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Introducing the Interstitium

There's a new organ called the interstitium, according to a new study in Scientific Reports. A network of fluid-filled spaces in connective tissues all over the body, including below the skin's surface; lining the digestive tract, lungs and urinary systems; and surrounding muscles, has now been classified as an organ, Scientific American says.

Previously, researchers had thought these tissue layers were a wall of collagen. But the new study shows that the tissue contains interconnected, fluid-filled spaces that are supported by a lattice of thick collagen bundles, according to cosenior author Neil Theise, a professor of pathology at New York University Langone School of Medicine. 

These fluid-filled spaces had been missed for decades because they don't show up on the standard microscopic slides that researchers use to looks at cells, Scientific American says. When scientists prepare tissue samples for these slides, it drains away fluid and causes the newfound fluid-filled spaces to collapse. Rather than using such slides, the researchers discovered these fluid-filled spaces by using a new imaging technique that allows them to examine living tissues on a microscopic level. The technology is called "probe-based confocal laser endomicroscopy" or pCLE, according to the article. It combines an endoscope with a laser and sensors that analyze reflected fluorescent patterns and gives researchers a microscopic view of living tissues.

The designation of the interstitium as an organ now needs scientific concensus to become official. But Thiese says the findings could help to explain why cancerous tumors that invade this layer of tissue can spread to the lymph nodes, as the interstitium is a source of lymphatic fluid.