A new “protective shield” in the brain helps remove waste from the organ and serves as a watchtower for vigilant immune cells that watch for signs of infection, scientists reported in a study of mouse and human brains.
The study, published Thursday, January 5 in the journal Science (opens in a new tab)describes a thin sheet of fabric that measures only a few cells thick and divides an overall compartment in the brain called the subarachnoid space into two halves horizontally. Several distinct layers of tissue lie between the inner surface of the skull and the outer surface of the brain, and the subarachnoid space lies between two of these layers of tissue. Space itself is not empty; it contains a cobweb-like network of connective tissue that extends between neighboring layers of tissue, major you sang vessels and a colorless liquid called cerebrospinal fluid (CSF), according to the online medical resource StatPearls (opens in a new tab).
The CSF surrounding the brain acts as a shock absorber, similar to the padding inside a bicycle helmet. However, this fluid does not just hang around in the subarachnoid space. Instead, it circulates through various tubes and compartments in and around the brain, delivering nutrients to the organ while flushing out its waste products into the bloodstream. The newly discovered “shield” likely helps control these important CSF functions, the study authors concluded.
“The discovery of a new anatomical structure that separates and helps control the flow of cerebrospinal fluid in and around the brain now allows us to appreciate much better the sophisticated role that CSF plays not only in transport and removal of waste from the brain, but also in supporting its immune defenses,” lead author Dr AS Maiken Nedergaard (opens in a new tab)co-director of the Center for Translational Neuromedicine at the University of Rochester and the University of Copenhagen, said in a statement (opens in a new tab).
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The shield, which the authors call the lymphatic-like subarachnoid membrane (SLYM), divides the subarachnoid space into an upper compartment, closer to the skull, and a lower compartment, closer to the brain. Experiences in mouse suggested that the thin membrane blocks most protein to pass from one compartment to another, although it allows very small molecules to pass. (The team also found evidence of SLYM in tissue samples from adult human brains.)
The new membrane can help separate fresh CSF from contaminated CSF containing waste and potentially harmful proteins, such as amyloid plaques associated with Alzheimer’s disease, and help direct these substances out of the brain, the authors theorized. Understanding how this works in a healthy brain and what happens if the shield suffers damage “will require more detailed studies,” they noted.
The study also revealed that a large number and variety of immune cells can be integrated into the shield and have been shown that these immune cells increase in number in response to inflammation and advanced aging in mice. This finding suggests that the SLYM serves as an “immunological watchdog” site, from which immune cells monitor the CSF for signs of infection and inflammation and can invoke additional defenses if needed, the authors concluded. .
However, if the SLYM ruptures, immune cells from the bone marrow of the skull can then flood the surface of the brain, an area they would not normally reach. This finding could help explain why traumatic brain injury often triggers prolonged inflammation in the brain and disrupts the normal flow of CSF through and around the organ, the authors suggested, although these hypotheses will need to be tested.
Traumatic brain injury is also linked to an increased risk of developing Alzheimer’s disease down the line, the authors added, and this increased risk may be partly explained by the trauma introducing new cracks in the body’s protective shield. brain – the SLYM, the authors theorize.