The short answer is no, the interstitium is not an “undiscovered organ,” but scientists are now reporting important and previously unrecognized roles for a network of connective tissue and fluid known as the interstitium.
This “interstitial lining” has been included in medical textbooks for years, but the tissue, which surrounds the lungs, GI tract, bladder, skin, and blood vessels, was previously thought to be a passive reservoir for fluid and a physical support system for important organs. Thanks to new technology allowing investigators to take a microscopic look at tissues in living subjects, researchers are getting a closer look at the interstitium than ever before.
Areas of interstitium, previously interpreted either as unassuming “spaces” between bordering tissues or as densely-packed walls of fibrous tissue, were found to be networks of connective tissue filled mostly with fluid. This allows these areas to function as “shock absorbers” for nearby organs which routinely stretch and compress in daily function — like the lungs, GI tract, bladder, and muscles.
Based on a unique structure in which surrounding tissues directly touch this fluid-rich space (rather than encountering a barrier of dense connective tissue or cells), scientists proposed that the interstitium may enable the early spread of cancers via the body’s lymphatic system, a process which is currently not well understood.
This mechanism may solve the mystery of cases in which the cancer is found to have spread to a nearby lymph node — without evidence that it ever invaded the neighboring lymphatic vessels on the way there. Further research is expected to help researchers understand why some cancers (particularly melanoma, breast, lung and gastrointestinal tumors) spread through the lymphatic system rather than the bloodstream, how to detect this spread early, and how to prevent them from doing so in the first place.
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The cells lining the fluid and fibers of the interstitium are also unique — they most closely resemble collagen-producing cells. Researchers suggest that these cells likely respond to mechanical forces surrounding a surgical wound or injury, and act in the process of wound healing and scar formation. Of particular interest is whether these cells contribute to the development of auto-immune conditions which cause scarring (or “fibrosis”) of involved organs, such as scleroderma or inflammatory bowel disease. They also have a potential role in the formation of keloids or exaggerated scars that form underneath the skin’s surface.
These discoveries highlight the opportunities available to further investigate the function of the interstitium in cancer spreading and scarring diseases — and we can bet that this tissue will receive more than its usual passing mention in the next generation of medical textbooks.