The Mesentery Isn’t the Organ You Think It Is

By Carl Engelking | January 6, 2017 4:29 pm

The kale-like structure you see here in this 1839 illustration is the mesentery. (Credit: Wellcome Trust)

In January 2017, a story begging to go viral fell onto writers’ laps: We have a new organ called the mesentery, which is a broad, fan-shaped fold that lines the guts. Here at Discover we pounced on the story, and so did CNN, the Washington Post, LiveScience, Smithsonian, Vice News Tonight, Jimmy Kimmel and many, many more.

We got it all wrong, and it’s time for us to spill our guts.

In our reporting, one burning question we wanted answered was who, or what, determines when a hunk of tissue “officially” becomes an organ. So we posed the question to J. Calvin Coffey, the Limerick University Hospital researcher who presented evidence in The Lancet Gastroenterology and Hepatology to “justify designation of the mesentery as an organ.”

“That’s a fascinating question. I actually don’t know who the final arbiter of that is,” he told us.

Intrigued, I set out to find the official, legislative organ of organ designation and fell into a rabbit hole that grew deeper and deeper. For one, there is no legislative body that determines what an organ is, but there probably should be (more on that later). But in my search for this illusory organization, I soon discovered there’s so much wrong with the mesentery narrative that developed this week.

As a remedy, I’m going to break down all the problems with the following sentence:

Scientists discovered the mesentery, the human body’s 79th organ.

Scientists discovered the mesentery…

This one is easy. We’ve known about the mesentery for thousands years, says Gray’s Anatomy editor Susan Standring. However, researchers have debated whether our body contained a single mesentery or mesenteries. Coffey and colleagues, in their new study, clarified the anatomic understanding of the mesentery, suggesting that it is indeed a continuous, singular entity that spans the gastrointestinal tract.

But Coffey didn’t discover the mesentery, nor was he the first to describe its contiguous structure. Leonardo Da Vinci depicted it as a single organ. In 1878, Carl Toldt echoed Da Vinci’s findings, which then were echoed by anatomist Edward Congdon in 1942 and again by Wylie J. Dobbs in 1986.

But these findings were largely ignored in mainstream literature, and British surgeon Sir Frederick Treves’ description of a fragmented mesentery — from 1885 — enjoyed a long shelf life, but no longer. Gray’s Anatomy has been updated in light of mounting evidence like Coffey’s, and the mesentery is whole once again.

“Their findings have very significant clinical implications and necessitate a re-evaluation of mesenteric anatomy,” Standring wrote in an email to Discover. “It will be fascinating to follow their future research on the subject.”

…the human body’s 79th organ

Whoever came up with this notion that humans have 78 organs — 79 if you count the mesentery — anatomists are looking for you.

“How anyone would come up with 78 organs as a definite number is mystifying,” says Thomas Gest, a medical education professor at Texas Tech University. He’s also an advisor for the Division of Gross Anatomy with the Federative International Programme for Anatomical Terminology (FIPAT). FIPAT sets the international standard for anatomical terms, and is a program of the International Federation of Associations of Anatomists.


J. Calvin Coffey (Credit: Limerick University Hospital)

“It’s a silly number,” says Paul Neumann, also a FIPAT officer and professor of medicine at Dalhousie University in Canada. “If a bone is an organ, there’s 206 organs right there. No two anatomists will agree on a list of organs in the body”

Neumann challenged me to find the definitive source of this magic number 78. Although it’s cited copiously on the web, I couldn’t trace it to its ultimate source. Neumann and I had a laugh when he pointed out that the Wikipedia page, which states there are 79 organs — with the addition of the mesentery — goes on to list more than 79.

“How we settled on 78 is a good question, and is probably lost in history,” says Shane Tubbs, vice president of the Seattle Science Foundation.

About that word ‘organ’  

So what hell is an organ? Tom Broman, a science historian at the University of Wisconsin-Madison, laughs when I ask him.

