Body-wide network of fluid-filled compartments could be our next new “organ”

Scientists use a new microscopic technique to look at the interstitium, the tissue surrounding our organs, and bring its true nature to light.

A fresh look at the middle layer of the bile duct led Neil Theise to a discovery. He didn’t see what he was expecting to see, a dense wall of connective tissue. Rather he saw an open, fluid-filled space that was supported by thick collagen bundles. This, he now describes in a study, is the true nature of our interstitium, the tissue that wraps many of our organs, and may be considered an organ itself.

ResearchGate: What is the interstitium, and what does it do?

Neil Theise: The interstitium has been defined historically as the "third space" (after the cardiovascular system and the lymphatics). It has generally been described as merely "the space between cells," though occasionally the concept that there is a larger interstitial space has been generally referred to, though its anatomic or histologic features have never been described.

It is a space where extracellular fluid gathers, i.e. the fluid of the body that is not contained within cells. Some such spaces are obvious: the cardiovascular system containing the fluid of blood, the lymphatics themselves, the space within skull and spine containing cerebrospinal fluid. These other spaces, however, are estimated to contain only about one quarter of the extracellular fluid. The majority, approximately 20 percent of the fluid volume of the body, comprising approximately 10 liters, is contained within the interstitium.

This interstitial fluid is conceived as being the "pre-lymph" that eventually becomes the fluid in the lymphatic system, and so the space is in direct continuity with the lymphatics and the lymph nodes.

Little else has been known of it until now.

RG: What led you to investigate it?

Theise: Drs. Petros Benias and David Carr-Locke showed me pictures of the wall of the bile duct that they had obtained using a new kind of endoscope. Endoscopes are the snake-like tools that clinicians can use to reach into the body, examining internal organs such as the upper and lower digestive tracts and to take samples of tissue as biopsies for diagnosis. As a pathologist, I had examined many of the specimens obtained by my two colleagues.

This new scope however had a new capacity: after injecting a little fluorescent dye into the vein of the person undergoing endoscopy, the scope could examine living tissue at the microscopic level, similar to what I do with the biopsies at the microscope. In most places they and other clinicians using the scope had looked — the esophagus, stomach, small and large intestines — nothing unexpected was revealed. But in the bile duct a pattern of spaces was revealed that did not match any known anatomy of the bile duct.

So they came to me, as an expert in microscopic examination of tissues — and specializing in the liver and bile ducts — to see if I can explain what they saw. To my dismay, I could not.


"I quickly realized that every dense connective tissue layer of the body was like this: open, fluid filled spaces supported by a collagen bundle lattice."


RG: How did you come to realize the interstitium was different than previously thought?

Theise: We finally devised an approach observing the bile duct in patients who were about to have cancer operations in which some of their normal bile duct would be removed. Rather than process the sampled bile duct tissue as usual, with dehydration and chemical fixation to make slides, we quickly froze the tissue, keeping the resected piece as close to the normal living tissue as possible.

Then we saw the unexpected. The middle layer of the bile duct, long thought to be essentially a wall of dense connective tissue, was actually an open, fluid-filled space supported by a lattice made of thick collagen bundles.

After recognizing this surprise in the bile duct, I quickly realized that every dense connective tissue layer of the body — the linings of all the visceral organs, the dermis, all the fascia between and around muscles, all the connective tissue around every blood vessel (arteries and veins of every size) — was like this: open, fluid filled spaces supported by a collagen bundle lattice.

RG: Why wasn’t the interstitium correctly described before now?

Theise: Standard processing of tissue for making slides usually involves dehydration at some or several points of the process. Just taking a bite of tissue from this space allows the fluid in the space to drain and the supporting collagen bundles to collapse like the floors of a collapsing building.

We would often see little "cracks" between collagen bundles in these layers. I was taught, and in turn taught many of my trainees, that these cracks were artifacts of processing. We had pulled the tissue too hard in preparing the slide and separations had formed. But these were not artifacts: these were the remnants of the collapsed spaces. They had been there all the time. But it was only when we could look at living tissue that we could see that.


"This new ability to look microscopically at living tissue made all the difference."


RG: What qualities makes the interstitium — or any organ for that matter — an organ?

Theise: The definition of "organ" is imprecise, but usually implies that there is a unity and uniqueness of structure or of function. This space has both: unique properties and structures not seen elsewhere and functions that are highly specific and dependent on the unique structures and cell types that form it.

RG: Another “new” organ, the mesentery, was designated just last year. Is there a reason for this recent boom in new organs?

Theise: New techniques for examining tissues always lead to new concepts not thought of before. For the mesentery, the tissue was recognized, but it was thought of as "just fat," inert and uninteresting.  New techniques for studying physiology revealed a highly organized functional organ.

In the case of the interstitium, this new ability to look microscopically at living tissue made all the difference.

RG: What impact could a better understanding of the interstitium have on medicine?

A newfound organ, the interstitium, is seen here beneath the top layer of skin, but is also in tissue layers lining the gut, lungs, blood vessels, and muscles. The organ is a body-wide network of interconnected, fluid-filled compartments supported by a meshwork of strong, flexible proteins.

Illustration by Jill Gregory. Printed with permission from Mount Sinai Health System.

Theise: One can't understand the mechanical properties of any tissue without understanding the lubricating and shock absorber potential of the interstitium. These line or surround parts of the body that move: skin and muscles as you move your body, peristalsis as food moves through your GI tract, the expansion and contraction of your lungs with breathing, the squeezing of the bladder during urination, the pulsing of arteries and veins. We've never asked how dense connective tissue layers survive such continual stress without tearing or rupturing? Now we know: they are not dense connective tissue, they are distendable and compressible fluid filled spaces.

We have known for decades that invasion of cancer into these layers is the moment cancer is at risk for spreading outside the organ, particularly to lymph nodes. Why would invasion into a dense wall of collagen potentiate that? Because that isn't the anatomy. The space is a fluid filled highway, often under pressure, that flows directly into the lymphatics and, thus, to the lymph nodes. Tumor metastasis is dependent on this space and its qualities.


"More questions arise than are answered, but that's true of all the best, most exciting science!"


Macrophages, the cleanup crew of white blood cells, traffic in this space. Like tumor cells, they always wind up in the lymph nodes. But unlike the tumor cells, they are performing a normal immune function. Inflammatory cells of all kinds are likely to travel through this space during injury or disease; in direct connection to the lymphatics, they probably play an important role in inflammation.

There is a novel cell type in the organ as well: cells that mix features of the fibroblasts that make collagen (and scar) and endothelial cells that line vessels. This hybrid combination seems unique to the interstitium. There are several lines of investigation that suggest they may be a long-sought, but not yet identified source of scar in diseases where fibrosis plays a dominant role. These same cells also share features of mesenchymal stem cells, an adult stem cell that can be isolated from nearly all tissues, but whose location in most tissues has remained a mystery.

There are many complementary medicine techniques that have been proven to have therapeutic efficacy, but in the absence of mechanistic explanations of the sort prized in Western medicine, remain poorly understood, or even scoffed at, overall. Acupuncture, pulse diagnosis in Tibetan and Chinese medicine practices, myofascial release therapy, for example, are all techniques that may find some mechanistic explanations in the interstitial structure and properties.

More questions arise than are answered, but that's true of all the best, most exciting science!