Illustration by Jill Gregory/Mount Sinai Health System
What on Earth is this thing? That is the question David Carr-Locke, MD, and his colleagues asked when they discovered a microanatomical network of interconnected spaces, filled with fluid and lined by collagen with a unique arrangement, that appears to be present throughout the body.
Whether or not this network is actually a newly discovered human organ is debatable. What is clear, however, is that this discovery is very widespread throughout the body and contains a lot of fluid, said Carr-Locke, clinical director of the Center for Advanced Digestive Care at Weill Cornell Medical College and New York Presbyterian Hospital, and the president of the International Society of Endomicroscopy. He first noticed the new structure in the clinical arena, which is somewhat unusual these days.
Researchers had never before identified the structure using standard histological techniques, but a probe-based Confocal Laser Endomicroscopy (pCLE) system from Mauna Kea Technologies made it possible. The Cellvizio enables direct visualization of human tissues at the cellular scale and allows doctors to detect anomalies invisible with standard techniques, particularly within the gastrointestinal, urinary, and pulmonary tracts, according to the company's CEO and co-founder, Sacha Loiseau, PhD.
The inventors of the device wanted to use miniature probes to encase the instrument as opposed to a larger endoscope-based system. The Cellvizio pCLE is now routinely used to diagnose things like gastroesophageal reflux disease, Barrett's esophagus, early gastric cancer.
"The technology is designed to provide instantaneous, microscopic visualization of human tissues," Loiseau told MD+DI. "So basically you stick this very advanced super miniature microscope into the patient and when the probe touches the tissue we re-create a video of the microscopy structure of the underlying layers of tissue that the probe is touching."
Instead of waiting several days for results of a biopsy, doctors can touch various tissues with the tiny probe and usually make diagnoses instantly because they are able to look at microvessels, cells, and other microstructures that can only be seen under a microscope.
Earlier this year the company received an expanded FDA clearance for the system, recognizing the ability to image the internal microstructure of tissues including the identification of cells and vessels and their organization or architecture. But Carr-Locke has been using the Cellvizio system in the clinical setting since it first became available years ago. He is particularly interested in diseases of the bile duct and pancreas.
"So we had been using the probe that's designed for going into the bile duct when it became available and we started to see images and so on, but it was quite clear that the images we were getting from the bile duct were very different from the images we got from anywhere else in the GI tract, Carr-Locke told MD+DI. "And we didn't really understand why, and of course other people had found the same. So we were all asking the same question, 'what on Earth is this, and why does it look this way?' Basically what we were seeing with the system was a meshwork, a sort of honeycomb of dark bands that were a particular size, which we could measure, with spaces in between them, and did not represent anything that we knew was there from pathology work for the last few hundred years, so it didn't quite make sense."
Taking a Closer Look
Initially, Carr-Locke and his fellow researchers wondered if this was some sort of artifact that was being produced in the bile duct, but that theory didn't quite make sense because the unidentified structure was always there, even in normal non-diseased portions of the bile duct. So they designed a study to look at the tissue itself, which is difficult to do in a living patient because they can't just take bits out to get a closer look. There are, however, times when surgeons do remove parts of the bile duct because when they remove cancers of the pancreas, for instance, they always take part of the bile duct and that bile duct is relatively normal, Carr-Locke explained. So in order to investigate this previously unknown structure, the researchers brought in a surgeon to participate in the study, which was recently published in Nature Group's Scientific Reports.
In order to use the Cellvisio system physicians have to inject fluorescein, so they figured out that they could inject the study patients during surgery, just before the specimen was removed, and that sample would have enough fluorescein to scan it with the same probe after it was removed and the images that they had been seeing in the clinic were still there, even after the tissue was removed. That enabled them to perform "all sorts" of further investigation with it in the lab, Carr-Locke said.
What they learned is that the honeycomb appearance of what they were seeing with the Cellvisio system was due to a network of collagen fibers and the spaces in between were filled with fluid.
