Like many modern major scientific breakthroughs, this was discovered because of technological advances in research and diagnostic tools. This was found using new imaging technology called Probe-based Confocal Laser Endomicroscopy (pCLE). What's important about this new form of microscopy is that it can visualize things that are inside a living person in real time. That's pretty rad. The interstitium has never been found before because all previous forms of microscopy involved taking tissue samples, and taking tissue samples means cutting tissue, which means that the fluid drained from these vessels before the tissue was viewed. Until now, it has been thought that these tissues were thick, hard, protective collagen layers; it seems now that these were actually the drained and collapsed collagen vessels of the interstitium. The structure was confirmed by taking tissue samples from patients having pancreatic surgery, rapidly freezing the samples, and looking at slices of the frozen samples using more traditional microscopy. When frozen before allowing the interstitial fluid to flow out, the vessels retained their form.
This could have implications for cancer migration, since it seems that the interstitium is totally connected. Potentially, a cancer cell that makes it into this fluid might be able to effectively travel down this water slide-like tube to another tissue to form another tumor site.
Of course, this is a huge discovery, and it has been peer-reviewed, but now that it's out there, other scientists will certainly try to poke holes in this conclusion/"organ" classification, or to provide a better explanation of what the study's authors showed. This is the beauty of the scientific method. Stay tuned!
A colleague of mine commented quite appropriately:
“Newly-discovered human organ may help explain...” Wait hold up, I’m sorry, did I just read the words “newly-discovered human organ”? What in the... I can’t... when I was deciding what area of biology to pursue, I didn’t know that discovering new organs was an option! *mind blown*