Humans are made mostly of water. In fact, approximately 75 percent of an infant's body mass and up to 60 percent of that of an adult is made up of water.
To store all this liquid, our bodies devised clever ways of compartmentalizing. The "interstitial space" is one such compartment.
Medical professionals have long known about the interstitium, a network of tissue generally known to reside within the lungs, and about the interstitial space, which stores fluid.
But now, for the first time, researchers — co-led by Dr. Neil Theise, a professor in the Department of Pathology at New York University's School of Medicine in New York City — defines the interstitium as an actual organ, and it is one of the largest in the human body.
In their paper — now published in the journal Scientific Reports — Dr. Theise and his colleagues explain further why the newfound organ was "missed" all this time, as well as what some of the additional implications of their discovery are.
The interstitium: Redefining an organ
The researchers explain that the predominant visualization technique in the medical field involves "fixing" a layer of tissue and analyzing it on microscope slides.
The "fixing" process uses chemicals that drain the tissue of liquid. This makes the connective "lace" that forms the interstitial tissue collapse.
"This fixation artifact of collapse has made a fluid-filled tissue type throughout the body appear solid in biopsy slides for decades."
"And," adds Dr. Theise, "our results correct for this to expand the anatomy of most tissues."
What prompted the researchers to study bile ducts in specific? Three years prior to the study, two co-authors were examining the bile ducts of people with cancer to see whether or not the tumors had metastasized when they stumbled upon this intermeshed tissue of fluid-filled cavities that did not resemble any known anatomical part.
The new technology allowed the scientists to recognize the same structure throughout the entire body.
"In sum," the authors write, "while typical descriptions of the interstitium suggest spaces between cells, we describe macroscopically visible spaces within tissues — dynamically compressible and distensible sinuses through which interstitial fluid flows around the body."
The study authors propose "a novel expansion and specification of the concept of the human interstitium."
Toward 'dramatic advances in medicine'
"Our findings," say the study authors, "necessitate reconsideration of many of the normal functional activities of different organs."
As they explain, the findings challenge a long-standing scientific narrative. It was previously thought that the digestive tract, lungs, and urinary systems, as well as the intermuscular fascia and the immediate layer beneath the skin's epidermis, are all lined with thick connective tissue.
Instead, as the new study reveals, these are lined with interstitial tissue, which is made of interlaced compartments filled with lymphatic fluid.
Given that the lymph fluid is filled with infection-fighting immune cells, the discovery may help us to understand why cancer that spreads to the interstitial tissue is so much more likely to metastasize.
As the authors explain, "These [newly discovered] anatomic structures may be important in cancer metastasis, edema, fibrosis, and mechanical functioning of many or all tissues and organs."
Also, the collagen lined by cells in the interstitial space tends to deplete with age, so the newfound organ may contribute to wrinkling and the skin aging process.
"This finding has potential to drive dramatic advances in medicine, including the possibility that the direct sampling of interstitial fluid may become a powerful diagnostic tool."
Dr. Neil Theise