In 2018, a research team from NYU School of Medicine published a paper in the journal Scientific Reports, entitled : Structure and Distribution of an Unrecognized Interstitium in Human Tissues.

The paper described what is now known as the Interstitium - a contiguous network of interconnected fluid-filled spaces below the skin's surface. It lines the digestive tract, lungs and urinary systems, as well as surrounding arteries, veins, and the Fasciaplugin-autotooltip__plain plugin-autotooltip_bigThe Fascia

Fascia are whitish bands or sheets of connective tissue - mainly composed of collagen and fat, which stabilise, enclose, and separate muscles and other internal organs.

An exact definition has been much disputed over the years. Aside from 'compartmentalising' the various organs etc, its functions are poorly described and not well understood.

For many years, anatomists had overlooked the interconnection of interstitial spaces - largely because of their dependence on the examination of dry, fixed tissue on microscope slides.

The fluid in the Interstitium - comprised of water and solutes, cytokines, hormones and other proteins etc. - may account for as much as 20% of the body weight.

It's now thought that the interstitium is :

• a reservoir and transportation system for nutrients and solutes among organs, cells, and capillaries.

• a conduit for signaling molecules communicating between cells.

• a route for antigens and cytokines participating in immune regulation.

It's also thought that the interstitium may be of crucial importance in the progress of disease - e.g. in the transport and metastasis of cancer cells.

Much more research. however, is needed to clarify its full list of functions and their medical implications.

Note: Some research teams have suggested that the interstitium should be thought-of as a bodily 'organ' - though the definition of an 'organ' is at present quite vague (ref.), and some other researchers have rejected the idea (ref.)

Also see : The Mesenteryplugin-autotooltip__plain plugin-autotooltip_bigThe Mesentery

"The mesentery is a continuous set of tissues which is formed by the double fold of peritoneum that attaches the intestines to the wall of the abdomen. ['..] It has been proposed for reclassification as an organ due to research at the University of Limerick in the 2010s" (another recently discovered 'organ')