Move over skin, there’s a new largest human organ in town, and it comes with a fancy name to boot: the interstitium. This newfound organ has been identified for the first time by researchers from New York University’s School of Medicine, who just published a study in the journal, Scientific Reports, detailing its anatomy and functions.
The paper defines the newly discovered organ as a “fluid-filled space within and between tissues” that spreads throughout the body and can be found nearly everywhere: under the skin, between our organs, around arteries and veins, along the fibrous tissue between muscles, and even around the digestive tract and urinary system.
Medicine is already acquainted with interstitial tissue (the tissue found between our cells) and interstitial fluid (the liquid that fills the space between the cells), but the interstitium ties everything together in a network of fluid-filled compartments strung together by collagen and elastin, the study explains.
Co-senior author, Dr. Neil Theise, a pathologist at NYU Langone Health in New York, points out that the interstitium may actually be the largest organ in the human body.
“I think it’s bigger than the skin,” Dr. Theise told CNN.
He estimates the interstitium makes up 20 percent of the body’s volume, “which is equivalent to about 10 liters in a young adult.”
By comparison, the skin — until now considered to be the largest human organ — only holds about 16 percent of the body mass.
Despite its size and widespread presence in the human body, the interstitium has been overlooked in anatomical studies until now due to the procedure previously used by researchers to analyze tissue.
According to National Geographic, the NYU researchers stumbled upon the newly identified organ purely by chance, while investigating a bile duct for cancer spread. The accidental discovery was made with the help of a probing technique called confocal laser endomicroscopy (pCLE), which the scientists were using to examine tissue samples from 13 patients with pancreatic cancer.
Traditionally, tissue is dehydrated and thinly sliced before going under the microscope for analysis. However, pCLE enabled the scientists to examine living tissue, allowing the structure of the interstitium to remain visible.
Intrigued by what they had found, the researchers froze the biopsies and took a closer look at them under the microscope. Thus, they were able to observe for the very first time the anatomy of this vast network of channels that accumulate fluid and drain in into the lymph nodes.
The scientists then compared the biopsies with healthy tissue samples, yielding the same result: the network of fluid-filled compartments was unequivocally there.
“The more tissues I saw, the more I realized it’s everywhere,” said Dr. Theise.
At first, his team thought that what they were observing was no more than dense connective tissue, but later it became apparent that the interstitium has both a unitary structure and a unitary function — each a prerequisite for the definition of an organ, Dr. Theise shows.
“This structure is the same wherever you look at it, and so are the functions that we’re starting to elucidate,” he said.
In his opinion, the interstitium acts as a source of lymph, the fluid vital to the functioning of immune cells and which carries infection-fighting white blood cells. In addition, the fluid-filled compartments that make up the interstitium take on the role of protecting tissues during daily functions, by operating as shock absorbers, Dr. Theise revealed.
LiveScience notes these discoveries are also reflected in a 2011 study, which signaled the existence of a network of dark fibers in association with several cases of bile duct cancer, but didn’t offer an explanation as to what it was.
“I was pleased that they substantiated our impression that this network exists,” said Dr. Michael Nathanson, chief of the digestive diseases section at Yale University School of Medicine, who authored the 2011 paper.
Nevertheless, Dr. Nathanson is not convinced the interstitium is actually an organ, but rather “a new component that is common among a variety of organs,” he told CNN.
Even so, he points out this discovery may have crucial implications in the way certain diseases are diagnosed, particularly cancer.
“In my opinion, this has the potential to change our understanding of the human body because this ‘pre-lymphatic region,’ as the authors refer to it, may undergo changes in certain diseases states such as certain types of cancer,” Dr. Nathanson stated.
Dr. Theise and his colleagues also suggest the vast network of fluid-filled compartments that make up the interstitium could better explain how cancer spreads to the lymph nodes, as well as other areas of the body.
“These anatomic structures may be important in cancer metastasis, edema, fibrosis, and mechanical functioning of many or all tissues and organs,” they conclude in their paper.