[Music]

Stefan: Every year around January 1st, you'll hear people say, "New year, new me." And while it's not tied to the calendar, there might actually be some truth to that statement. Like when your body   literally makes new parts of itself: From regrowing your liver after donating part of it or when you grow a new spleen that you didn't even know you grew. True story. We don't regenerate lost limbs like some other animals do, but we can regenerate a lot more of ourselves than you might realize, and that can happen naturally or a little more intentionally. See there's a cool sci-fi future where you can use stem cells to regenerate all sorts of body parts, and that future is getting closer with every new year. Here's Savannah with  a progress report on regeneration thanks to stem cells acquired from the unlikeliest of places like period blood.

[slide: "Can We Treat Alzheimer's With Period Blood?"]

Savannah: So let's talk about stem cells: What are they and why do they matter so much. Stem cells are the most flexible kind of cells in your body and as they reproduce, they can differentiate into specific and different kinds of tissues. They're kind of like an Eevee -  they'll turn into something different based on the environment   they're subjected to but instead of a Fire Stone, the stem cells get like a liver cell stone or something.

These little cells are all the buzz in the world of regenerative medicine - the field  focused on healing or replacing tissues that  have been damaged by trauma, disease, congenital abnormalities, or even just aging. Basically, regenerative medicine is the stuff we do to try to help a body heal its own tissues and for a while the most promising thing we've been able to throw at this problem has been stem cells. So the hope in stem cell therapies is that you can take your stem cells and turn them into say neurons, and then put those healthy cells into someone whose  neurons are diseased. Theoretically, embryonic stem cells - or ESC's - offer the most potential. They're so great because of their extreme plasticity - they're able to become any tissue or organ in the body, which makes sense since embryos need to go on to differentiate into all those bits as they grow.

In practice, however, embryonic stem cells have had limited use because of two major problems: First the ethics of destroying an embryo even in the name of life-saving research are still hotly debated; second, fresh embryonic stem cells are just hard to come by. Most are embryos left over from in vitro fertilization which means that of the very small pool of IVF embryos to harvest from, science only gets the extras. With this dubious future for embryonic stem cells, the hunt for cells with similar plasticity, more renewable sourcing, and better publicity began.

Enter mesenchymal stem cells or MSCs - cells that are harvested from adults and thus avoid the ethical dilemmas and bad PR of the embryonic ones. When it comes to plasticity, not all stem cells are created equal. Embryonic stem cells are valued because they are  pluripotent, meaning they can turn into any adult cells no matter what part or layer of the body that tissue comes from. But not all stem cells are like this. There are some that can only turn into a few kinds of cells, and those less-adaptable stem cells just don't have as many potential applications.

Mesenchymal stem cells aren't pluripotent but they're pretty darn close. They can become bone, muscle, blood vessels, and connective tissue cells, and even liver cells, which is kind of a huge deal. But while mesenchymal stem cells are ethically less complicated and demonstrate similar plasticity to embryonic stem cells, they're not exactly easy to get into your petri dish. You mostly get them through invasive procedures like bone marrow donation, liposuction, or apheresis which is blood filtering.

Or at least, that was the case until a new source of mesenchymal stem cells was discovered -  menstrual fluid. Scientists had a   hunch that the uterus might be utilizing stem cells for its monthly redecorating. So in 2007, they collected menstrual fluid, isolated the cells, and got to work testing them to see if they were actually stem cells. There are two basic tests that confirm whether a cell is a stem cell or not. Can it clone, and can it differentiate into other types of cells? The answer to that was "yes" on both accounts. The isolated menstrual cells were not only able to double over 68 times, they doubled faster than the mesenchymal stem cells isolated from other body parts.

Bone marrow stem cells have been the gold standard for mesenchymal stem cell research, but they take anywhere from 2 to 8 days to double their population. In contrast to this, the menstrual cells took an average of just over 19 hours to double, which means they can double way faster than bone marrow derived MSCs. And this also means they double faster than the mesenchymal stem cells derived from other places like umbilical cord blood, adipose tissue, or Wharton's Jelly -  a thick gelatinous tissue that cushions the blood vessels of the umbilical cord.

But while cloning is important, it's only half of what it means to be a stem cell. The other half deals with differentiating into many types   of tissues which is that plasticitywe've been talking about. The plasticity of mesenchymal stem cells can vary depending on where they're derived from. For example, studies suggest that those from   the umbilical cord can't differentiate into fat cells and those from the placenta can't become bone cells. Menstrual stem cells, however, were able to differentiate into all nine different tissue   types the researchers tested, developing into everything from neurons and liver cells to fat and bone cells. So in terms of being stem cells, menstrual MSCs not only meet the criteria, but also outperform MSCs derived from other bodily locations. And it turns out that these overachieving cells can also do a lot of what we want them to do in regenerative medicine, too.

