What Is the Interstitium?

Medically Reviewed by Jennifer Robinson, MD on August 17, 2022
2 min read

The study of human anatomy reaches back thousands of years, to the Romans and Greeks. Herophilus, the Greek anatomist, is considered the first to take a scalpel to skin to see how our bodies work. That was about 300 B.C., give or take a decade or two.

With all that time poking around in the human body, you’d think we’d have a decent idea of what’s where and what’s not. That’s why the discovery of what some scientists call a previously unrecognized organ is so fascinating.

In a new study, researchers claim that interstitium -- fluid-filled spaces in tissue that are connected throughout the body -- should be considered a new “organ” and that these spaces may play a major part in a lot of what goes on in your body.

Experts have long thought parts of the body (like skin, veins and arteries, and the lining around muscles) contain walls of collagen, the main component of connective tissue. Instead, as the new study points out, those "walls" aren't walls at all. Instead, they're spaces filled with fluid that are simply supported by collagen. Those spaces are the interstitium.

This new "organ" -- it’s not officially recognized as one yet -- acts as a kind of shock absorber for the body, the researchers say. The scientists found interstitium in tissue from the lungs and aorta, the digestive tract and bladder, in the skin, and in many other spots -- all places that expand and contract, where a “shock absorber” is important to protect tissue.

This “highway” of interconnected spaces filled with moving fluid -- the interstitium -- may also explain how cancer spreads. The study says the newfound network is where lymph comes from. Lymph is the fluid that your immune cells need to work well.

Scientists have known about the interstitium for a long time. But according to the study, they thought it was much denser, almost solid. That’s because the way they looked at it -- slicing it on fixed slides and seeing it under a microscope -- drained all the fluid from the spaces.

A new method of looking at it within living tissue, called probe-based confocal laser endomicroscopy, lets researchers see these spaces filled. The fact that they're connected raises the possibility that sampling the fluid in those spaces could help experts examine the spread of diseases.

Whether it's recognized as an official organ or not, the research suggests the importance of the interstitium and the need to continue to study how these spaces work.