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We spend a ton of time and energy studying the human body. Globally, hundreds of billions of dollars a year go into health research.

The vast majority of that research asks questions broadly within the realm of physiology, or how our body functions. But discoveries in anatomy — or the study of the body’s structure and form — still happen all the time. Like, even in the last decade — or since 2010, for our purposes — researchers have found multiple new body parts, including some you could even see with the naked eye.

Here are four of them. First, there’s the interstitium. And for you etymology fans, yep, interstitium literally means “in between”.

But in between what? Well, kind of everything. The interstitium is a sort of nebulous network of fluid-filled compartments spread all around the body, but we didn’t identify it until 2015.

Now, we’ve known about interstitial fluid for a long time — fluid that isn’t inside cells, but between them. [interstitial fluid: fluid in between cells]. And before finding this new structure, we thought this fluid just kind of drained into the empty space between cells. But this discovery finally gave the space itself some structure — and turns out, that might be more important for understanding disease than we thought.

Here’s how it went down. Around 2014 to 2015, doctors at Mt. Sinai Beth Israel stumbled upon the interstitium while looking for signs of cancer in a patient’s bile duct. That’s a tube that takes bile from the liver to the small intestine.

Using a really tiny microscope on a wire, they were able to get a super close and precise view of the area. And they noticed that the spaces around the bile duct were filled with connected, fluid-filled compartments, which was a total surprise.

That’s because when doctors had taken out this tissue in the past, they’d had to remove the liquid from the tissue to look at it under a microscope, so the tissue condensed on itself. But with this microscope on a wire, they didn’t have to drain anything. Recognizing that this structure might be a new thing, the team took biopsies of 12 more patients’ bile ducts and found that they all had this tissue, too.

And in fact, as they kept looking around, they found more of this same tissue around different, compressible body parts, like the gut, bladder, and skin. It was mostly fluid, but it also had tough collagen proteins and flexible fibers. And this new tissue was the interstitium.

As we’ve studied this network, we’ve started realizing that all these new fluid-filled spaces might act like shock absorbers to other organs. But also, we’ve learned that the interstitium acts as a holding place for fluids en route to the lymphatic system, the system that moves waste and fluid around your body. So, if cancerous cells get into the interstitium, they might drain into the lymphatics and spread elsewhere. Which, well, would make the interstitium pretty important to understand.

If you’ve ever been putting in contact lenses and accidentally poked your eyeball — or if you’ve, you know, just had a really bad day and poked yourself in the eye — then congratulations, you’ve poked your cornea. The cornea is made of transparent layers of tissue that protect the eye from physical harm.

It’s a tough, resilient tissue that comes in at about half a millimeter thick. But until recently, we didn’t know what all of its layers were. Like the Dua’s layer, which we identified in 2013.

In our defense, the Dua’s layer is especially thin, coming in at only 15 microns thick. It’s thinner than some human hairs. Before finding it, we thought the cornea had five distinct layers.

But then, during some transplants, surgeons noticed something new. So, during procedures like this, a surgeon might separate the cornea’s big, middle layer, called the stroma, from a thin, deeper layer called Descemet’s membrane. And to do that, they might use the Big Bubble Technique.

It involves injecting bubbles between Descemet’s membrane and the stroma to delicately peel away the desired tissue. With bubbles, surgeons are able to take out only what they need and leave everything underneath untouched. And aside from being delightful to say, Big Bubble also leads to fewer transplant rejections.

But during some of these procedures, some surgeons noticed separation of the deepest layers of the stroma. And they wondered if this was just, like, a delicate part of the stroma or something new. So, researchers from the University of Nottingham collected 31 human eyes from cadavers and performed mock transplants on them.

They separated the layers of the cornea with air bubbles like they normally would. But by using even tinier bubbles, the team was able to peel away this new layer. Then, they followed that up with a super powerful electron microscope to study it in further detail.

And yep, this was a new piece of anatomy! Today, we know this layer — the Dua’s layer — is mostly collagen, which makes it tougher than some of the other layers. And that’s actually really helpful for us.

Because of its toughness, surgeons can inject their bubbles between the stroma and Dua’s layer to prevent anything underneath from tearing. Hopefully, this will help us improve the quality of corneal surgeries and transplants going forward, especially for surgeries on the deeper part of the cornea.

Next are the trans-cortical capillaries. These capillaries are extremely thin blood vessels found in long bones like the femur, and they help transport things like stem cells between the bone marrow and outside of the bone. Specifically, inside that bone marrow are stem cells calls HSCs, which can mature into different types of blood cells and different immune cells. And on the outside of the bone, you’ve got a tissue called the periosteum, which wraps around the bone and has plenty of blood vessels that hook up to the rest of your bloodstream.