In a 2010 study, researchers simulated stroke conditions in neurons from rats to see how menstrual MSCs might affect outcomes in oxygen-deprived rat neurons. This involved researchers harvesting   human menstrual stem cells, then placing them in a media perfect for growing neurons in the hope that they would differentiate into exactly that. They then injected them into the brains of rats who   had suffered a stroke. And sure enough, they found that the rats who were given these menstrual stem cells had fewer behavioral and motor deficits than the control group that got no treatment.

And a 2018 study looking at Alzheimer's in mice found that  injections of menstrual mesenchymal stem cells into the brain could not only correct learning and memory deficits and diseased mice, but even helped to remove the plaques in their brains. Researchers have also studied how menstrual stem cells can treat mice with diabetes and found that those stem cells can step in for the pancreas to make insulin. Menstrual MSCs have even been used to restore liver function, improve COVID-19 outcomes, reduce inflammation from hernia meshes, diminish infertility, and accelerate wound healing. Plus, surveys show that people are already willing to donate. And most perceive their periods more positively knowing the incredible positive impact their menstrual donation could make.

But before you show up to the Red Cross with your Diva Cup, it's important to note that menstrual fluid donation sites are not widely available yet. For those of you that are eager beavers though, there are a few options: There's a facility in India that has already begun menstrual blood banking, and one company in Florida charges for private menstrual blood banking as well. There are even some clinical trials in the United States that are enrolling participants to collect menstrual blood during gynecological visits.

Periods are no walk in the park, but the next time you find yourself   dreading your next visit from "Aunt Flo," remember that someday it might be your menstrual fluid that goes on to cure somebody else's Alzheimer's. 

Stefan: If your doctor's saying, "New year, new organ," you might be able to benefit from the progress in stem cell research one day. But if you're part of a special 20% of people, your doctor could  also give you the opposite news - "You already made yourself a whole new spleen." Here's how you could be making a new spleen right now.

[slide: "20% of Humans Have An Extra Spleen"]

We generally know that there are organs in the human body that we can - quite literally - live without: your appendix, your gallbladder, your spleen - these organs are kind of optional, but there's a case where the reverse is also true. Not only can you do without your  spleen, you can do with multiple. Kind of. Because you might have been born with more than one. Or you might even grow another over the course of your life. It comes down to how the spleen both  forms in the developing body and recovers from injuries. So let's take a look at what makes it so unique. Or I guess the opposite if you have more more than one. Let-, let's just have a look.

First, let's go over what a typical singular spleen does. It's part of your immune system. It serves as a gathering point for white blood cells, which help fight off infection. It also filters out old, damaged red blood cells, which keeps your blood healthy. But when your spleen gets damaged either through disease or injury, doctors can   safely remove it, and you can go on to be pretty much fine.

Though you will have to be a little more cautious since not having a spleen does make you more susceptible to illness and infection. Some doctors recommend long-term antibiotic therapy after a splenectomy, maybe even for life. And your doctor will want you to be vaccinated against certain bacterial illnesses that your spleen is   otherwise pretty good at protecting you from. But given that the spleen isn't critical the way your heart and lungs are, that only makes it strange that you can sort of end up with extras. In fact, there are two ways you can end up with an extra spleen:

First, you can have an accessory spleen, which is an extra spleen that you're born with. And having an accessory spleen is surprisingly not that uncommon. In fact, according to estimates in the medical literature, anywhere from 10-30% of people are born with an accessory spleen, which means one out of every five or so people you know is likely to have one. What's more, an estimated  26% of people with accessory spleens have more than one accessory spleen.

Now accessory spleens aren't exactly extra spleens though. The average healthy adult has a spleen that measures about 13 cm long by 7 1/2 cm wide while an accessory spleen is typically much smaller than that - roughly 1 to 3 cm in diameter. Normally when a spleen forms during embryonic development, several buds of  tissue link up to form the mature organ, which maybe sounds like a silly way to build one thing, but hey, that's how we get pineapples - just a fun pineapple fact for your spleen video. But anyways, sometimes those buds don't link up all the way. When this happens some of the tissue can end up in other places, usually nearby, but sometimes in more distant locations like the stomach or small  intestine.

