Your appendix is not, in fact, useless. This anatomy professor explains

It was the first day of spring break in 1992 in Phoenix, and 12-year-old Heather Smith was excited for her family's upcoming ski trip.

But before Smith and her family had even packed their snow pants, she realized she didn't feel good. "I woke up feeling just a little bit nauseous, and I wasn't sure why. Throughout the course of the day, I started to feel worse and worse and started to develop pain in the abdomen," she says.

By about midafternoon, her father took her to urgent care. She ended up getting emergency surgery to have her appendix out.

Smith still has a small scar from the appendectomy. And after the surgery, she found herself intrigued by the part of her body she had so suddenly lost. "It inspired me to wonder: Why do we have this weird little organ in the first place? What does it do? Why does it get inflamed?"

Smith grew up to be a professor of anatomy at Midwestern University and editor-in-chief of a journal called The Anatomical Record. And all these decades later, Smith has made a mark in the field by studying the very organ that threw off her family's vacation plans in 1992.

She acknowledges that the appendix has a bad rap as a useless organ that can cause you pain and require emergency surgery. "But it turns out recent research shows it does have functions that can help us," she says.

NPR's Short Wave spoke to Smith about what the appendix is good for and how a future where appendicitis can be prevented or treated without emergency surgery could be on the way.

This conversation has been edited for length and clarity.

What and where is the appendix?

The type of appendix that humans and some primates and rodents have looks like a little worm. It's about the size of your pinky finger, and it projects off the cecum, which is the first part of the large intestine.

You can identify the location based on a landmark called McBurney's point. So if you draw a line between your bellybutton and the part of your pelvis that sticks out [on the right], two thirds of the way down, that's about where the appendix is.

How did scientists get the idea that the appendix was useless?

There had been a lot of discussion about what the appendix might do as a function, whether it served a function, prior to [Charles] Darwin's time. The [fact] that we can live without it does provide some support for the idea that it's vestigial and it doesn't really do anything. And so Darwin's interpretation of it as a vestige was, was reasonable at the time, given the information that he had.

But now with modern technology, we can see things like the microanatomy and the biofilms in the appendix, and we have a better understanding of what it is and what it's doing.

How has the appendix evolved over time?

If you map the distribution of appendices across a phylogeny — a tree of mammal life — you can interpret that the appendix has actually evolved independently. It has appeared independently multiple times throughout mammalian evolution. So that is evidence that it must serve some adaptive function. It's unlikely that the same type of structure would keep appearing if it wasn't serving some beneficial role.

So what are the appendix's beneficial roles?

It turns out that the appendix appears to have two related functions. The first function is supporting the immune system. The appendix has a high concentration of immune tissue, so it's acting to help the immune system fight any bad things in the gut.

The second function that it serves is what we refer to as the safe house. So this was a hypothesis that was put forward by a team from Duke University in 2007. And they argued that the appendix may serve as a safe reservoir for the beneficial gut bacteria that we have.

During times of gastrointestinal distress — you know, a diarrhea episode where all of your good gut bacteria is getting kind of flushed out of the system — the appendix is kind of this blind tube with a very narrow diameter and narrow lumen, so the good bacteria doesn't get flushed out of the appendix. The idea is it's safe during this time of gastrointestinal distress and it can then exit the appendix and recolonize this good bacteria throughout the rest of the gut.

So the appendix is kind of helping us in two ways, both within the gut: It's helping to fight off invading pathogens, but also to repopulate the gut with this beneficial bacteria after gastrointestinal issues.

Why do some people get appendicitis?

Appendicitis is predominantly happening in the industrialized nations of the world — areas where fiber content of the diet tends to be lower. So one hypothesis is that, with the lower fiber content, we're more likely to get little pieces of food that's being digested stuck [inside] the appendix and cut off blood supply and cause this inflammation.

The other hypothesis that doesn't seem like it's quite as plausible these days has to do with an old idea called the hygiene hypothesis. The idea is that these days we do so much oversanitization, with all of our antibacterials and all our antibiotics that we take, that our immune systems are not developing properly because they don't have exposure to the full range of pathogens that we would otherwise. And so the immune system overreacts and panics. And because the appendix has so much immune tissue, it's one of the areas where this manifests.

Could this new understanding lead to new treatments?

I think there's some promising treatments out there. People are looking into antibiotics and other ways of treating appendicitis without completely removing it, given the evidence that is accumulating that it is in fact helpful for your health to have an appendix. Studies have shown that infections with the really bad, nasty bacteria C. diff tend to be higher in people who have had their appendix removed.

So there are health benefits to retaining the appendix. In an ideal world, we would have a future where we wouldn't have to always remove it.

What have you gained from studying this "weird little organ"?

I think this study has shown me the importance of looking into small anatomical details. Anatomy is just the study of the body, so you'd think that it's a dead science. You'd think we know everything about the body, especially the human body.

But it turns out that there's actually a lot more variation and function and microanatomical adaptations that haven't been fully realized. So doing just descriptive studies of exotic animals that have never been described or looking at small parts of our own bodies that haven't been well documented are absolutely worthwhile.

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