"So we came to the conclusion that this was the normal structure, not of the surface tissue of the bile duct, but just underneath it," Carr-Locke said.
That was a key part of the discovery because it explained why nobody had noticed it in other tissues because the surface tissue in other parts of the body is much thicker than the bile duct surface tissue.
"Then we asked, 'if this is always there and there's this network of collagen and fluid spaces, why haven't pathologists seen this for years?' Then we realized that there's one big difference between the real-time living tissue that we're now looking at with the probe compared to what the pathologist sees," Carr-Locke said.
That difference is that when specimens are taken from a patient and examined in the lab setting, the sample is dehydrated, and without that fluid filling, the honeycomb-like structure collapses. "So nobody's ever seen it in fixed tissue," he said. "It's only now, when we have a way of looking at real-time living pathophysiology that we started to see this."
From there, the researchers began to look at other tissues, starting with other parts of the GI tract but then expanding their investigation to areas of the body outside the gut.
"Sure enough, the more we knew where to look the more it was apparent that this was there," Carr-Locke said. "So then it started to make sense that this is a collagen-structured network with fluid-filled spaces that communicate with different parts of the same system, different organs."
That's why "someone somewhere" called it a new organ. Whether it is, in fact, an organ by itself is a bit controversial, he said, but whatever this is, it is very widespread throughout the body and contains a lot of fluid.
How This Discovery Could Impact Cancer Research
"So that was a big step, we realized we'd hit on something that was unrecognized before," Carr-Locke said. "And then it continued to develop. We know that if you inject something into this space, which we do sometimes for marking tumors, for instance ... a lot of those particles end up in the lymph nodes. Nobody ever questioned why that should be, but now we know it's because it communicates with this interstitium space. There's a direct connection between the fluid we were looking at and the lymph nodes."
That actually makes a whole lot of sense, he added, because researchers have known for a long time that cancers that start out on the surface, as they do in many parts of the GI tract, usually spread beneath the wall of wherever it is (the esophagus, the stomach, the intestine, etc.) and as soon as that cancer reaches the layer underneath the skin it begins to spread to other places, particularly the lymph nodes.
"So now we think we've hit on something that explains a lot of how cancers spread and why it's important to recognize how deeply the cancer goes because that makes a big difference of how you treat it," Carr-Locke said. "That will certainly be a big area of further research, not necessarily by us but by people who are interested."
Hey Wait, What About That Funny-Looking Cell?
The last step of the study was to look at the structure of this network in much more detail, so Carr-Locke's team involved Rebecca Wells, MD, whose lab at the Universit of Pennsylvania has an advanced electron microscopy system that enabled further investigation of the newly discovered collagen-structured network. The technology at the Wells Lab enabled the researchers to see that the collagen fibers are arranged very nicely in this matrix, Carr-Locke said, and on one side of them there is a "very funny-looking, thin cell."
The team doesn't quite know what to call this cell yet, he said, but it is always there, and it is always just on one side, which means one side of the collagen fiber is open to the fluid, which he said is also slightly unusual and means that things can cross between the fluid and the collagen. There's also a cell on the other side of the fiber, he said, although the researchers are not quite sure what it does. It probably makes the collagen in the first place, Carr-Locke said, "but it must do other things too."
"So now we have a structured system of collagen, we have fluid going through it, we have a cell that is sitting there doing something, and that's what we call the interstitium," Carr-Locke said.
The interstitium is not a new term, of course, it simply is used to describe the space between things.
The researchers speculate that this is a very important part of the whole lymphatic system, which Carr-Locke said is important not only for cancer but also for immunology. One other group of disease states that this discovery could impact is scar tissue. While fibrous tissue that we call scars are usually formed after an injury or surgery, there are many internal diseases that produce collagen as part of the disease process, and researchers have never quite understood where that collagen comes from in those cases.
"It may well be that these funny little cells that we've described are actually the sources of this fibrosis, which maybe give us a target to treat certain things," he said.