The human brain is fundamentally so complex that we do not yet understand all the processes taking place within 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 in greater depth, researchers have discovered a previously unknown thin membrane. Described for the first time, this anatomical part of the brain would play the role of a protective barrier, an immune cell platform and a regulator of cerebrospinal fluid (CSF) flow.
Under the cranial box, our brain is enveloped by several layers of membranes containing the cerebrospinal fluid (CSF), and separating its different hemispheres. These membranes are three in number and form what is called the meninges. From the inside out, we distinguish the pia mater, the internal membrane that adheres to the brain and the spinal cord. The arachnoid then adjoins it in the form of an intermediate membrane, and the dura mater is the outermost and strongest.
However, researchers from the University of Medicine in Rochester (New York) and Copenhagen recently noticed a fourth membrane that has never been described so far. Extremely thin — ranging from just one to a few cells thick — the membrane has never been noticed, in part because it disintegrates when the brain is removed during an autopsy. Its low thickness would also have made it invisible to the eyes of brain scanners.
Discovered for the first time in mice, the researchers were able to detect the membrane thanks to a technique of genetic marking by fluorescence. It was then observed in humans, when researchers dissolved the skulls of bodies that had been donated for research. Called the subarachnoid lymphatic-like membrane (SLYM), it sits below the arachnoid membrane, as the name suggests. According to the study, described in the journal Scienceit would divide the space under the arachnoid membrane in two, so that it separates the CSF into two distinct liquid strata.
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The SLYM membrane would be located below the arachnoid membrane, and separates the CSF into two. © University of CopenhagenThe membrane mediates the flow of CSF around the brain
According to the researchers of the new study, the SLYM would be a mesothelium-like membrane – those that line 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 once morest each other. The SLYM would reduce the friction of the brain once morest the skull and would also house immune cells.
Namely that the brain has a population of immune cells that is specific to it. The integrity of the SLYM membrane therefore makes it possible to act as a barrier to immune cells coming from outside. In addition, the membrane would be so tight that it would only let through very small molecules, less than 3 kilodaltons. This function would thus make it possible to maintain the biomolecular and functional integrity of the CSF.
In addition, because it separates the CSF into two distinct layers, the researchers of the study believe that it would act as a barrier between “clean” and “dirty” CSF, and would regulate its flow. ” The discovery of a new anatomical structure that separates and helps control the flow of cerebrospinal fluid (CSF) in and around the brain now allows us to better appreciate the sophisticated role the CSF plays, not only in transport and l elimination of waste from the brain, but also by supporting its immune defenses », noted Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester and the University of Copenhagen, and co-lead author of the new study.
According to the study researchers, the SLYM membrane would mediate the flow of “fresh” CSF around the brain, removing toxic proteins that may be linked to neurological diseases. Supporting this hypothesis, laboratory observations have shown that immune cells congregate in greater numbers and with greater diversity at the membrane level, during brain inflammation or symptoms of aging. And when the membrane ruptures following a traumatic lesion, the flow of CSF is altered and carries immune cells that do not normally belong to the central nervous system.
However, as the discovery is still preliminary, the exact roles of the SLYM membrane might be much more complex. Nevertheless, his discovery is a promising lead and should be taken into account for the development of new therapies once morest neurological pathologies such as Alzheimer’s or multiple sclerosis.