A new anatomical structure has been discovered in the human brain that could be central to the organ’s fluid balance and immune protection. The fourth layer of the brain’s meninges prevents larger molecules from entering the brain and contains immune cells. Researchers from the University of Rochester Medical Center and the University of Copenhagen stumbled upon this previously unknown structure while studying how cerebrospinal fluid moves through the brain’s membranes. The newly discovered layer, called the Subarachnoidal Lymphatic-like Membrane (SLYM), is only a few cell layers thick and acts as an essential barrier function, preventing the exchange of most peptides and proteins between the outer and inner parts of the subarachnoidal space.
The SLYM membrane also contains a significant amount of immune defense cells, including dendritic cells for enemy recognition and macrophages. This suggests that the SLYM may also serve as a niche for immunological surveillance. Additionally, the SLYM membrane could act as a sliding layer, reducing friction between the skull and the brain during strong head movements. If the protective SLYM membrane of the brain is damaged, it could impair the brain’s waste disposal system and immune defense. Potentially harmful contaminants that occur in old cerebrospinal fluid could then enter the inner subarachnoidal space and even the brain. A tear in the protective layer would also interrupt important flow channels.
The researchers believe that a damaged SLYM membrane could explain the disrupted flow patterns of the glymphatic system after a traumatic brain injury. They also suggest that the increased risk of Alzheimer’s or neuroinflammatory complications after a severe concussion may be related to a tear in the SLYM membrane. In animal experiments, they found that damage to the protective layer led to inflammation and a local accumulation of defense cells. This discovery highlights the important role of cerebrospinal fluid in the brain and the potential consequences of damage to the SLYM membrane.
