Our body comes with an inbuilt drainage system called the lymphatic system. The tissues in our body lie in a pool of fluid called the interstitial fluid. While most of this fluid is directly circulated and recirculated from and into the blood vessels (called the body’s circulatory system), about 1% of this fluid is re-circulated through a different route. Interstitial fluid enters a network of vessels called lymph vessels, which in turn drain excess proteins and waste material from this fluid and into larger blood vessels for recirculation or destruction. The lymphatic system is thus an accessory system that acts in parallel with the blood circulatory system to remove excess proteins and solutes from tissues.
Fluid Flow in the Brain
Apart from blood vessels, the brain also has a fluid called the cerebrospinal fluid (CSF) circulating in its outer parts. This fluid maintains brain pressure and protects the brain from physical injury. However, a system analogous to the body’s lymphatic system has not been seen in the brain till date. This is surprising because the brain has a very high metabolic rate and brain cells are particularly sensitive to the balance of chemicals in their environment. So how does brain tissue drain waste? It has been speculated that the CSF could perform the drainage role in the brain.
Injecting Fluorescent Molecules into the Brain
How does waste from the brain tissue get out of the brain? If it is through the CSF, then the question becomes—how do tissues release their waste into the CSF which has so far been found only in the sub-arachnoid space, the outer areas of the brain? Researchers from the University of Rochester injected small amounts of fluorescent ‘tracer ‘molecules into the brain’s CSF, and as the name suggests, traced the destinations of these tracers by brain scans. They found new channels through which the CSF flows, right into the brain tissue, called brain parenchyma. They could trace the paths of these molecules, and using molecules of different sizes, they could estimate the volumes of these paths through the brain.
They found that the CSF traverses the inner parts of the brain including the space around brain tissue in hitherto unknown channels that lie parallel to and on the boundaries of the brain’s arteries and veins, formed by cells called astrocytes. Moreover, when a water-transport gene called AQP4 was deleted in mice, fluid flow through this system was suppressed, meaning that water-transport helps build up the pressure to move things along in this system using bulk-flow or convection. The water-pressure creates a pressure allowing waste to be drained away faster. The brain’s lymphatic system has thus been found, and is being referred to as the ‘glymphatic system’, called so because cells called glial cells help create pressure.
Alzheimer’s Molecule Takes This Route
The researchers went one step further and traced the path of fluorescent-tagged amyloid b, the protein responsible for Alzheimer’s disease. They found that this protein travels along this route of ‘glymphatic’ blood vessels. This insight provides therapeutic possibilities. Improving flow through this system could speed up clearance of neurodegradative molecules like amyloid-beta from the brain. Conversely, impeding or reducing flow through this system might help retain vaccines or drugs in the brain tissue for a longer period of time. If these vessels are also routes for migrating cells, could the metastasis of cancerous tumour cells be dependent on this system too?