Researchers from the University of Copenhagen and the University of Rochester have discovered a previously unknown layer of tissue in the human brain.

The newly identified layer, only a few cells thick, is thought to protect the brain’s gray and white matter. The study outlining the discovery was published on January 5 in the journal Science.

According to the researchers, this membrane appears to be responsible for managing the exchange of tiny, dissolved substances between different brain regions. The tissue may also play a role in aiding the brain’s waste-removal (glymphatic) system.

The newly identified layer has been named the Subarachnoid Lymphatic-like Membrane (SLYM) by the team responsible for the discovery.

At present, most of the findings around the SLYM have been gleaned by studying mice. However, using two-photon microscopy — a 3D live cell imaging technique — the study’s authors verified that the structure similarly exists in humans.

The SLYM sits between two other membranes of the brain. According to the team, the SLYM separates the brain fluid space into two sections. It is suspected that the membrane acts as a barrier to filter molecules found in the brain that are three kilodaltons or bigger.

“The discovery of a new anatomic structure that segregates and helps control the flow of cerebrospinal fluid (CSF) in and around the brain now provides us much greater appreciation of the sophisticated role that CSF plays not only in transporting and removing waste from the brain but also in supporting its immune defenses,” suggested Maiken Nedergaard, a neuroscientist at the University of Rochester.

The researchers identified numerous immune cells in the SLYM that appear to monitor the brain. In mice, these cells react to stressors, like swelling, suggesting that they may play a part in disease pathologies.

The SLYM was also found to share molecular markers with the mesothelial membrane, which can be found lining every organ except the brain. The authors believe the SLYM may act as the brain’s mesothelial membrane, ensuring it moves smoothly in the body, acting as a lubricant.

“Physiological pulsations induced by the cardiovascular system, respiration, and positional changes of the head are constantly shifting the brain within the cranial cavity,” according to the paper. As a result, it is hypothesized that the “SLYM may, like other mesothelial membranes, reduce friction between the brain and skull during such movements.”

It is also thought that ruptures in the SLYM may drive symptoms of traumatic brain injury by permitting immune cells originating in the skull to cross into the brain.

Damage to the SLYM and disrupted cerebrospinal fluid flow patterns may also explain why brain waste removal can be hindered following brain trauma.

This newly discovered membrane may also play a role in the general immunity of the central nervous system, but further research will be needed to investigate the theory.