An Acupoint-Originated Human Interstitial Fluid Circulatory Network

An Acupoint-Originated Human Interstitial Fluid Circulatory Network

The concept of acupoints and meridians has been a cornerstone of Traditional Chinese Medicine (TCM) for centuries. The Tian Sheng Bronze Statue, introduced in 1027, was the first to depict acupoints and meridians, evolving into the atlas used today in TCM. Acupoints are defined areas on the body surface relative to certain landmarks, particularly in the extremities, where they connect anatomically with various visceral organs or tissues, forming a Meridian and Collateral network. Each main meridian is typically a virtual line connecting a group of adjacent acupoints associated with specific visceral organs. Despite their significance in TCM, the anatomical structures of these meridians remain largely unclarified in modern medical science.

In TCM, an acupoint is considered a gateway for substances or bio-signals to enter or escape the meridians. Over the decades, efforts have been made to track the transport processes of imaging tracers from acupoints along the meridians. However, no conduit-like structures, other than blood or lymph vessels, have been found to connect acupoints on the body surface with different visceral organs. This has led to a physiological question initiated by Ernest Starling in 1896: how does fluid flow in interstitial connective tissues and circulate throughout the body like blood and lymph?

Explorations of fluid flow from an acupoint began in the early 1960s when hypodermic injections of colored dyes into animal acupoints revealed conduit-like structures named “Bonghan ducts.” In the 1990s, these structures were re-investigated using Trypan blue or Alcian blue and renamed “primo-vessels” or the “primo vascular system.” These primo-vessels were described as micro-conduits lined with endothelial cells, supporting fluid flow and containing multiple channels surrounded by loose collagenous matrices. However, the anatomical and histological structures of these primo-vessels remained ambiguous, and their relationship with human acupoints was unclear.

From the 1950s to the 1990s, hypodermic injections of isotopic tracers into human acupoints in the hands or feet allowed visualization of long-distance migration channels using radionuclide imaging techniques. Meng et al. investigated the migration channels of technetium-99m from acupoints of the 12 main meridian channels, finding that the isotopic tracer migrated over long distances in either the upper or lower limbs. The radiotracer migration channels from the Neiguan acupoint were distinct from those originating from non-acupuncture and non-meridian points. However, comprehensive data confirming the existence of these isotopic channels for all acupoints on the meridian lines is lacking. The trajectories of radioactive tracers from acupoints differed from those of intravenously injected isotopes and lymphotropic isotopes, suggesting that the isotopic migration channels from acupoints are unique interstitial fluid (ISF) flow pathways rather than conduit-like vessels. Although somewhat similar to the 12 main meridian lines on the TCM atlas, it is difficult to draw a relationship between acupoint-originated isotope migration channels and the meridians in humans. Other studies showed that radiotracers from acupoints entered the venous system via lympho-venous anastomoses, denying the existence of meridian lines. Additionally, the histological structures of the isotopic pathways could not be clearly identified due to the poor spatial resolution (approximately 1 cm) of scintigraphic images.

Since 2006, contrast-enhanced magnetic resonance imaging (MRI) has been used to investigate fluid flow originating from acupoints in healthy volunteers. Hypodermic injections of a paramagnetic tracer (gadolinium diethylenetriamine pentaacetic acid [Gd-DTPA]) into acupoints of the hands or feet allowed high spatial resolution (approximately 1 mm) MRI to visualize two types of long-distance fluid flow pathways originating from acupoints: smooth and non-smooth pathways. The smooth pathways showed continuous trajectories, while the non-smooth pathways showed discontinuous trajectories. Neither the smooth nor the non-smooth pathways colocalized with lymphatic vessels visualized by iodized oil. If the injection point was not an acupoint but near a vein, only smooth pathways were observed. If the injection point was an acupoint, both smooth and non-smooth pathways were observed simultaneously. Neither the smooth nor non-smooth pathways matched the 12 main meridian lines on the TCM atlas, raising the question of the relationship between meridians in TCM and the fluid flow pathways enhanced by tracers originating from acupoints.

To identify ISF pathways from acupoints in physiological conditions, fluorescent imaging was adopted in an ex vivo human leg sample. In a patient with severe foot gangrene preparing for lower leg amputation, a fluorescent tracer was injected hypodermically into the Kunlun acupoint at the ankle before amputation. The lower leg was dissected carefully, revealing several long-distance ISF flow pathways extending from the Kunlun acupoint to the upper end of the amputated leg. Histological analysis identified four types of pathways: (1) a cutaneous pathway in the dermis and hypodermis tissues; (2) a perivascular and adventitial connective tissues (PACT) pathway along venous vessels; (3) a PACT pathway along arterial vessels; and (4) a neural pathway composed of the endoneurium, perineurium, and epineurium of a peripheral nerve. Compared with MRI results, cutaneous pathways were probably the non-smooth pathways, and PACT pathways were probably the smooth pathways. All four types of pathways were composed of fibrous connective tissues rather than endothelial cells, providing evidence that acupoint-originated pathways are distinct from blood or lymph flow and represent long-distance ISF flow pathways.

