Until now, the human brain and other parts of the central nervous system were believed to have three layers – dura, arachnoid, and pia mater – that are collectively called the meninges. A new study has shown the existence of a fourth meningeal layer, called the subarachnoid lymphatic-like membrane (SLYM) in mouse and human brains.

The SLYM divides the subarachnoid space – which exists between arachnoid and pia mater, and consists of the cerebrospinal fluid (CSF), blood vessels, and cisterns – into an outer superficial compartment and an inner deep compartment lining the brain.

SLYM also showed the presence of podoplanin (PDPN). The PDPN protein is found in mesothelium, membranes that line body cavities and internal organs. Mesothelium is present where tissues slide against each other and is believed to act as a boundary lubricant to ease movement, and SLYM too may reduce friction between the brain and skull during movements like positional changes of the head as well as physiological functions, the study noted.

The study was conducted by researchers at the University of Rochester and the University of Copenhagen.

“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 defences,” said Maiken Nedergaard, co-director of the Centre for Translational Neuromedicine at the University of Rochester.

SLYM only allows the exchange of a few small molecules through it. The newly-discovered membrane acts as a barrier that limits the exchange of most peptides and proteins between the upper and lower subarachnoid space compartments. It also seems to separate “clean” and “dirty” CSF, possibly playing a role in the glymphatic system – a waste-clearing system that promotes efficient elimination of soluble proteins and metabolites from the central nervous system.