Despite being being smaller than spleens, accessory spleens are still pretty close to the real thing; they have smooth muscle to help move blood along and get their blood supply from the spleenic artery - features that mirror the actual spleen. And the existence of an accessory spleen can even provide some residual function for   someone who's had their spleen removed. In fact, it may even get bigger after a splenectomy, suggesting it may be taking on more spleeny work. Though to be fair, it won't be as effective as having the full-sized version.

It's also not necessarily a good thing to hold on to that spleen function. if you had a splenectomy because of a deleterious  condition like hypersplenism, any accessory spleens probably have the exact same problem, and doctors will want to have them out, too. Shich means they'll need to find them. Not always easy.

Now the other way to get an extra spleen is through splenosis, though in this case, we're being a little more fast and loose with the idea of an extra spleen, - it's more like having little bits of spleen tissue in assorted non-spleen parts of your body. Unlike an accessory spleen, these bits of tissue don't have smooth muscle, and they aren't connected to the splenic artery. Instead, the tissue gets its blood supply from whatever blood vessels are nearby.

Splenosis also happens over the course of a person's life. Specifically, it can happen after an injury to the spleen when little  bits of spleenic tissue get separated from the primary spleen and travel to other parts of your body. They can even end up in distant parts of the body, like across the diaphragm, in the chest cavity, or even in the brain, though that's pretty rare. And this tissue can have some function. Though, again, it isn't going to stand in for an actual spleen. Both splenosis and accessory spleens are benign. However, if an unknown lump of tissue shows up on any kind of scan, doctors will want to rule out stuff like cancer. In practice, extra spleens also rarely cause problems other than weirding you out when you think about all the tiny spleens making themselves at home in your body. But especially in the case of splenosis, ending up in the wrong place can result in complications.

Now, all of this does beg an interesting question: If you don't even strictly need your spleen, why is it so good at growing in and even regenerating? I mean, why didn't someone send a memo to the human body that it would be much nicer if you could do this with like a lung or something. And scientists don't really seem to have a great answer to that question; they're only just starting to understand how the spleen is able to do this.

In 2017, researchers in Japan and Australia identified a type of stromal cell that functions as a spleen organizer. Stromal cells are   the cells that connective tissue is made from. In the spleen, these cells are thought to help control tissue regeneration. But as for the "why," well, that's even more mysterious. though it's worth noting that the liver - another organ that helps filter old red blood cells - is also good at regenerating. In fact, scientists have demonstrated that a healthy liver can regenerate itself after a loss of around 2/3   of its mass.

Understanding exactly how and why splenic regeneration works will help doctors develop ways to regenerate spleens for people who  need them, and it could also provide clues for how to produce other lab-grown organs, too. Until then though, having extra spleens is just one of those bizarre, mostly harmless things that the human body comes up with to confuse us from time to time.

[transition]

So, this year you might find out you have one extra spleen, but that's not the only organ you can make more of. In the last video, I mentioned that you can regenerate up to 2/3 of your liver, but that   doesn't mean you can keep donating an organ like you donate blood over and over again, and Hank is here to tell you why.

[slide: "Can You Keep Donating and Regrowing Your Liver?"

Hank: Here at SciShow, we recently learned a fun fact - you can donate over half your liver to someone who needs a transplant, and the tissue will grow back within a year. So, then we got super curious: Could you just keep cutting chunks off and donating them and regrowing your liver, kind of like donating blood except it's an organ. It turns out the answer is "no," which is kind of a bummer because that would have been cool. But the reason why is actually really interesting - it's because of how the liver cells get replaced.

Liver regrowth in humans is pretty amazing. In a study of 27 living liver donors. it took only about a month after donation for their liver function to return to normal and less than a year for their liver to regrow its normal mass. And you could probably regrow your liver more than once. In one study a very unlucky rat had part of its liver removed again and again, and supposedly, it grew back 12 times. That being said, rodent livers have a different structure than ours, so it might not work exactly the same in us.

But even if it's possible to donate part of your liver more than once, transplant doctors probably would not recommend it. First of all, even though it's pretty safe if you're under age 60 and healthy, it's a major surgery. And the bigger problem is the regrown liver chunk likely wouldn't do any good because the liver regrows but it technically doesn't regenerate. True regeneration occurs in some animals like salamanders; if a salamander loses a leg, cells near the cut dedifferentiate. In other words, they revert to an earlier  developmental state; they basically become the cells in an embryo that have no specific job, but they have the potential to turn into tons of cell types - from bone to skin to muscle - depending on what molecular signals they get. Then they multiply, divide up into groups, and respecialize into the cells that make up a leg. It's essentially a redo of what happens as the salamander embryo  develops, so the leg structure is the same, and the scientists consider it regeneration - good as new.

