Searching for lymphatic vessels in the brain

You probably remember your Biology teacher talking about the skeletal system, cardiovascular system, nervous system, muscular system and so on. But do you remember us humans have a lymphatic system as well? For some of you this may ring a bell in your head, but for others not so much…

The lymphatic system consists of a network of vessels that carry a milky white fluid called lymph. This system has three main functions: to remove the interstitial fluid from tissues; to transport fatty acids from the digestive system; and to help defend our body, since it contains immune cells, such as lymphocytes, neutrophils and other white blood cells. The two main organs that comprise this system are the bone marrow and thymus, but the spleen and tonsils also make part of the lymphatic system.

It is textbook knowledge that the lymphatic system is not present in the central nervous system (i.e. brain and spinal cord), even though we recently discovered there is a constant immune surveillance that takes place in the meninges (the membranes that cover the brain). In the search for the entrance and exit of immune cells from the central nervous system, scientists from the University of Virginia developed a technique to mount the meninges in such a way they could examine it as a whole. When they examined the piece under the microscope, they found vessel-like patterns in the distribution of the immune cells. Prompted by their discoveries, they tested for markers associated with lymphatic endothelial cells, discovering they matched the distribution of the cells. The lymphatic vessels lining the central nervous system are connected to deep cervical lymph nodes and it was probably due to their complex location that they weren’t discovered up until now.

This fortuitous finding sheds a new light on the pathology of the innumerous neurodegenerative and neuroinflammatory diseases we all hear about everyday: Alzheimer’s, Parkinson’s, Huntington’s, multiple sclerosis and ALS. Malfunction of the meningeal lymphatic vessels could be a root cause of these neurological disorders. New studies will elucidate the association between these lymphatic structures and the etiology of the various neurological diseases, but this discovery certainly crushed some of the dogmas in Medicine about the immune privilege the brain possesses. After all, this just goes to show the mapping of the human body is not complete as most of us thought. New discoveries await us, we just need to keep looking.

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