“The question isn’t stupid, but the answer is kind of crazy,” he says. “In antiquity, you already have writers like Galen talking about the kidneys, liver and spleen as unified structures that do stuff.”

Before our interview, Broman had dusted off early 20th-century internal medicine textbooks to see if he could find a clear-cut definition of an organ — he didn’t.

“The author in one of these books is talking about the nipple as an organ, the esophagus as an organ, and he does it without explicitly defining what an organ is,” Broman says. Based on his survey of the literature, he arrived at a pretty unsatisfying answer to my question.

“It’s pretty clear that an organ is any solid thing in the body that does something,” Broman concluded.

I dug into the question myself, and the modern definition of an organ is a small improvement on Broman’s: An organ is composed of two tissues, is self-contained and performs a specific function.

That still leaves a lot of wiggle room, and anatomists are taking it. Neumann said some scientists argue that an organ must provide a vital function not just a function, while others say organs can only be located inside the body.

“We don’t even have complete uniform acceptance that the skin is an organ,” says Neumann. “They say it doesn’t have a function, it’s just the body envelope.”

As I exchanged emails with the experts, I got a taste of the discord in the anatomist community. Gest said that, by definition, each bone in the body would be an organ. In the same email chain, Ian Whitmore, a professor of surgery at Stanford, disagreed.

“I would not have called a bone an organ. Although I now accept that the skin is the largest organ,” he wrote. “In previous discussions the largest organ was the liver.” 

Gest summed up the state of organ nomenclature neatly.

“Defining any part of the body as an organ is pretty much in the mind of the definer,” he wrote.

Yes, It Does Matter

The anatomical world is just beginning to standardize definitions, with the first attempts being made by the Clinical Anatomical Terminology Committee of the American Association of Anatomists, says Whitmore. The process has been slowed by the contentious nature of the field. 

“Whenever an item was discussed it became quickly clear that the 20 or so senior anatomists from all over the world could not agree,” Whitmore says. 

Although FIPAT standardizes international terminology for human anatomy, it doesn’t determine what’s an organ and what isn’t — no one does, apparently. Neumann wants to change that. He says the rise of computer science necessitates a change of focus for FIPAT.

A closer look at the mesentery. (Credit: J Calvin Coffey/D Peter O’Leary/Henry Vandyke Carter)

Neumann has worn many hats over the years, and got his start in academia as an artificial intelligence researcher at Columbia University in the 1970s. Today, we have early versions of AI-enhanced search engines — IBM’s Watson or Semantic Scholar — that comb through science papers and build connections based on the language in a study. We’re also in the age of electronic health records.

These computerized functions are dependent on data. AI-driven semantic searches need to be trained with large, annotated datasets that describe what a thing is and how it’s related to other things. In computer science this is called ontology, or the process of formally naming the types, properties and relationships between entities. Neumann wants to move FIPAT to a more ontological approach and modernize its work to suit the age of computers.

“With the rise of computer sciences, a bunch of biomedical ontologists have decided to set up their own terminologies, so we have a rift now between computer science and biomedical science,” says Neumann. “If you set up a database with an entity in it, you have to name it. You have to decide, for example, is the mesentery an organ? That’s something anatomists have avoided doing for a long time.”

If the definition of an organ is relative to every database, or Latin terms are translated differently, there are going to be search engines and health records with noticeably different hierarchical arrangements, and that might alter an AI’s outputs. So Neumann is hoping to close the gap, and he’s building momentum to standardize this process.

“I want to turn our organization around so it’s not just names and implied definitions, but get absolute definitions and develop a modern ontological approach,” Neumann said. He believes the time has come, and is working to build consensus for his approach, making stops around the world convincing other ontologists in the process.

If Neumann’s momentum continues and he ultimately succeeds, we might finally have a clear-cut answer to the question, “What is an organ?”

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  • Jean

    Well, if it’s an organ, what does it do?

