Scientists have discovered lymphatic vessels present in the leathery outer layer of the brain, which is known as dura. This finding was made by the researchers from the National Institutes of Health (NIH).
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This is the first evidence which leads to the speculation that our brains may drain some waste out through lymphatic vessels, which act as the body's sewer system.
This discovery would aid in better understanding of how neurodegenerative ailment is linked with the immune system. The results state that these vessels might act as a pipeline between the brain and the immune system.
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"We literally watched people's brains drain fluid into these vessels," said Daniel S. Reich, M.D., Ph.D., senior investigator at the NIH's National Institute of Neurological Disorders and Stroke (NINDS) and the senior author of the study published online in eLife.
"We hope that our results provide new insights to a variety of neurological disorders," Dr Reich added.
Reich is a radiologist and neurologist, who carried out this finding along with two post-doctoral fellows -- Martina Absinta, PhD and Seung-Kwon Ha, PhD.
Our body's circulatory system consists of lymphatic vessels, which are a kind of drain pipes present alongside the blood vessels. These vessels carry colourless fluid called lymph which comprises waste and immune cells to the lymph nodes.
The lymphatic system plays the role of removing the white blood cells (WBC) transported to an organ by the blood vessels and it recirculates them through the body. This process helps the immune system in finding out if any organ in the body is being attacked by microbes or is wounded.
The lymphatic vessels were first discovered on the brain's surface by an Italian anatomist in the year 1816, but it was forgotten about for two centuries. Scientists from the modern era didn't find any evidence about the presence of the lymphatic system in the brain and they wondered how the brain ducts out the waste, some researchers ended up tagging the brain as an exceptional organ.
The proof about the presence of the lymphatic system was discovered in the dura of the brain of mice after two studies were carried out in 2015.
That very year a presentation was given by Jonathan Kipnis, PhD, who is a professor at the University of Virginia and also an author of one of the studies conducted on the brain of mice. This presentation was attended by Dr Reich.
"I was completely surprised. In medical school, we were taught that the brain has no lymphatic system," Dr Reich stated, as quoted by a press release.
"After Dr. Kipnis' talk, I thought, maybe we could find it in human brains?"
To look for the vessels, Dr. Reich's team used MRI to scan the brains of five healthy volunteers who had been injected with gadobutrol, a magnetic dye typically used to visualise brain blood vessels damaged by diseases, such as multiple sclerosis or cancer. The dye molecules are small enough to leak out of blood vessels in the dura but too big to pass through the blood-brain barrier and enter other parts of the brain.
At first, when the researchers set the MRI to see blood vessels, the dura lit up brightly, and they could not see any signs of the lymphatic system. But, when they tuned the scanner differently, the blood vessels disappeared, and the researchers saw that dura also contained smaller but almost equally bright spots and lines which they suspected were lymph vessels. The results suggested that the dye leaked out of the blood vessels, flowed through the dura and into neighbouring lymphatic vessels.
To test this idea, the researchers performed another round of scans on two subjects after first injecting them with a second dye made up of larger molecules that leak much less out of blood vessels. In contrast with the first round of scans, the researchers saw blood vessels in the dura but no lymph vessels regardless of how they tuned the scanner, confirming their suspicions.
They also found proof for blood and lymph vessels in the dura of autopsied human brain tissue. Moreover, their brain scans and autopsy studies of brains from non-human primates confirmed the results seen in humans, suggesting the lymphatic system is a common feature of mammalian brains.
"These results could fundamentally change the way we think about how the brain and immune system inter-relate," said Walter J. Koroshetz, M.D., NINDS director.
Dr. Reich's team plans to investigate whether the lymphatic system works differently in patients who have multiple sclerosis or other neuroinflammatory disorders.
"For years we knew how fluid entered the brain. Now we may finally see that, like other organs in the body, brain fluid can drain out through the lymphatic system," said Dr. Reich.
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Inputs taken from NINDS press release
