Scientists have just discovered a whole new layer of brain anatomy: ScienceAlert

The human brain is a ridiculously complex organ that doesn’t give away its secrets easily. Thanks to advances in imaging technology, the hidden forms and functions of neurological anatomy continue to emerge, from new types of nerve cells to brand new pieces of tissue.

Now researchers from the University of Copenhagen and the University
of Rochester have identified a layer of tissue that helps protect our gray and white matter, a layer 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 source of brain-specific immune cells, not to mention help remove waste from the brain (glymphatic) the system.

Molecular biologist Kjeld Møllgård and his colleagues from the University of Copenhagen 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, confirmed the presence of SLYM in the brain of an adult.

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

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

Therefore, the team suspects that the cerebrospinal fluid may take over part of the role of the immune system in the brain. The presence of SLYM could explain how this works.

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

Møllgård and team found several types of immune cells – including myeloid cells and macrophages – camping out in SLYM, keeping watch over the brain. In mice, cell types changed in response to swelling and natural aging, suggesting that this site may play an important role in disease pathology.

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

The mesothelium also plays the role of a lubricant between organs that slide against each other.

“Physiological pulsations caused by the cardiovascular system, breathing and changes in head position constantly move the brain inside the cranial cavity,” researchers explain in your work. “SLYM can, 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 to gain direct access to the brain, cells that are not calibrated for brain conditions. This could explain the constant inflammation.

The flow of waste products from the brain may also be suppressed long after brain injury, and altered cerebrospinal fluid flow patterns due to membrane rupture may explain this.

Since this additional layer of brain armor has only just been discovered, there is still much to work out. The researchers wonder if 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 the mesothelium by acting as an immune barrier that prevents the exchange of small solutes between the compartments of the external and internal subarachnoid spaces and by covering blood vessels in the subarachnoid space,” piss Møllgård and colleagues.

This research was published in Science.