Scientists say they have found the first evidence that our brains may drain waste, a surprising finding that has eluded discovery for two centuries.

A paper describing the research was published in eLife. The team said they saw lymphatic vessels removing waste and fluid from the brain, a function that researchers were not sure the brain possessed.

This idea was actually first postulated in 1816 by Italian anatomist Paolo Mascagni, who claimed to have seen lymphatic vessels in cadavers he was dissecting. However, nobody had since been able to discover them.

In this latest experiment, the team led by Dr Daniel Reich from the National Institutes of Health used an MRI to scan the brains of five healthy volunteers. These people were injected with a magnetic dye called gadobutrol, which is made of molecules that are too big to enter the brain but small enough to leak out of blood vessels into the dura, the brain’s leathery outer coating.

Using MRI scans, the team saw that the dye had leaked out of the blood vessels, flowed through the dura, and entered into neighboring lymphatic vessels, lighting up in the scans. To confirm their theory, they used a second dye with larger molecules that were unable to leave the blood vessels, and found the lymph vessels did not light up in the same way.

Lymphatic vessels transport immune cells and waste around the body. Blood vessels deliver white blood cells to organs, and the lymphatic system removes these and recirculates them. The discovery therefore suggests the brain has a sort of drainage system to remove waste. Now, the team hope to work out whether the lymphatic system works differently in patients with multiple sclerosis or other neuroinflammatory disorders.

“I was completely surprised. In medical school, we were taught that the brain has no lymphatic system,” Dr Reich said in a statement. 

He added: “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.”

The finding still needs to be verified of course. The study was fairly small, as MRI scans are expensive. But if confirmed to be true, it could fundamentally change what we know about how our brains and immune systems work together.