    • Valareos

      Plays music… ~sagely nods~

      In all seriousness, a definition of an Organ will need to start with what everyone agrees is an organ (heart, lung, brain, liver, kidneys, stomach.. ect ect). and make a definition that meets that criteria, then see how it applies to other parts. A Hierarchical system would be a good start. I do see the eventual result would be splitting the body into systems (muscular, digestive, skeletal, circulatory, ect), then label organs as part of these systems.
      Calling an organ as a unified, specialized group of tissue that performs a single, critical, non structural purpose would be reasonable. It removes calling bone, muscles, ligaments and such organs, and allows a simple test to determine if it IS an organ: If you remove the item in its entirety, can your body survive without it. If no, then its an organ.

      • Christopher Hubert

        You can live without a spleen. Is it no longer an organ?

        • DreamStain

          We need a glass of cold water here!

      • Sal Baptist

        Which would make the appendix, gall bladder, spleen, eyes, uterus, ovaries, testicles, and single kidneys not organs?

        • Valareos

          You just hit on the issue here :) there is no exact definition of what is an organ. Different definitions may cause different results. I suspect that no matter what definition is used, there will be some items we currently classify as organs that will lose that definition. As Erik Bosma half joked about in another comment.. there stands a good chance that “Classical” organs will be downgraded in the creation of a scientific definition of an organ, much like Pluto lost its classical planet status when a scientific definition was accepted. Unfortunately, I foresee an issue where definitions accepted are partly influenced by a desire to NOT see certain body parts labeled as organs as much as creating a scientific description, much like how Pluto was robbed of its status through an arbitrary clause.

    • Electric Bill

      Jean: I heard a discussion of the mesentery’s function on the radio.

      Uf I understood them correctly, at least one of the functions they attributed to the mesentery is that it gave structure and form to the intestine and/or other organs, keeping them from shifting about inside, which can unduly stress them when jumping, falling, or tumbling about, helping to prevent rupture or tearing… holding them is place. In that respect, it is similar in function to the bones. They mentioned no other function, but I suspect it serves a variety of other functions, such as, perhaps, providing infrastructure for the flow of blood, lymph, or other fluids.

      Maybe you and I should do some googling to gain a greater understanding of it.

      • Richard Young

        Then wouldn’t the entire body skeleton be considered an organ based on its function? I don’t believe so. Organs have to perform a function other than structure IMO.

        • Diego

          Bones have very vital functions, not only for structure of the organism but also for protection as well as for being a vessel for the bone factory. For me they actually are organs. I don’t know about the mesentery, however; it plays a similar though not identical role as meninges, and I don’t think they are considered organs.

    • Gillian McAllister

      Don’t you know, organs make music – from very low to very high sounds – if you listen closely you’ll hear your own making music after a meal :) In actuality, it keeps the intestines from twisting on themselves and causing a blockage with the potential to rupture. Similar to what dogs get called torsion. It can be critical because if it doesn’t untwist by itself, it needs immediate surgery to prevent rupture and potentially gangrene

      • Jean

        Ohhhh, so that’s what that sound was. I thought it was gas.

  • Stephen Andrew White Jr.

    I think this is also how we lost Pluto. Well, not that we misplaced it or anything, but we couldn’t define “planet,” in a way that kept the original 9 without adding a bunch of new ones that nobody wanted to call a planet.

    What other things do we just accept as is, that we haven’t actually properly defined?

  • Electric Bill

    S.A. White, Jr., who also commented here, mentioned Pluto, which is what I was going to mention as well, since naming organs, planets, species, clouds, storms, and many other things have a similar problem— where does one draw the line?

    There are those organs (or planets, etc.), such as the heart, lungs and kidneys that everyone would agree on as being organs. As we continue on with the list of 78, 79 or so organ “candidates”, at some point we encounter more and more disagreement among anatomists. Among those arguing over planets and non-planets, they have at least agreed upon other categories of other bodies— minor planets, planetoids, asteroids comets, etc.,

    If there is general acceptance that there are 79 organs, we can stop there and simply declare others… bones, for instance… as “organ-line bodies”, “organoids”, “disputed organs”, or some other such terminology, so as to assure they would at least be considered in such discussions.

