“What is an organ? Anatomy textbooks are rather fuzzy about what defines an “organ,” requiring one to have primary tissue—parenchyma—and “sporadic” tissue, called stroma, which can be nerves, vessels, and other connective tissue. Organs are the necessary building blocks of organisms (hence, the name), and can be gigantic or microscopic. So long as cells clump together to form tissues, and these tissues organize themselves into organs that perform specific functions in the survival of an organism, that mass of tissues and cells can be called an organ.
“Theise, Carr-Locke, and Benias weren’t sure what to call this space with its collagen bundles and fluid. The fluid itself appeared rich in proteins typical of lymphatics and serum, but the space was neither lymphatic nor vascular (meaning that it contained neither veins nor arteries), so what could it be?
“That’s when it dawned on them that what they’d stumbled upon was actually talked about in medical textbooks, but that they were the first to actually define it.
“This thing they were looking at, struggling to understand with its bizarre structure and rule-breaking form, was the interstitium, a space vaguely described in textbooks as where “extracellular fluid” is found, the fluid that isn’t contained within cells. What doctors had defined as “dense connective tissue” wasn’t dense connective tissue at all. In fact, they were all fluid-filled structures that only appeared to be densely compacted when tissues were made into slides, the fluid draining away, the collagen lattice collapsing onto itself.
“They had a theory—that the space was the interstitium—and a way to prove it. They were on to something.”
Read Tanya Basu’s full article at Daily Beast: The Interstitium, the Largest Organ We Never Knew We Had