Scientists discover new anatomic structure in the brain that monitors and shields cells

"The discovery of a new anatomic structure that segregates and helps control the flow of cerebrospinal fluid in and around the brain now provides us a 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," Nedergaard said in a statement.

Newly discovered part of the brain's waste disposal system

Though the team largely explains the function of SLYM in mice, they do report its presence in the adult human brain as well.

The SLYM is made of the mesothelium, which lines organs such as the heart and lungs. Møllgård, the first author of the study, suspected that a similar membrane might exist in the central nervous system, considering mesothelium protects organs and harbors immune cells. The membrane is too thin to be seen in people through brain-scanning machines.

The SLYM is a thin and delicate membrane but acts like a tight barrier, permitting only very few molecules to transit, making it seem like separating "clean" and "dirty" cerebrospinal fluid (CSF). 

This particular function also points out the role played in the glymphatic system, "which requires a controlled flow and exchange of CSF, allowing the influx of fresh CSF while flushing the toxic proteins associated with Alzheimer's and other neurological diseases from the central nervous system". The discovery is imperative in understanding the workings of the glymphatic system.

Identifying the SLYM is crucial for further study of its role in diseases as diverse as multiple sclerosis, central nervous system infections, and Alzheimer's that might be triggered or worsened by abnormalities in SLYM function. The researchers also suggest that the delivery of drugs and gene therapeutics to the brain may be hindered by SLYM function, which needs to be on the radar as new generations of biological therapies are being developed. 

Study Abstract:

The central nervous system is lined by meninges, classically known as dura, arachnoid, and pia mater. We show the existence of a fourth meningeal layer that compartmentalizes the subarachnoid space in the mouse and human brain, designated the subarachnoid lymphatic-like membrane (SLYM). SLYM is morpho- and immunophenotypically similar to the mesothelial membrane lining of peripheral organs and body cavities, and it encases blood vessels and harbors immune cells. Functionally, the close apposition of SLYM with the endothelial lining of the meningeal venous sinus permits the direct exchange of small solutes between cerebrospinal fluid and venous blood, thus representing the mouse equivalent of the arachnoid granulations. The functional characterization of SLYM provides fundamental insights into brain immune barriers and fluid transport.