The 'organ' can be found throughout our body, including the connective tissue under our own skin. Kevin Mackenzie, University of Aberdeen/Wellcome Collection

You would think, after thousands of years of study, we’d have nailed down what was going on inside us. But researchers are arguing that they have accidentally discovered an entirely new organ.

The new structure could even help explain where much of the fluid in our body sits, and may even be the source of lymph, the fluid that is essential to the functioning of our immune system. As such this network, or structure, of fluid-filled channels could be playing a significant role in maintaining our health, as well as propagating disease.

It's not even limited to the obscure parts of us – it is found wrapped around many of our other, more well known, organs. You see, while we thought our lungs, digestive tract, and even our skin were surrounded by tough and dense connective tissue, it turns out that this tissue is in fact riddled with spaces supported through a meshwork of strong connective proteins to prevent them from collapsing, and which freely allow fluid to flow.

Jill Gregory/Mount Sinai Health System

This could help explain where so much of our body’s fluid goes. While our cells contain most of the fluid, and the circulatory system carries a whole load more, over a third went unaccounted for and was simply said to be “interstitial”, or just floating around between organs and cells. The researchers claim, in a paper published in Scientific Reports, that the “interstitium” should be defined as an organ in its own right.

They think it may explain why some forms of cancer can spread so rapidly and between unrelated organs. It might also help explain why wrinkles develop, as when you age these channels in the tissue under your skin may fold in on themselves.

It was while conducting routine endoscopies of patients that doctors noticed that the tissue surrounding the bile duct, which should have been fairly solid and dense, was actually covered in an intriguing pattern. When one then decided to take a look using the same device under the skin of his nose, he surprisingly found exactly the same effect.

The key, it seems, was looking at living tissue at such fine magnification. Up until now, medical researchers have relied on fixed tissue microscope slides when studying the human body, say the authors. To do this, they take thin slices of tissue, treat it with chemicals, and dye the structures so that they can be easily identified when placed under a microscope.

The problem, however, is that the fixing drains the tissue of all fluid, and it is expected that this process collapses all the once fluid-filled compartments of the interstitium. Because of this, researchers looking at slides of organs have simply assumed the flattened compartments were tears in the tissue.

It now seems that they will need to do more work to define this structure and convince others that it is an organ in its own right, rather than just a new type of tissue.

“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," explained co-author Neil Theise.