A thin layer of tissue called the subarachnoid lymphoid membrane (SLYM) keeps fresh cerebrospinal fluid separate from fluid containing waste products from brain cells.
health
January 5, 2023
Cross-section of the skull (top) and the outer layer of the brain. The subarachnoid lymphoid membrane is shown in green. University of Copenhagen
A new anatomical structure has been discovered that is part of the brain’s waste disposal system.
This tissue is a thin membrane that envelops the brain and separates the newly-made cerebrospinal fluid that circulates in the brain from the “dirty” fluid that contains cellular waste products.
It was already known that there are three membranes between the skull and the brain. The new structure is her fourth membrane above the innermost membrane called the subarachnoid lymphoid membrane (SLYM). It is very thin, only a few cells wide, sometimes even a cell.
SLYM has gone unnoticed until now, in part because the membrane breaks down when the brain is removed from the skull after death, said Maiken Nedergaard of the University of Rochester Medical Center in New York. It’s also too faint to be seen by a living person via a brain scanning device.
The tissue was first discovered in mice after Nedergaard’s team used a genetic tagging technique that caused SLYM’s cells to glow fluorescent green. Later, it was also observed to cover human brains by melting the skulls of corpses donated for research.
In 2012, Nedergaard also helped discover a network of thin tubes that collect waste fluid from brain cells. This is known as the glymphatic system. These tubes can drain into the outflowing cerebrospinal fluid, says Nedergaard.
The waste products of brain cells contain beta-amyloid and a protein called tau, and excessive accumulation is thought to be involved in Alzheimer’s disease.
In both mice and humans, SLYMs also contain immune cells, Nedergaard said, which could potentially detect signs of infection present in the cerebrospinal fluid. “It’s filled with immune cells.”
Marios Politis from the University of Exeter (UK) said:
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