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So you probably have organs that you could literally live without, but did you know that the reverse can also be true? You may, in fact, be living with multiple spleens already!

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Head to D-R-I-N-K-L-M-N-T dot com/scishow for a free sample pack of electrolyte drinks with any order. [♪ INTRO] 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 than one? Just, let’s 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 is 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 stranger 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 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 ten to 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 centimeters long by 7.5 centimeters wide.

While an accessory spleen is typically much smaller than that, roughly 1 to 3 centimeters 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 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 splenic 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 spleen-y 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 onto 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. Which 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 splenic 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 two thirds 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 the human body comes up with to confuse us from time to time. Now, lab-grown organs can’t solve all of our problems… yet.

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Thank you to LMNT for supporting this SciShow video! [♪ OUTRO]