The human brain is an incredibly complex organ, and its secrets are not easily revealed.
Advances in imaging technology are continually revealing hidden forms and functions of neuroanatomy, including new types of nerve cells and entirely new tissue nodules.
Researchers at the Universities of Copenhagen and Rochester have now identified layers of neurons in the brain.
The University of Rochester has discovered a previously undistinguished layer of tissue that protects gray and white matter.
Only a few cells thick, this membrane appears to mediate the exchange of small dissolved substances between brain compartments.
It also appears to underpin specific immune cells in the brain, as well as help the brain’s waste disposal system (glymphatic).
Molecular biologist Kjeld Møllgård and colleagues at the University of Copenhagen have named this discovery the ‘subarachnoid lymphatic membrane (SLYM)’.
Much of the research on this structure has so far been done in mice using two-photon microscopy and dissection, but the presence of SLYM in the adult human brain has now been confirmed.
SLYM is located between two other membranes that protect the brain. It is known that there are a total of four membranes that cover the brain, dividing the cerebrospinal fluid space into two.
It appears to function as a barrier to cerebrospinal fluid molecules larger than 3 kilodaltons, comparable to very small proteins.
Unlike other parts of the body, the central nervous system does not have lymphatic (immune) vessels, and is regarded as an “immunostimulatory site,” which refers to places such as the eyes and testes where immune reactions are strictly controlled.
So the researchers suspect that cerebrospinal fluid picks up some of the work of the immune system in the brain. The existence of SLYM may explain this mechanism.
“The discovery of a new anatomical structure that secretes and regulates the flow of cerebrospinal fluid (CSF) allows CSF to transport and remove waste products from the brain,” said Mayken Nedergaard, a neuroscientist at the University of Rochester.
Not only that, but it is becoming more clear that it plays a sophisticated role in supporting the brain’s immune defenses.”
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