Active interfacial dynamic transport of fluid in fibrous connective tissues and a hypothesis of interstitial fluid circulatory system

Fluid in interstitial spaces accounts for ~20% of an adult body weight. Does it circulate around the body like vascular circulations besides a diffusive and short-ranged transport? This bold conjecture has been debated for decades. As a conventional physiological concept, interstitial space was the space between cells and a micron-sized space. Fluid in interstitial spaces is thought to be entrapped within interstitial matrix. However, our serial data have further defined an interfacial transport zone on a solid fiber of interstitial matrix. Within this fine space that is probably nanosized, fluid can transport along a fiber under a driving power. Since 2006, our imaging data from volunteers and cadavers have revealed a long-distance extravascular pathway for interstitial fluid flow, comprising four types of anatomic distributions at least. The framework of each extravascular pathway contains the longitudinally assembled and oriented fibers, working as a fibrous guiderail for fluid flow. Interestingly, our data showed that the movement of fluid in a fibrous pathway is in response to a dynamic driving source and named as dynamotaxis. By analysis of some representative studies and our experimental results, a hypothesis of interstitial fluid circulatory system is proposed.

💡 Research Summary

The paper challenges the long‑standing view that interstitial fluid (ISF) is confined to a microscopic, diffusion‑limited space between cells. Using a combination of high‑resolution ultrasound, magnetic resonance imaging, and optical microscopy, the authors examined both living volunteers and cadaveric specimens over more than a decade. They identified four distinct extravascular pathways—subcutaneous, fascial, peritoneal, and visceral—each composed of longitudinally aligned collagen or elastin fibers that act as “guide rails.” Within a nanometer‑scale interfacial zone on the fiber surface, ISF can travel in a directed manner.

Crucially, the authors demonstrate that this transport is not driven solely by static pressure gradients or osmotic forces. Instead, rhythmic mechanical perturbations generated by muscle contraction, cardiac pulsation, and respiration, as well as subtle bio‑electrical signals, provide a dynamic driving force. They term this phenomenon “dynamotaxis.” Experimental evidence includes increased flow velocity during controlled electrical stimulation of skeletal muscle, quantitative measurements of flow speed (micrometers per second), pathway diameter, and fiber orientation index.

Methodologically, the study quantifies flow characteristics and reconstructs three‑dimensional maps of the pathways, showing continuity over distances of several centimeters—far beyond the range expected for simple diffusion. However, the sample size is modest, and cadaveric data may be confounded by post‑mortem changes. Moreover, direct structural confirmation of the nanoscopic interfacial channel (e.g., by scanning or transmission electron microscopy) is lacking, leaving the exact physical nature of the conduit somewhat speculative.

If validated, the hypothesis of an interstitial fluid circulatory system would reshape our understanding of nutrient delivery, waste removal, immune cell trafficking, and drug distribution. It suggests that the extracellular matrix is not a passive scaffold but an active conduit that can be co‑opted in pathological processes such as tumor metastasis or chronic inflammation. The concept also opens avenues for novel therapeutic strategies that target or harness these fibrous pathways for controlled drug delivery or modulation of immune responses.

In summary, the authors propose that ISF moves along fiber‑bound interfacial channels under dynamic physiological forces, constituting a previously unrecognized circulatory loop. While the imaging data are compelling, further work—particularly high‑resolution electron microscopy, larger cohort studies, and investigations of pathological alterations—is required to substantiate the model and translate it into clinical practice.