A cross-section of the skull (top) and the outer layer of the brain, showing the subarachnoid lymphatic-like membrane in green
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.
The SLYM hadn’t been noticed before, partly because the membrane disintegrates when the brain is removed from the skull in post-mortems, says Maiken Nedergaard at the University of Rochester Medical Center in New York, who helped discover the structure. It is also too thin to be seen in living people via brain-scanning machines.
The tissue was first discovered in mice, after Nedergaard’s team used a genetic labelling technique that made the SLYM’s cells glow fluorescent green. It was then also observed covering human brains by dissolving away the skull in bodies donated for research.
In 2012, Nedergaard also helped to discover a network of thin tubes that collect waste fluid from brain cells, known as the glymphatic system. These tubes may drain into the outgoing cerebrospinal fluid, says Nedergaard.
The waste products of brain cells include proteins called beta-amyloid and tau that are thought to be involved in Alzheimer’s disease when they build up in excessive amounts.
In both mice and people, the SLYM also contains immune cells, so it may allow them to detect signs of infection present in the cerebrospinal fluid, says Nedergaard. “It is loaded with immune cells.”
“This is a fascinating discovery that will have significant implications for our understanding of the glymphatic system,” says Marios Politis at the University of Exeter, UK.
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