Science

Opening of the thinnest membrane surrounding the brain. It will play a key role in the immune system.

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The human brain is fundamentally so complex that we do not yet understand all the processes that take place in it. All the layers and compartments that make up its structure are also partly unknown. However, the latter play a fundamental role in the regulation of neurobiological processes. By studying the structure of the brain more deeply, the researchers found a previously unknown thin membrane. Described for the first time, this anatomical part of the brain will play the role of a protective barrier, a platform for immune cells, and a regulator of cerebrospinal fluid (CSF) flow.

Beneath the skull, our brain is enveloped in several layers of membranes containing cerebrospinal fluid (CSF) and separating its different hemispheres. There are three of these membranes, and they form the so-called meninges. From the inside out, we distinguish the pia mater, the inner membrane that is adjacent to the brain and spinal cord. Then the arachnoid membrane adjoins it in the form of an intermediate membrane, and the dura mater is the outermost and most durable.

However, researchers from the Medical University of Rochester (New York) and Copenhagen recently noticed a fourth membrane that has not yet been described. The extremely thin – one to a few cells – membrane has never been seen, in part because it breaks apart when the brain is removed during an autopsy. Its thinness would also make it invisible to brain scanners.

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Discovered for the first time in mice, researchers were able to detect the membrane through a genetic marking technique using fluorescence. This was then observed in humans when researchers dissolved the skulls of bodies that had been donated for research. Called the subarachnoid lymphatic membrane (SLYM), it sits under the arachnoid membrane, as the name suggests. According to the study, described in the journal Science, the space under the arachnoid membrane will be divided into two parts, so that the cerebrospinal fluid separates into two separate layers of fluid.

The SLYM membrane will sit below the arachnoid and separate the cerebrospinal fluid into two parts. © University of Copenhagen

The membrane mediates the flow of cerebrospinal fluid around the brain.

According to the researchers of the new study, SLYM will be a mesothelial-like membrane that lines other organs such as the heart and lungs. These membranes surround the organs in such a way as to lubricate them so that they can slide over each other. SLYM will reduce the friction of the brain against the skull and will also contain immune cells.

Namely, that the brain has a specific population of immune cells. Thus, the integrity of the SLYM membrane allows it to act as a barrier to incoming immune cells. In addition, the membrane would be so dense that only very small molecules, less than 3 kilodaltons, would pass through. Thus, this function allows maintaining the biomolecular and functional integrity of CSF.

Also, because it separates the cerebrospinal fluid into two separate layers, the researchers believe it will act as a barrier between “clean” and “dirty” cerebrospinal fluid and will regulate its flow. “The discovery of a new anatomical structure that separates and helps control the flow of cerebrospinal fluid (CSF) into and around the brain now allows us to better appreciate the complex role that CSF plays in not only transporting and removing waste from the brain, but also supporting its immune system. defense,” says Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester and the University of Copenhagen and co-author of the new study.

According to the researchers, the SLYM membrane will promote the flow of “fresh” cerebrospinal fluid around the brain, removing toxic proteins that may be associated with neurological disease. In support of this hypothesis, laboratory observations have shown that immune cells assemble in greater numbers and with greater diversity at the membrane level during brain inflammation or symptoms of aging. And when the membrane ruptures after a traumatic injury, the flow of cerebrospinal fluid changes and carries immune cells that do not normally belong to the central nervous system.

However, since the discovery is still preliminary, the precise roles of the SLYM membrane may be much more complex. However, his discovery is promising and should be taken into account when developing new treatments for neurological pathologies such as Alzheimer’s disease or multiple sclerosis.

Science.

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