This is not what happens when part of your liver gets cut off. Instead of all this dedifferentiation, what mostly happens is a variety of mature liver cells just multiply to make up for the loss. The  regrown tissue has some structure, but it doesn't replicate the exact layout of a fresh liver from cell organization to the arrangement of blood vessels. So even though the regrown liver is fully functional, it's not true regeneration. Technically, what your liver does is called "compensatory hyperplasia."

This different structure would make it harder for transplant surgeons to safely remove a portion of a repeat donor's liver. It would also be harder to connect that regrown chunk to the  transplant recipient's blood vessels so they could actually use it. So even though a liver donation can save someone's life, doing it more than once isn't a good idea for you or the person who needs the help.

[transition]

Stefan: So there are limitations to even our most famously regenerating body parts; you can only really donate your liver once, just like you can only grow in a new set of teeth once. But wait. Why can't you grow more teeth? Here's me with the answer.

[transition]

Now to start, not all teeth are created equal; well, except for when they are all created equal. So, let's go back. Some tooth-having animals, like many sharks, are "homodonts," which means that   every tooth in their mouth is more or less the same shape -  generally bigger at the bottom and pointier at the top. But other animals have fancier teeth with a whole bunch of different shapes, and these animals are "heterodonts," including me.

Humans and our relatives being heterodonts is the reason that anthropologists can find a single tooth and tell exactly where it went in its owner's mouth. Each tooth is so different in shape that it looks totally unique. And having all those different shapes in your mouth is pretty useful because each one can have a shape that suits a specific purpose - like having molars for crushing and canines for  slicing. It's almost like keeping a multi-purpose tool in your pocket all the time, but instead of your pocket, it's in your face.

But that's not to say that homodonts aren't doing cool stuff with  their teeth. Sharks, for example, are constantly regrowing their teeth on a cycle that results in an alternating pattern of old and new teeth along the road. What that basically means is that if a shark gets decked in the jaw and loses a bunch of teeth at once, there's a fresh set just under the gum line that's ready to pop up, so the shark never spends too much time without its chompers. And when one tooth gets broken or damaged, the replacements are only so many weeks away from coming in. That wave-like pattern also results in the teeth that are mature and sticking out having somewhat staggered heights, giving them a kind of organic saw-tooth shape along the whole row. So, there are some perks to being a little matchy-matchy with your teeth, but if there weren't upsides to having specialized teeth, nobody would would be doing it. And the kinds of stuff heterodons can do with those specialized tooth   shapes can get really specific.

Take carnivores - they've got a pair of teeth called their "carnassial teeth" that are basically Nature's scissors. Their last upper premolar and their first lower molar have long, sharp ridges that rub against each other in a way that's perfect for slicing through meat. And dental specializations aren't just for slicing and dicing. Certain monkeys, like baboons as well as a few apes, have what's called a "C-P3 honing complex," which means that their lower canine and premolar are always rubbing against the top canine, keeping it sharp. And they need these sharpened canines, not to chow down, but to throw down.

See, these animals are pretty territorial, and they fight over access to females for mating. One study even found that the more often the males of a species fight, the bigger their canines are. So, having two giant sharp knives in your mouth is a great way to defend your territory or your girlfriend.

And there are a few animals that use their teeth to keep themselves looking neat and tidy. Lemurs, colugos, and hyraxes all have what are called "tooth combs," which they mostly use to groom their fur and pick off parasites. So ,the basic idea is that in order for all these highly specialized tooth shapes and functions to work, all the teeth need to be the correct size so they fit together.

As mammals' teeth gained complexity, they lost the ability to   regenerate. Like, imagine if those carnivores lost a piece from their carnassial complex; they'd be stuck with one half of a pair of scissors, which is not that great. And nobody likes a broken comb. If you're waiting for a tooth to grow in, that's what you're stuck with. Basically, at some point along the evolutionary track of most mammals, it turned out to be so much better to have specialized tooth shapes that the ability to grow more sets of teeth had to be sacrificed.