Somehow, those stem cells need to get from inside the bone to the periosteum and into circulation. Prior to this discovery, we knew that there were a few blood vessels at either end of the long bones that moved blood in and out of the marrow — at least, in mice. But there was a problem.

Past experiments in mice had shown that cells from the bone marrow get into circulation more quickly than what you’d expect if they only used those vessels we knew about. So there had to be something else going on. Some previous studies had found extremely tiny holes in the bone that might act as a path between the marrow and periosteum — holes, like, 10 to 20 microns wide. But they didn’t find blood vessels.

But thanks to some newer imaging technology like X-ray microscopes, the researchers were able to track them down. In a study published in 2019, researchers used a three-dimensional scan and saw that the holes in the mice bones went all the way from the outside of the bone to the inside.

But more than that, they also saw tiny blood spots around the holes. And that implied that blood vessels ran through them. Now, like we’ve said loads of times, mice are not humans.

You might notice that we have different anatomy. So, next, this team looked at images from human surgeries, and lo and behold: They found little bleeding spots where they suspected our capillaries to be. They also followed that up with a different imaging technique and saw little canals in human shin bones, and blood vessels similar to the mice’s.

Except, the canals in the human bones were much wider because, you know, we’re a little bigger than mice. That was the evidence they needed to confirm that trans-cortical capillaries also exist in us! Now, that’s great for our general understanding of our bodies and all, but this discovery also came with some big implications.

See, the researchers also noticed osteoclasts in the middle of these canals in mice. These are cells involved in bone remodeling — an ongoing process of building and trimming bone. And this made the research group think that these canals and blood vessels are linked to bone turnover, and keeping bones strong as we age.

As we get older, the number of mature bone cells we have declines, which makes our bones weaker and more likely to break. Maybe not coincidentally, we now know that we also have less of those trans-cortical capillaries as we age. The researchers think that losing the capillaries may be one of the factors that predisposes us to fractures as we get older. So future research may look into preserving the capillaries to help heal fractures.

Finally, your brain has a lymphatic system, little vessels that help transport waste and immune cells around your body. And while that might sound like a given, we didn’t know this for sure until 2015, which says a lot, considering how many studies we’ve done on the brain.

The conventional view said that any waste products the brain made would flow into the cerebrospinal fluid before being moved to the bloodstream. But over the years, a few experiments made scientists question if maybe the lymphatics were involved after all.

Like, in 2015, by sheer coincidence, two separate labs found lymphatic tissues in mice brains — specifically in the dura mater, a tough layer of tissue between the mouse’s brain and skull. So if these mice brain lymphatics showed up in mice… well, hey, maybe they existed in us, too. Historically, imaging techniques make blood vessels and lymphatic vessels look similar.

But in 2018, thanks to new imaging and staining technology, we could finally get a more precise picture of the lymphatics in living bodies. That year, researchers published a paper about an experiment they did on monkeys and human volunteers. In the experiment, the subjects were injected with a special dye-like substance.

The dye molecules were tiny enough to leak out of blood vessels and into any sort of potential lymphatic system, but they were still too big to travel to the rest of the brain through the blood-brain barrier. The hope was that, if the human brain did have a lymphatic system, we’d be able to find it by following the dye. And we did!

Using a special MRI, the researchers saw the dye had pooled into a network of vessels that sat next to the blood vessels in the dura. Then, to be sure, they repeated the imaging with a similar substance that doesn’t leak out of the blood vessels at all. And as expected, they didn’t see the new lymphatics.

Finally, to really confirm what they were seeing on the MRI, they autopsied some brain tissue and found cells and proteins that were unique to lymphatics. Now, this is a big deal because it means we need to rethink how normal brain and lymphatic physiology work. But by understanding that, we can hopefully come up with better treatments for neurological diseases.

For instance, something like Alzheimer’s disease has a lot of moving pieces — one of which is the buildup of certain proteins. We know from mice studies that lymphatic activity slows down with age, which means they won’t be able to clear some of those harmful proteins as efficiently. So if we can keep track of someone’s lymphatic activity in their brain, we could spot Alzheimer's disease in that person before it becomes problematic.

Or at least, that’s the hope. That’s what makes new discoveries in anatomy so exciting! Whether it’s a network of vessels or a whole new organ, each of these new body parts represents a new opportunity to advance medical research. The anatomy itself is cool, but what we do with this new body of knowledge, pun very intended, is the exciting part.

Speaking of exciting things…. This episode has a President of Space! It’s Joshua Furman, who’s one of our patrons on Patreon. So, thanks, Joshua! We’re glad to have you.

And we’re grateful for all our patrons, too. They make up an awesome community, and thanks to them, we can keep releasing SciShow videos every day of the week. If you want to learn more about becoming a patron — maybe even our next President of Space — you can go to patreon.com/scishow. {♫Outro♫}.