To detect systemic distributions of ISF flow pathways in visceral organs or tissues, a mechanical automatic chest compressor was used on human cadavers to simulate heartbeats and chest movements. Repeated chest compressions were performed for 2.5 hours after hypodermic injection of a fluorescent tracer into the Shaoshang acupoint in the first knuckle of the thumb. Fluorescent imaging revealed a cutaneous and perivenous pathway from the Shaoshang acupoint to the right atrium. The cutaneous pathways were found in the hand and lower forearm but not in the skin above the cubital fossa. The cutaneous pathway contained dermic and hypodermic tissues and superficial and deep fascial tissues from the thumb but not from the index finger. The PACT pathways were observed along the veins of the arm, axillary sheath, superior vena cava, and into the pericardium and epicardium tissues over the right atrium. Micro-CT imaging showed that the interlobular septum of adipose tissues in the cutaneous pathways was longitudinally assembled toward the direction of ISF flow, while the interlobular septum of skin tissues outside the pathways was irregular. Under a confocal laser microscope, the intrinsic framework of the ISF pathways comprised abundant micron-sized fibers assembled longitudinally toward the direction of ISF flow, with their surrounding gel-like interstitial matrix fluorescently stained, indicating long-distance ISF flow.

Connective tissues are one of the four basic types of animal tissue and are continuously distributed throughout the body. The identification of ISF flow pathways from acupoints in humans strongly indicates that ISF may flow systematically in interstitial connective tissues, comprising diverse types of ISF pathways. Since Ernest Starling described the net flow of fluid between connective tissue spaces and the capillary lumen in 1896, the transport pattern and mechanical mechanism of ISF flow through extravascular spaces or interstitial connective tissues have been explored. Using porous matrix theory and measurements of hydraulic conductivity of various tissues, it was found that ISF diffuses mainly for short distances through the tissue gel between capillaries and adjacent cells. Occasionally, ISF can flow freely as small rivulets and vesicles for short distances in the interstitium. When a dye is injected into circulating blood, it often flows along the surfaces of collagen fibers or cell surfaces. Observing ISF flow with imaging tracers is more direct than measuring intricate interstitial pressure gradients.

In the 1950s to 1970s, linear fluorescent pathways were observed along the elastic fibers of interstitial connective tissues after the passage of fluorescent dye through the microvascular wall in the mesentery of rabbits and cats. The fluorescein transport occurred much faster than diffusion, and elastic fibers might have a passive transport or guide rail function for fluid flow between arterial and venous regions of capillaries and lymphatic vessels. The fluorescein transport pathway in interstitial connective tissues was marked as a “low-resistance” pathway or described as an extravascular fluid pathway. The regional ISF flow after deriving from capillaries was further studied by incomplete dark field transillumination and electron microscopy. The movements of marked ISF in interstitial connective tissues were named prelymphatic or interstitial tissue channels, believed to connect continuously with initial lymphatic vessels, forming a random network of converging drainage pathways in interstitial connective tissues. Illustrated by India ink suspension, fluorescein-isothiocyanate, and ferrocyanide precipitates, the positions of tracers were found near the vessel wall or in skeletal muscle tissues or intestinal wall, representing water-rich regions in interstitial connective tissues. However, the spatial structures of such interstitial tissue channels and their surroundings in the gel-like interstitial matrix for fluid flow have not been adequately stained and identified.

The above imaging studies suggested that: (1) ISF can flow through interstitial connective tissues, but the spatial structures where ISF flows need to be identified. (2) Revealing the spatial structures of the fibrous matrix for fluid flow, as well as their boundary structures and compositions, might be key to understanding the mechanical mechanism of long-distance ISF flow in interstitial connective tissues. (3) ISF flow may not only be a diffusive process but also guided by fibers, indicating a unique dynamic mechanism to drive ISF besides pressure and concentration gradients. The findings on diverse types of ISF flow pathways from acupoints, especially the PACT pathways along vascular vessels, might reveal the spatial structures and driving mechanisms of long-distance ISF flow in interstitial connective tissues.

Using in vivo imaging techniques and histological identification, ISF flow along vascular adventitia has been identified in several veins and arteries of systemic and pulmonary circulation. In rabbits, peripheral ISF in the ankle dermis was found to flow constantly along the veins in the lower extremity veins, inferior vena cava in the abdomen and thorax, and into the epicardium, forming pericardial fluid. The velocity of the constant adventitial ISF flow was estimated as 3.6 to 15.6 mm/s. Meanwhile, peripheral ISF was found to enter capillaries or lymphatic vessels and merge into blood circulation. A constant ISF flow along the veins and arteries of lower limbs was also observed in mice. In rabbit pulmonary circulation, ISF flow was found along the pulmonary vein from the lung toward the heart.

