Scientists Discover An Added Layer Of Complexity To Brain Anatomy, Literally

Scientists have discovered a new layer of brain anatomy that both protects and monitors the brain.

The meningeal layer that surrounds the brain was previously thought to consist of three individual layers: dura, arachnoid, and pia matter. However, researchers from the University of Copenhagen and the University of Rochester utilized advanced imaging technology to discover an additional layer they call the Subarachnoid LYmphatic-like Membrane (SLYM), ScienceAlert reported.

The Brain’s Membrane with SLYM in green (University of Copenhagen)

Most of the research on SLYM has been conducted on mice, but the researchers confirmed the layer’s presence in the human brain, too. The SLYM is just a few cells thick and plays a role in the back-and-forth flow of cerebrospinal fluid (CSF), acting almost like a filter for the good and bad CSF.

“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,” said Maiken Nedergaard, a neuroscientist at the University of Rochester.

Immune cells were discovered in SLYM, indicating that it is a type of mesothelium, which is a membrane that surrounds other human organs such as the heart and lungs. What makes SLYM unique is that it is incredibly thin and delicate, yet it is a tight barrier, EurekAlert! noted. The immune cells in SLYM keep the central nervous system’s own immune cells out of the brain while simultaneously scanning the brain’s CSF for any signs of infection or other abnormalities, and subsequently flushing them out.

The study, published in Science, indicates that SLYM responds to aging and inflammation. When damage is detected, “…the number of [immune cells] increases in response to inflammation and aging,” researchers wrote.

More specifically, “SLYM fulfills the characteristics of a mesothelium by acting as an immune barrier that prevents exchange of small solutes between the outer and inner subarachnoid space compartments and by covering blood vessels in the subarachnoid space,” wrote professor of neuroanatomy at the University of Copenhagen Kjeld Møllgård, M.D., and colleagues.

Additionally, the brain may receive a bit of protection from the skull thanks to SLYM. Various cardiovascular, respiratory, and head movements subject the brain to potential impacts and friction with the skull, but “SLYM may … reduce friction between the brain and skull” during such jarring physiological activity.

The discovery of SLYM opens up a wide breadth of opportunities to investigate the relationship between the brain, central nervous system, and various diseases. Given SLYM’s response to aging and inflammation, diseases like Alzheimer’s, multiple sclerosis, and central nervous system infections are of particular interest to how the newly discovered membrane may play a role in them. These observations also raise points of reconsideration for how drugs and gene therapeutics are delivered to the brain.

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