That said, there are a few mammals who have managed to find  workarounds to the whole "one set of adult teeth" thing, and oddly enough, figuring out how to do that has also meant growing your teeth in a different direction, too. Take elephants. All living species of elephants have gigantic molars that look more like car batteries than teeth: They're huge, rectangular blocks made up of thin sheets of enamel, and when the elephants use them to chow down, they really only chew on the front part of the tooth. As the plates of enamel at the front wear down, they fall off the molar, and out of the elephant's mouth. They have about six or seven molars in each quadrant of their mouth over the course of their lives, and as the first one gets worn down, the next one in line moves forwards - like a conveyor belt. This system means that elephants have a fresh grinding surface for their entire lives; as in the elephants will starve to death once their last molar wears out all the way.

And silvery mole rats manage to grow in new molars at the back of the mouth as the ones at the front wear out without an apparent   limit on how many they can grow. Unlike elephants, they don't just form a whole set and wait to bump them into place; they actually generate a new batch of teeth when they're needed, which is really rare for mammals.

So there we have it; the reason you have to suffer through root canals is the same reason you can bite a chunk off of an apple with your front teeth and crunch it to bits with your molars. And while some may say it was worth it evolutionarily, I'd like them to tell that to my dental bills.

[transition]

It might seem like you should be able to grow more teeth and you shouldn't be able to grow dark hair once it starts going gray, but the human body is bizarre like that, and those two body parts work in opposite ways. So here's the science behind reversing your gray hair.

[slide: "Can Grey Hair Be Reversed?"]

Savannah: If you're over 30 and haven't started going gray yet, it might be because of your genes. After all, in identical twins it's pretty uncommon for one twin to have gray hair while the other doesn't, but if you're lucky enough to live a long life, you'll earn that silver medal eventually. Unless you preempt the graying before it happens by keeping the cells that color your hair alive and well.

Those cells are called "melanocytes," and their death is one of  the things that turns your hair gray as you get older. Well, not just the melanocytes; the whole hair follicle gets damaged by a process called "oxidative stress," and really anything with the word "stress" in the name can't be good. See, when your melanocytes make their melanin pigment to keep your hair dark, they also make hydrogen peroxide as a byproduct. Having that hydrogen peroxide in your hair is fine when you're young, but as you age, you make fewer antioxidants to neutralize it. So when you get older, the large amount of hydrogen peroxide in your hair mutates your DNA and damages cells, which means this byproduct of making melanin   destroys the very cells that create it until eventually you end up with a bunch of hydrogen peroxide-filled gray hair.

But researchers in the UK and Germany are working on ways to prevent hydrogen peroxide buildup around these cells. So far, they're at the point where their interventions work in a petri dish, but hey, that's progress. Their goal is to break down hydrogen peroxide and prevent oxidative stress from damaging hair cells, and they managed to find an amino acid called L-methionine that does exactly that. Exposing your hair cells to L-methionine works as long as it exceeds the amount of hydrogen peroxide floating around. So in the future, with the approval of a healthcare professional, you might be able to take L-methionine pills to keep your color.

But if you're already gray, there may still be a way to reverse it, and this time it's not about keeping your melanocytes alive but rather keeping them moving. See, your melanocytes start out as stem   cells - you know, the cells that can grow up to be whatever they set their mind to. And when you're young, those stem cells move back and forth inside each strand of hair across the bulb, the follicle, and the bulge right above it. Because, as it turns out, the location of your melanocytes is what determines if they can produce color or  not. There are chemical signals that help melanocyte stem cells turn into color producing melanocytes, and melanocytes only come into contact with these signals in certain parts of each strand of hair, like the bulb. So when melanocytes travel down to the bulb, they become color producing cells. Then these cells travel back to the bulge to regenerate, losing their color and starting the process all over again.

But as you get older, your melanocytes don't move around quite as much as they used to. They get stuck in the bulge, so they can't access the proteins that would otherwise activate them to create pigment, and that's another cause of gray hair. But that doesn't necessarily mean you have to stay gray for the rest of your life.  

The researchers that discovered this melanocyte movement pattern think you might be able to reverse graying by dislodging your melanocyte stem cells from their rut; that could help them differentiate into pigmented melanocytes. And this isn't the first time people have considered melanocyte differentiation as a way to add color back to hair.

In fact, the first time was kind of an accident. A study published in 1986 was inspired by treatments for patients with vitiligo, which is a depigmentation of the skin and hair that's not necessarily from advanced age. One vitiligo treatment called PUVAsol uses UV light and chemical therapy to restore pigment. The chemical part can be consumed 2 hours before noon; then, when midday comes along,  patients soak up the sun's rays for 10 to 15 minutes. While researchers aren't entirely sure how this treatment restores color, one hypothesis is that it has to do with helping your melanocytes  move around, which makes sense given the more recent information we have about how important melanocyte movement is for pigmentation.

