Speaker 4: During evolution, the cave dwelling version lost its ability to
Speaker 4: regenerate their hearts, and it is because they entered the caves,
Speaker 4: they got thropped there and they had to continue living
Speaker 4: in a complete darkness with no enough food and no
Speaker 4: external food sources. They had to minimize their metabolism, and
Speaker 4: they had to find a way to compromise from the
Speaker 4: things that they don't really need to survive, and heart
Speaker 4: regeneration happened to be one of them.
Speaker 1: So basically we're seeing the whole theory of why we
Speaker 1: can't regrow limbs play out right in front of us.
Speaker 1: Some of the tetrafish have migrated to live deep inside
Speaker 1: caves and because there is little food, low oxygen, and
Speaker 1: not as many predators, we can see evolution in real
Speaker 1: time basically say, eh, we don't really need to regenerate hearts.
Speaker 1: It's better to throw out that skill and save energy. Wow,
Speaker 1: that's so interesting that they would lose that ability. Do
Speaker 1: they know how they lost it?
Speaker 4: There are some genes that are responsible, but there are
Speaker 4: so many we only started exploring these.
Speaker 1: And this is where we get to the edge of
Speaker 1: current signs, because if we can figure out how exactly
Speaker 1: we lost our ability to regrow limbs, like if we
Speaker 1: figure out which genes got turned off or lost, then
Speaker 1: maybe we can get that ability back. I asked our
Speaker 1: experts if they thought this was possible. All right, last question,
Speaker 1: doctor Wolf. If you could enable humans to regenerate limbs,
Speaker 1: parts of our organs, how would you do it?
Speaker 2: Ooh, okay, Well, technically, I will say we do have
Speaker 2: some regeneritive capacity, so parts of your liver can regenerate.
Speaker 2: There's also a lot of research and information about the
Speaker 2: tips of your fingers, so kids could regrow the tips
Speaker 2: through your fingers.
Speaker 1: I've had that happen to me.
Speaker 4: Yeah.
Speaker 1: Yeah, So we do have.
Speaker 2: Some ability, but over time you just begin to lose
Speaker 2: that ability, so we're not quite the worst. I believe
Speaker 2: birds are famously not that regenerative. But if I had
Speaker 2: to make a human regenerate, I think there's several things
Speaker 2: that need to happen. One is, scarring needs to be minimized,
Speaker 2: Inflammation needs to be minimized. We need to mobilize those
Speaker 2: cells enough.
Speaker 1: What you said that it's probably the same kind of
Speaker 1: signaling that happens when we're first growing to art. Originally, yes,
Speaker 1: that would be the idea.
Speaker 2: So how is it that digits were made in an embryo,
Speaker 2: take some of that information and see if those sorts
Speaker 2: of things can be turned on again because we have
Speaker 2: them now.
Speaker 3: There might be a sheer genetic program in all forlimp
Speaker 3: vertebrates that is still present to a certain degree.
Speaker 1: Meaning it might still be hidden somewhere in our DNA,
Speaker 1: but somehow it's not activated. How would that work, like
Speaker 1: through gene editing, go into human DNA and like, oh wait,
Speaker 1: here's something that needs to be tweaked, and this here
Speaker 1: and there and there, and oh, now we have the
Speaker 1: ability to regenerate limbs.
Speaker 3: So it will require a lot of different scientists coming together,
Speaker 3: which will be like the biomedical people, the molecular biologists,
Speaker 3: people working specifically with Excel models and other organisms as
Speaker 3: model organisms to really understand all the details of the processes.
Speaker 3: So I think in the end, all these lines of
Speaker 3: evidence will come together to know what the process is
Speaker 3: triggered by and how to maybe implemented in humans at
Speaker 3: some point in time.
Speaker 1: All right, So the answer to the question why can
Speaker 1: we regrow limbs seems to be who says we can't?
Speaker 1: That ability might be hidden inside of us and all
Speaker 1: we have to do is regrow it. Thanks for joining us.
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