What is the largest organ in a human body?

Krishna: You usually get the answer ‘skin’ (external organ) or ‘liver’ (internal organ) to this question.

But another previously unknown ’organ’ has been identified (1) in 2018 which astonishingly, the discoverers puts forth the idea that this new organ is the largest by volume among all organs—if what the researchers found is, in fact, an organ!

The new organ was a thin layer of dense connective tissue throughout the body, sandwiched just under our skin and within the middle layer of every visceral organ. The organ also made up all the fascia, or the thin mesh of tissue separating every muscle and all the tissue around every vein and artery, from largest to smallest. What initially seemed to be a solid, dense, connective tissue layer was actually a complex network of fluid-filled cavities that are strong and flexible, yet so tiny and undiscerning that they escaped the attention of the brightest scientific minds for generations.

Scientists discovered the new organ, which consists of fluid-filled spaces, in the body's connective tissue, including in the skin's dermis, which is shown above as the light pink layer at the bottom of this image. (Image credit: Eric V. Grave/Getty)

Called “interstitium”, these fluid-filled spaces were discovered in connective tissues all over the body, including below the skin's surface; lining the digestive tract, lungs and urinary systems; and surrounding muscles, according to a study detailing the findings, published  (March 27, 2018) in the journal Scientific Reports.

Previously, researchers had thought these tissue layers were a dense "wall" of collagen — a strong structural protein found in connective tissue. But the new finding reveals that, rather than a "wall," this tissue is more like an "open, fluid-filled highway”.

The interstitial fluid is a reservoir and transportation system for nutrients and solutes distributing among organs, cells, and capillaries, for signaling molecules communicating between cells, and for antigens and cytokines participating in immune regulation. The composition and chemical properties of the interstitial fluid vary among organs and undergo changes in chemical composition during normal function, as well as during body growth, conditions of inflammation, and development of diseases, as in heart failure[4] and chronic kidney disease. (3)

However, this is an unofficial distinction; for a body part to officially become an organ, a consensus would need to develop around the idea as more researchers study it. The presence of these fluid-filled spaces should also be confirmed by other groups (2).

So controversy still surrounds this interstitium. Until it is resolved scientifically, skin (external) or liver (internal) stays.

Footnotes:

1. Structure and Distribution of an Unrecognized Interstitium in Human...
2. Meet Your Interstitium, A Newfound 'Organ'
3. Interstitium - Wikipedia