    • Erik Bosma

      Plus, after reading some of the stuff produced by the writers of these Discovery articles I wonder if the brain is even an organ.

      • Electric Bill

        I don’t like to be too critical of any such publication, knowing that subscriptions to all varieties of physical media– magazines and newspapers— have dropped considerably over the last several years, making it very difficult to stay in business against free, online competition. I am grateful that they continue to survive… many have been driven out of existence.

        • Erik Bosma

          In all seriousness, as a comedian, they ARE discovering functions of some organs that would have got someone burned like a steak several years ago. Who knew that the gut is the organ that produces serotonin. Yes, the same serotonin which throws moods out of control and which Prozac and other SSRI’s are designed to control. It’s a good reason to explain why we feel like we’ve been punched in the gut when an extremely emotional incident affects us. Or why we have ‘gut feelings’ about stuff. Or why people suffering from intestinal diseases are often very depressed. Or why you sometimes feel like a million bucks after you’ve had a big sh*t. So…. who knows what else the good old Mesentery (I’ll capitalize it from now on and, oh…I’m not really a comedian) is good for. Stay tuned and stay regular.

  • Erik Bosma

    It would be more interesting if it functioned like the Hammond B-3. Well, perhaps the B-3 micro mini.

  • Erik Bosma

    Next thing you know they’ll stop calling Pluto an organ. Then what??

    • Jean

      That’s hysterical!

    • Mike Shefler

      Maybe we could call the mesentery a dwarf organ.

  • polistra24

    Some vague definitional questions are important. Defining a country is difficult in the same way as planet or organ, but country is hugely important because defining or undefining a country can prevent or cause a war. (See Taiwan, Confederacy, Scotland, East Timor, ad infinitum.)

    Organ and Planet are trivial. The job of a surgeon will not change now that the mesentery is an organ. The job of an astronaut or a satellite course planner didn’t change when Pluto stopped being a planet.

    • J. Kevin Dix

      Quite right. A quick search reveals that are four, seven, or six continents, and even those who say there are six can divide the world in two different ways.

  • kader kochi

    Omentum is 80 th ofgan .prepuce is 81 st organ. medical research is fast growing science.

  • kader kochi

    Omentum is 80th, prepuce is 81st organ. let us read shortly

  • grande pescador

    The placenta is a temporary organ and unique to the female body during gestation only.

  • Jim Abbott

    My Jr. College Physiology prof. made us memorize: “Ontology is merely a brief but rapid recapitulation of the phylogony”. This, in 1958, covered all the bases.
    Jim A.

  • Tubbery

    So just to be sure, (and get to the Crux of this) the published article about the mesentery is correct and accurate. Its the fact that there is no definition of an organ that’s the real story.

  • Jim Speidel

    I’m with Eric, the Hammond B3 is the best organ ever. I never considered bones to be an organ before, but that is where blood cells are made. Since they perform this vital function, I think that makes them an organ. Do all bones make blood cells? Maybe just the ones that do are organs and the rest are just part of our support structure, like cartledge.
    And how come i did not get this email discussion until it was five days old already….

  • J. Kevin Dix

    If it is so difficult to define the term “organ”, perhaps we should examine whether the term is useful. Anatomists are expected to know the names of all the bones in the body, regardless of whether they would be considered organs individually, or the skeleton would be considered an organ collectively, or if they were not considered organs at all. Perhaps all the parts of the body do not need a categorical name to collect them; they’re just parts.

  • ECarpenter

    The need for standardizing is a practical matter, why is that so hard for some anatomists to stomach?

    There needs to be a category of body parts at that level of specificity, and ‘organ’ will do. Get a small committee together, give them the authority, and accept whatever they come up with. In twenty years no one will care about today’s controversy.

  • Dee Gee

    I am confused, bones are not organs, yet are functional.

  • shashwat

    if its function is to provide support to intestines to be in their place then bones should also be considered as 90th human organ…silly i think an organ performs something that maintains the functioning of body? is it or not


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