Originally, PUVAsol was used as an effective treatment to add color back to the skin and hair of vitiligo patients. Years later, the treatment was extended to people who didn't have vitiligo to see if it could add color to hair that had lost pigment for other reasons. And it worked. At least for some people. Most participants including those with and without vitiligo diagnoses had more color in their hair than before treatment, and it was still going strong months later.

We're going to need more research to understand why PUVAsol  doesn't work the same way for everyone, but with the progress being made in our understanding of melanocyte longevity and movement, it may just be a matter of time.

[transition]

Stefan: So, people are awesome. We can grow new spleens, we can regenerate our livers, and we might even be able to regrow dark hair after losing pigment. But if you think we're cool, other animals - like salamanders that can regrow entire limbs - kind of have us beat. So, here's how they do that incredible feat and why we still can't.

[transition]

Hank: Starfish can regenerate arms, certain lizards can grow back severed tails, some flat worms can recreate their entire body from a single adult cell, and my skin will grow back together after a paper cut. On some level, we can all do a little regeneration. But if I cut my arm off, I'm not going to expect to grow a new one, right? The wound would eventually be covered over with a rugged, fibrous  matrix of the protein collagen, forming what we know as scar tissue, with a layer of skin on top, but I'm not going to grow a new appendage.

But that is not true for everyone; the real superstars of limb regeneration are the members of the order Caudata -  salamanders. Cut off a salamander's leg - don't actually cut off a salamander's leg you, know what I mean - and cells at the wound site won't form  scar tissue. Instead, they'll transform into what researchers call a "wound epidermis," which activates a wave of chemical instructions to the cells below. Soon, the nerves in the stump begin to grow again, and mature muscle and tissue cells actually revert back to their immature state - sort of going back in time before they were specialized cells. They then start streaming toward the wound, forming a mass called a "blastema."

These undifferentiated cells are a lot like stem cells or what embryonic cells are like during development before a gene is activated that tells them what they're going to be, like a liver or a heart or a skin cell.  But these undifferentiated cells were mature,  and they have a terrific memory of what they used to be, like a muscle cell in a forelimb or a cartilage cell in a leg joint. This is how they take up their specific positions and form new muscle, connective tissue, cartilage, and bone until - boom - the animal has a whole shiny new leg.

We understand the basic method here, but researchers are still puzzled by the details; like, why does the wound epidermis form in the first place, and how does it trigger that reversion to the cells  below it? And just how do all those regenerating cells know where they should be and what shape to take on? The truth is we just don't know. Yet. But researchers have recently pinpointed a cell  that seems to be responsible for salamander's remarkable regeneration capabilities.

All animals have a kind of repair cells called "macrophages." They rush into a wound site and eat up dead cells and pathogens while triggering the release of other immune cells.  Mammals also use them to repair muscle, which got Dr James Godwin of the Australian Regenerative Medicine Institute to thinkin'. When Godwin and his colleagues reduced the number of macrophages at a salamander wound site, they found that regeneration took much longer. And   when they removed all the macrophages, the poor guys could no longer regrow limbs but rather ended up  with a lot of scar tissue just like we do. This suggests that regeneration is possible because those macrophages cells release some vital chemical signal that might trigger the undifferentiated  cells to come in and do their thing. So, does that mean that we'll soon know how to help humans   regenerate lost limbs? Don't hold your breath.

Researchers say we're still a long way off from understanding the complexity of regeneration. Plus, considering that it takes some small salamanders over a year to regenerate a limb and larger ones  over a decade, even if a human could grow back a lost leg, it could take them over 20 years. Still, there are more immediate benefits and attainable goals that might come from this research, like how to make wounds heal faster and with little, if any, scarring. Not a new arm but still pretty awesome.

[transition]

Stefan: There's still a lot we don't understand about how other animals regenerate their body parts, so who knows ,there may be a future where scientists figure out how they do it and apply that knowledge to humans. But until then, I'll just keep my "New year, new me" statements focused on stuff like cooking healthier meals and nurturing my relationships and growing the biggest biceps  possible.

Thank you for watching this episode of SciShow, and you can support this channel by joining the SciShow patreon. As a patron of SciShow, you'll get all sorts of perks like Discord access, behind the-scenes bonus content, extra podcasts, and a little bit of bloopers. If that sounds interesting to you, you can go check it out at patreon.com/scishow, and here's to another year of awesome science.

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