Hank Green: As you may have noticed, I tend to be a pretty easygoing guy, but like most people I occasionally get somewhat irritable, and that usually happens when one or two things happened: either I'm hungry or I'm really stressed out. Note that you do not want to be around me when I am both hungry and really stressed out. There are a number of ways that I can deal with these situations. I eat, I exercise, sometimes I just scream.... One thing that I cannot do, though, which would be really nice, would be to revert back to a simpler time -- back to when I was eight-year-old Hank. But there is an organism in this great wide world of ours that can do that very thing. [intro music] This, my friends, is Turritopsis dohrnii, otherwise known as Turritopsis nutricula, other otherwise known as The Immortal Jellyfish. You heard me correctly. This little guy is the only animal that we know of that is capable of reverting back to a younger version of itself. And it does this, you've probably guessed, when it's facing starvation or other stresses in its environment. The question, though, is how does this happen, and how can I get me some of it? Okay, so I know you're super curious. Here's the typical life cycle of a jellyfish. We've got the fertilized egg, which becomes a larva which sinks to the bottom of the ocean, attaches itself to something, and becomes a polyp, which, over time, transforms itself into a free-swimming, bell-shaped Medusa, AKA jellyfish, with 80-90 tentacles. Simple, right? Since our friend the Turritopsis is a kind of plankton ("plankton" is, by the way, a word for anything that doesn't really move itself around in the ocean, it just gets moved around by the the ocean -- drifting organisms), and Turritopsis is a source of nutrition so it often gets eaten by other organisms or, you know, killed off by disease, so it's technically not immortal if it gets eaten, but the amazing stuff happens when this dime-sized jellyfish somehow avoids getting eaten but then can't find enough food, or is injured, or is otherwise stressed by the environment. This is the point that the jellyfish goes Benjamin Button on us, throwing its life cycle into reverse. The bell and all those tentacles deteriorate and it eventually becomes a cyst. The cyst has the ability to reactivate genetic instructions from earlier in its life cycle, and it actually starts creating new polyp cells. And from there, the life cycle process begins again as the polyp, through the beauty of asexual reproduction, releases many identical copies of itself. Scientists have said that, in theory, this process could repeat indefinitely, making the jellyfish technically immortal. It may seem like science fiction, like there can be only one jellyfish, but what's actually happening here is the jellyfish transforms some of its cells into a younger state. It's called transdifferentiation. This is when mature cells, that have already been assigned a function like lung cells or skin cells, become different kinds of cells. In the case of this jellyfish, that might mean converting a muscle cell into a nerve cell, or even a sperm or an egg cell. Among animals, transdifferentiation is not unique to these jellyfish; for example, if you take the lens out of the eye of a chicken, the iris cells will actually develop into lens cells. Unlike the jellyfish, though, the chicken cannot revert its entire body back into an egg, though yeah, that would be pretty awesome. The story of the Turritopsis is important to scientists because we're constantly looking for ways to regenerate new tissue. Usually, that process involves stem cells, or unassigned cells that haven't been given genetic instructions to become a nerve cell or a bone cell or a liver cell. Transdifferentiation doesn't require that. The problem is we have no idea what signals are being sent to these cells to change their type. We just don't understand how it works, and it's not entirely clear that it's even possible in mammals like us. Of course, that doesn't mean that studying this jellyfish isn't going to be tremendously useful for us -- if this thing has cells that can switch themselves on and off when they're in peril, could prove very useful for cancer research. Imagine if we could isolate a cancer cell and just turn on their "young and healthy" cell instructions again. For now, the Turritopsis remains the only animal that we know of that can regenerate its entire body, which might go a little ways to explain why the population of this jellyfish is exploding worldwide. Course it is; it's practically immortal. Always so much good information here at youtube.com/scishow. You can go and subscribe right now if you haven't, and you will get this good stuff in your subscription box constantly. If you have questions or suggestions for us, you can hook up with us on Facebook or on Twitter and always, of course, in the YouTube comments below. Goodbye! [outro music]