In brief: It’s called the interstitium, or a layer of fluid-filled pockets hemmed in by collagen and it can be found all over our bodies, from skin to muscles to our digestive system. The interstitium likely acts as a kind of shock absorber for the rest of our interior bits and bobs and the workings of the fluid itself could help explain everything from tumor growth to how cells move within our bodies. The authors stop short of saying “new organ,” but the word is certainly on everyone’s lips.

Is it just me, or are you feeling a bit of deja vu?

Well, maybe it’s just me, but that’s because I’ve been in this situation before. You see, just over a year ago, researchers announced that they’d discovered a different “new” organ — the mesentery. That particular collection of bodily tissue is a fan-shaped fold that helps hold our guts in place. It had been known about for centuries, but only recently discovered to be large and important enough to justify calling it an organ. It was to be the body’s 79th, but that number is entirely arbitrary.

As we discovered here at Discover, the definition of an organ is hardly settled (and we’re aware of what a church organ is, thankyouverymuch). As became apparent during the whole mesentery craze, there’s no real definition for what an organ actually is. And the human body doesn’t have 79 organs, or 80 organs, or 1,000 organs, because that number can change drastically depending on the definition. And you can bet scientists debate what an organ actually is.

“It’s a silly number,” said Paul Neumann, a professor of medicine at Dalhousie University in Canada and member of the Federative International Programme for Anatomical Terminology, in a Discover article from last year. “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”

Calling the interstitium a new organ, then, is a bit of a stretch. It’s there, it’s certainly important, but we need a better idea of what an organ is before we can start labeling things as such.

There is a definition of sorts, but it’s got more wiggle room than your large intestine. An organ is composed of two or more tissues, is self-contained and performs a specific function, according to most definitions you get by Googling “what is an organ?” But there’s no governing body that explicitly determines what an organ is, and there’s no official definition. Things like skin, nipples, eyeballs, mesenteries and more have crossed into organ-dom and back throughout history as anatomists debated the definition.

Source: The Interstitium Is Important, But Don’t Call It An Organ (Yet)