Researchers find a previously unknown lymphatic system in the brain.

Researchers at the Wihuri Research Institute and the University of Helsinki report a surprising finding that challenges current anatomy and histology textbook knowledge, they have found lymphatic vessels are present in the central nervous system where they were not known to exist. The team discovered the meningeal linings of brain have a lymphatic vessel network that has direct connections to the systemic lymphatic network elsewhere in the body.  The  study is published in The Journal of Experimental Medicine.

Previous studies show that lymphatic circulation forms a network that covers almost the whole body and is especially important to the tissue clearance of fluids and macromolecules as well as for immune defense mechanisms. Until now, the central nervous system has been considered an immune-privileged organ devoid of lymphatic vasculature.

Researchers have recently discovered that in the eye, which is another immune-privileged organ previously considered to lack lymphatic circulation, there exists a lymphatic-like vessel that is important for the regulation of eye pressure. This led the researchers to reinvestigate the brain in this regard.

The team say that they were stunned to find such an extensive network in connection to the brain. They go on to stress that this incredible finding completely changes the understanding of how to brain is cleared of excess fluid and gives a chance to look at brain diseases from a completely new angle.

The researchers performed a thorough job in characterizing the structure and function of these previously unknown vessels. They showed that these meningeal lymphatic vessels drain out of the skull alongside arteries, veins and cranial nerves. These vessels show all molecular hallmarks of the lymphatic vessels and function as a direct clearance routes for the brain and cerebrospinal fluid macromolecules out of the skull and into the deep cervical lymph nodes.

The team can also explain how the lymphatic vessels manage to escape notice until now.  The lymphatic vessels in the current study are collapsed structures between the meninges, and if one does not know how to look, they cannot be found. Although the group had already found the vessels, it took some time to develop good imaging methods to visualize these vessels.

The lab state that the discovery has raised several new questions concerning some fundamental brain functions and the mechanisms of brain diseases. They go on to add that researchers find it highly possible that lymphatic clearance of the brain proves to be important in neuro-immunological diseases as well as in diseases characterized by the pathological accumulation of misfolded proteins or fluid into the brain parenchyma, for example in Alzheimer’s disease, which affects tens of millions of people worldwide.

Source:  Wihuri Research Institute

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