Interstitium

interstitial fluid compartmentinterstitial space

the idea

The fluid-filled space running within and between the body's tissues — supported by collagen and draining into the lymphatic system, found in skin, gut and bladder linings, and the soft tissue around blood vessels and airways. The disputed claim is whether these spaces form one continuous, structured compartment large enough to count as an organ-like entity, or are simply a long-known property of tissue that newer imaging brought into view.

The fluid-filled compartment within and between human tissues — collagen-supported sinuses that drain to lymph, present in the submucosae of the gastrointestinal tract and urinary bladder, the dermis, the peri-bronchial and peri-arterial soft tissues, and fascia. Named as a previously unrecognised structured organ-like compartment in Benias et al. 2018 in Scientific Reports; the framing was partially walked back by the authors themselves (Theise: not an organ — a system) and contested by Mestres-Ventura 2019 as overreach over the existing anatomical record.

Etymology§

Interstitium from Latin interstitium, that which stands between (inter- + sistere). The general anatomical sense — the space between tissue cells filled with extracellular fluid and matrix — has been in continuous use since the nineteenth century and is uncontested. The contested move in 2018 was the proposal that the spaces are structured, continuous, and large enough to be considered a discrete anatomical entity rather than just a property of tissue.

The interstitium in the 2018 framing is a continuous compartment of fluid-filled, collagen-supported sinuses pervading the body's connective-tissue spaces. The argument is that the spaces exist in vivo at a scale and continuity that the standard histological slide-preparation process — which collapses the tissue by removing fluid before fixing — had been hiding. Probe-based confocal laser endomicroscopy, used during endoscopy with intravenous fluorescein, was the imaging technology that made the spaces visible in living tissue.

The literature has two poles. On one side: the Benias group at Mount Sinai Beth Israel and NYU Pathology — Petros Benias, David Carr-Locke, and Neil Theise as the principals — the press coverage that framed the finding as a new organ, and the follow-on work on the interstitium's possible roles in cancer metastasis, fibrosis, and fascia function. On the other: Mestres-Ventura's 2019 European Journal of Anatomy commentary and like-minded responses from inside anatomy proper, which argue the spaces were well-documented before 2018 and that the unrecognised framing depends on conflating not visualised by standard slide-preparation with not previously known.

Two claims have to be separated. The methodological claim — that pCLE imaging shows fluid-filled spaces in vivo that standard histology had collapsed — is uncontested and important. The ontological claim — that the spaces warrant the designation new organ or even new system, distinct from the existing concept of interstitial space — is contested and not yet settled. The conjectured functions (shock absorption during organ compression, lymphatic drainage routes, conduits for cancer-cell migration) are reasonable hypotheses awaiting empirical work. The reading list around the term is currently thin and will need to be extended into cancer-metastasis, fluid-mechanics, and fascia literatures before the term can do the work the new organ framing promises.

Interstitium

Etymology§

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