The Brain's Lymphatic System

theatlantic  |  Reich started his search in 2015, after a major study in Nature reported a similar conduit for lymph in mice. The University of Virginia team wrote at the time, “The discovery of the central-nervous-system lymphatic system may call for a reassessment of basic assumptions in neuroimmunology.” The study was regarded as a potential breakthrough in understanding how neurodegenerative disease is associated with the immune system.

Around the same time, researchers discovered fluid in the brains of mice and humans that would become known as the “glymphatic system.” It was described by a team at the University of Rochester in 2015 as not just the brain’s “waste-clearance system,” but as potentially helping fuel the brain by transporting glucose, lipids, amino acids, and neurotransmitters. Although since “the central nervous system completely lacks conventional lymphatic vessels,” the researchers wrote at the time, it remained unclear how this fluid communicated with the rest of the body.

Reich reasoned that since this fluid exists in human brains, and the conduits exist in mice, the conduits likely exist in humans, too. After two years of work and inordinately complex physics calculations, Reich’s team found the vessels. When Reich started telling colleagues what his team found, he got two reactions: “No way, it’s not true,” and “Yeah, we’ve known that.”

There are occasional references to the idea of a lymphatic system in the brain in historic literature. Two centuries ago, the anatomist Paolo Mascagni made full-body models of the lymphatic system that included the brain, though this was dismissed as an error. A historical account in The Lancet in 2003 read: “Mascagni was probably so impressed with the lymphatic system that he saw lymph vessels even where they did not exist—in the brain.”

No one had published definitive evidence of lymph vessels in any brain until the Virginia mouse study and a concordant Helsinki one in 2015. “You could say that was the discovery,” Reich said. “Did Newton discover gravity? I mean—not to equate these two discoveries—but obviously people knew that things fell before Newton’s apple.”

His team’s discovery, though, not only shows that the vessels exist in people, but just how elaborate the system is.