In 1816, an Italian anatomist reported finding lymphatic vessels on the surface of the brain—but the information went nowhere. However, 200 years later, NIH researchers believe they have confirmed that report with evidence that the human brain may drain some waste out through the body’s lymphatic “sewer system.”
Two animal studies in 2015 had showed evidence of a lymphatic system in the brain. Building on that, the researchers used magnetic resonance imaging to scan the brains of 5 healthy volunteers who had been injected with gadobutrol, typically used to visualize brain blood vessels. 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.
At first, the researchers say, the dura lit up brightly, but no lymphatic vessels were visible. When they turned the scanner differently, the blood vessels “disappeared,” and they saw that the dura also had smaller but almost equally bright spots and lines—possibly lymph vessels. The researchers’ results suggested that the dye leaked from the blood vessels and flowed through the dura into neighboring lymphatic vessels. “We literally watched people’s brains drain fluid into these vessels,” said Daniel Reich, MD, PhD, senior author of the study.
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The researchers tested the findings by doing another round of scans, using a dye made of larger molecules. This time they saw blood vessels but no lymph vessels, no matter how the scanner was turned.
The researchers also found evidence for blood and lymph vessels in autopsied human brain tissue. “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.
“These results could fundamentally change the way we think about how the brain and immune system interrelate,” said Walter Koroshetz, MD, the director of the National Institute of Neurological Disorders and Stroke.