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The Interstitium: Hiding in Plain Sight

Recently, researchers have announced the existence of what they believe to be a new organ labeled the “interstitium.” The article, published in Scientific Reports, received wide media coverage. In a press release announcing the study, it was reported: “The field has long known that more than half the fluid in the body resides within cells, and about a seventh inside the heart, blood vessels, lymph nodes, and lymph vessels. The remaining fluid is “interstitial,” and the current study is the first to define the interstitium as an organ in its own right, and as one of the largest of the body, say the authors.”1

Freezing biopsy tissue before fixation preserved the anatomy of this structure, demonstrating that it is part of the submucosa and a previously unappreciated fluid-filled interstitial space, draining to lymph nodes and supported by a complex network of thick collagen bundles.2

Throughout nature, fluid-filled spaces are known to be connective and communicative, insulating and protective. For a layperson, it’s thus useful to ask: what might else a fluid-filled space do? Well, it might “breath” (in its own way). It might “inhale” and “exhale,” exchanging nutrients, it might expand and contract (sensitive to pressure, temperature), it might react to its environment.

These bundles are intermittently lined on one side by fibroblast-like cells that stain with endothelial markers and vimentin, although there is a highly unusual and extensive unlined interface between the matrix proteins of the bundles and the surrounding fluid.

What else might a fluid-filled space do? Well, it might cleanse. It might keep things moving, circulating, washing away toxins and impurities. In this regard, it’s useful to ask: does the interstitium have healing properties? Perhaps it might: “Lastly, the protein bundles seen in the space are likely to generate electrical current as they bend with the movements of organs and muscles around them, and may play a role in techniques like acupuncture, he says.”

We observed similar structures in numerous tissues that are subject to intermittent or rhythmic compression, including the submucosae of the entire gastrointestinal tract and urinary bladder, the dermis, the peri-bronchial and peri-arterial soft tissues, and fascia. 

What else might a fluid-filled space do? It might lubricate. It might provide flexibility and fluidity in conjunction with a smooth, conformable surface. Aside from movement, there may be other ways the body benefits from such a design. What else might a fluid-filled space do? It might encapsulate, creating a structure for that which it surrounds and encompasses. Etc.

These anatomic structures may be important in cancer metastasis, edema, fibrosis, and mechanical functioning of many or all tissues and organs. In sum, we describe the anatomy and histology of a previously unrecognized, though widespread, macroscopic, fluid-filled space within and between tissues, a novel expansion and specification of the concept of the human interstitium.

So why is this an important finding? For those who handle injuries, from this point forward, investigation into whether an “injury to the interstitium” has occurred should be investigated. Long-term injury to this organ may help explain why chronic pain persists. Probably other consequences, such as certain types of inflammatory or autoimmune disease, may be implicated as well.

Physicians have known for decades of lymph, bile, cerebrospinal fluid, as well as other bodily fluids. What difference does it make in finding yet another small region (or regions) of fluid-filled space within the human body? Some have suggested this study is not that important, merely making explicit what has been implicitly known for decades. This finding, this new organ, could actually become quite significant, leading to breakthroughs, but only if researchers take the time to pause and think. They have to believe there’s a reason why nature created this small space and caused it to be filled with fluid in its precise location. Their task will be to study this, to unmask its secrets. By asking what potential uses––breathing, cleansing, healing, etc.––are at stake, they may be able to tease out the threads which shed new light on how the body actually works.

The more interesting point is how one of the largest organs in the human body has been “hiding in plain sight” all these years. Now physicians and anatomists may feel compelled to ask––what else have we missed?

1 Researchers Find New ‘Organ’ Missed by Gold Standard Methods for Visualizing Anatomy & Disease

2 Benias PC, Wells RG, Sackey-Aboagye B, Klavan H, Reidy J, Buonocore D, Miranda M, Kornacki S, Wayne M, Carr-Locke DL, Theise ND, Structure and Distribution of an Unrecognized Interstitium in Human Tissues, Sci Rep. 2018 Mar 27;8(1):4947. doi: 10.1038/s41598-018-23062-6