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Scientists have just uncovered a whole new layer of the brain’s anatomy: ScienceAlert

Scientists have just uncovered a whole new layer of the brain’s anatomy: ScienceAlert

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The human brain is an absurdly complex organ that does not reveal its secrets easily. Thanks to advances in imaging technology, hidden forms and functions of neurological anatomy continue to emerge, of new types of nerve cells to brand new pieces of fabric.

Now researchers from the University of Copenhagen and the Univ
of Rochester have identified a layer of tissue that helps protect our gray and white matter, one that was not previously distinguished.

Only a few cells thick, this membrane appears to play a role in mediating the exchange of small, solutes between compartments in the brain. It also appears to be the main base of brain-specific immune cells, not to mention helping to remove waste from the brain (glymphatic) system.

University of Copenhagen molecular biologist Kjeld Møllgård and colleagues have named their discovery the Subarachnoid Lymphatic Membrane (SLYM). Although much of their research on this structure is so far from mice, using two-photon microscopy and dissections, they have confirmed the presence of SLYM also in the brain of an adult.

SLYM is located between two other membranes protecting the brain. It divides the cerebrospinal fluid space into two, bringing the total number of known membranes surrounding our brain to four. It appears to act as a barrier to molecules in our cerebrospinal fluid that are larger than about 3 kilodalton; comparable to an extremely small protein.

Anatomy of the brain membrane including SLYM in green. (University of Copenhagen)

Unlike the rest of our body, our central nervous system has no lymphatic (immune) vessels and is considered immune privileged – a term that refers to places in our bodies where immune responses are highly controlled, such as our eyes and the testicles.

So the team suspected that the cerebrospinal fluid could take over some of the role of the immune system in the brain. The presence of SLYM may explain how this works.

“The discovery of a new anatomical structure that compartmentalizes and helps control the flow of cerebrospinal fluid (CSF) in and around the brain now gives us a much greater appreciation of the complex role that CSF plays in not only transporting and removing waste from the brain, but also to support its immune defenses,” says University of Rochester neuroscientist Maiken Nedergaard.

Møllgård and team found several types of immune cells – including myeloid cells and macrophages – camped out in SLYM, maintaining surveillance over the brain. In mice, cell types change in response to inflammation and natural aging, suggesting that this site may play an important role in disease pathologies.

SLYM shares molecular markers with mesothelial membrane which covers the rest of our organs, enveloping their blood vessels and storing immune cells. So the researchers hypothesize that SLYM is the brain mesothelium lining the blood vessels in the cavity between the brain and the skull.

The mesothelium also acts as a lubricant between the organs that slide against each other.

“Physiological pulsations induced by the cardiovascular system, respiration, and changes in head position constantly displace the brain within the cranial cavity,” the researchers explain in their paper. “SLYM may, like other mesothelial membranes, reduce friction between the brain and skull during such movements.”

Tears in SLYM may explain some of the long-term symptoms of traumatic brain injury, Møllgård and team speculate. Breaking this barrier would allow immune cells from the skull direct access to the brain, cells that are not calibrated for brain conditions. This may explain the ongoing inflammation.

Brain waste flow may also continue to be suppressed for a long period of time after brain injury, and altered CSF flow patterns due to membrane rupture may explain this.

Since this extra layer of brain armor has just been discovered, there is still work to be done. Researchers question whether this tissue may also be involved in more general central nervous system immunity and therefore play a role in related diseases such as multiple sclerosis.

“We conclude that SLYM fulfills the characteristics of mesothelium by acting as an immune barrier that prevents the exchange of small solutes between the outer and inner compartments of the subarachnoid space and by covering blood vessels in the subarachnoid space,” write Møllgård and colleagues.

This research was published in Science.


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