Brain Waste Disposal System Discovered

It’s not often that a new part of the brain is found.
This one keeps our cerebrospinal fluid clean and disease-free.

We’ve just discovered a new part of the brain’s waste disposal system (New Scientist, 5 Jan 2023). Reporter Clare Wilson uses the “royal we” in this Tontological headline, but she didn’t discover this: scientists at the University of Rochester and University of Copenhagen did.

A new anatomical structure has been discovered that is part of the brain’s waste disposal system.

The tissue is a thin membrane encasing the brain that keeps newly made cerebrospinal fluid – which circulates inside the brain – separate from “dirty” fluid containing cells’ waste products.

It was already known that there are three membranes between the skull and the brain. The new structure is a fourth membrane, lying on top of the innermost membrane, called the subarachnoid lymphatic-like membrane (SLYM). It is extremely thin, with a width of just a few cells or, in places, even one cell.

It had not been discovered previously, because it disintegrates when the skull is opened up, and is not visible in imaging techniques. The scientists found it in studies on mice, and figured out what it was doing.

Newly Discovered Anatomy Shields and Monitors Brain (University of Rochester Medical Center, 5 Jan 2023). This press release from one of the institutions discovering the SLYM tells more about it. The membrane is certainly “slim” as the name implies, but it serves important functions:

The new membrane is very thin and delicate, consisting of only a few cells in thickness. Yet SLYM is a tight barrier, allowing only very small molecules to transit and it also seems to separate “clean” and “dirty” CSF. This last observation hints at the likely role played by SLYM in the glymphatic system, which requires a controlled flow and exchange of CSF, allowing the influx of fresh CSF while flushing the toxic proteins associated with Alzheimer’s and other neurological diseases from the central nervous system.

The University of Copenhagen supplied the following graphic to illustrate its position under the skull:

A new study reveals a newfound anatomical structure in the brain that acts as a barrier and a platform from which immune cells can monitor the brain. (Arrow added) (Image credit: University of Copenhagen)

Newfound ‘protective shield’ in the brain is like a watchtower for immune cells (Live Science, 5 Jan 2023). Reporter Nicoletta Lanese says that the cerebrospinal fluid (CSF) is the skull’s shock absorber. But it needs to be replenished, and waste products in the fluid need to be kept separate from new CSF. The scientists also found that the SLYM is loaded with a variety of immune cells, so that it can serve as “a platform from which immune cells can monitor the brain.”

“The discovery of a new anatomic structure that segregates and helps control the flow of cerebrospinal fluid 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,” senior author Dr. Maiken Nedergaard (opens in new tab), co-director of the Center for Translational Neuromedicine at University of Rochester and the University of Copenhagen, said in a statement.

The paper about this was published in Science on 5 Jan 2023. The Abstract says:

The central nervous system is lined by meninges, classically known as dura, arachnoid, and pia mater. We show the existence of a fourth meningeal layer that compartmentalizes the subarachnoid space in the mouse and human brain, designated the subarachnoid lymphatic-like membrane (SLYM). SLYM is morpho- and immunophenotypically similar to the mesothelial membrane lining of peripheral organs and body cavities, and it encases blood vessels and harbors immune cells. Functionally, the close apposition of SLYM with the endothelial lining of the meningeal venous sinus permits direct exchange of small solutes between cerebrospinal fluid and venous blood, thus representing the mouse equivalent of the arachnoid granulations. The functional characterization of SLYM provides fundamental insights into brain immune barriers and fluid transport.

There is no mention in the paper of how this system might have evolved.

The Creator thought of everything. Let’s give credit where credit is due.