Unlike blood in a sealed cardiovascular conduit, the spaces in a PACT pathway for ISF flow are unique. In live rabbits, real-time fluorescence stereomicroscopy revealed that ISF in a venous PACT pathway flows in at least two layers: (1) between the covering fascia and adventitia and (2) through adventitia. Under a confocal laser microscope, the venous adventitia comprised abundant fluorescently stained fibers and their surrounding gel-like matrix, indicating ISF flow in PACT pathways. The fibrous matrix, neither the fibers nor the fascia nor the gel, can flow. The spaces in PACT pathways for constant ISF flow were named an “interfacial zone or interspace” between a solid phase (a fiber or fascia) and a liquid phase (the gel/liquid substance). The patterns of ISF flow in a PACT pathway were named “interfacial fluid flow.” When fluid entered such topologically connected interfacial zones or poly-interspaces along solid fibers or fascia in a PACT pathway, ISF would flow constantly along vessels under an actively dynamic driving power, such as heartbeats and respiratory movements. Two patterns of constant ISF flow were found in venous PACT pathways: (1) longitudinal flow along the vascular vessel toward a driving center and (2) diffusion from the vessel into surrounding tissues, like an irrigation system.

Based on the pattern of pulling ISF from a PACT pathway and the characteristics of the gel-like matrix to absorb and release water, a hypothetical “gel pump” has been proposed. The gel-like fibrous connective tissues covering the cardiovascular system can work as a driver, named a “gel pump,” to regulate ISF. When the heart contracts and relaxes repeatedly, the gel-like matrix covering the heart squeezes out fluid and releases it into the surrounding cavity. The matrix of the drivers might be a one-way “pump” that normally allows fluid to flow from ISF pathways into the gel pump while preventing fluid within the gel pump from flowing back into the pathways. In human cadaver experiments, the “gel pump” on the heart, repeatedly compressed by an automatic cardiac compressor, “pulled” peripheral ISF in the thumb via PACT pathways into the epicardium. In live rabbits, the “gel pump” on the heart “pulled” peripheral ISF via PACT pathways into superficial tissues on the heart and entered the pericardial cavity, causing pericardial effusion. The detailed driving mechanism needs further investigation. In short, the one-way gel pump mechanism suggests that ISF flow in the network of interstitial connective tissue is a phenomenon of capillary flow (or interfacial fluid flow) driven by an active force in biological and living systems.

An acupoint-originated interstitial fluid circulatory system throughout the body has been distinguished from rodents. Diverse types of long-distance ISF flow pathways originating from acupoints in the human body have been identified: cutaneous-, venous PACT-, arterial PACT-, fascial-, and neural-pathways. It is unclear whether cutaneous, fascial, and neural ISF may circulate systematically around the whole body like adventitial ISF flow pathways. A working hypothesis is proposed to comprehend the acupoint-originated ISF circulatory system: (1) There are one or more ISF flow pathways connecting with an acupoint on the body surface, including at least one cutaneous pathway. (2) An ISF circulatory network is constituted between different acupoints on the body surface and their associated visceral organs or tissues. (3) The acupoint-originated ISF circulation is regulated mainly by the cardiovascular system via heartbeats and respiratory movements. (4) The acupoint-originated ISF circulatory network is part of an ISF circulatory system, communicating and coordinating functionally with other systems of the human body, such as the nervous system and cardiovascular system. (5) The internal environment around cells in a visceral organ or tissue can be modulated from certain acupoints on the body surface via the ISF circulatory system.

Using a mechanical automatic chest compressor and ventilator to simulate heartbeats and respiratory movements, a network of acupoint-originated ISF flow pathways from each acupoint on all fingers and toes can be found in cadavers. Enhanced or other specific MRI techniques can visualize such a network in live human subjects. According to the 12 main meridians in TCM, the acupoint-originated ISF flow networks from all acupoints on each finger and toe might be divided into 12 groups. If a “node” is defined to correspond to an acupoint and the “lines” to the various types of ISF flow pathways, 12 groups of the node-lines networks can be aligned and constructed as an acupoint-originated Human Interstitial Fluid Connectome Atlas (HIFCA). The reflected projections of the 12 groups of diverse node-lines on the body surface might be compared with the 12 meridian lines on the TCM atlas. The connections with visceral organs or tissues of the 12 groups of node-lines networks might also be compared with the relations between acupoints on the body surface and corresponding internal organs described by the 12 main meridians in TCM.

Together with previous studies, an acupoint may become a historical intersection of ancient medical knowledge and modern medical science, where we could cherish ancestral treasures and inaugurate a new frontier for innovative medical applications.

doi.org/10.1097/CM9.0000000000001796

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