Newfound 'protective shield' in the brain is like a watchtower for immune cells

A newfound "protective shield" in the brain helps clear waste from the organ and serves as a sentry tower for watchful immune cells that monitor for signs of infection, scientists reported in a study of mouse and human brains.

The study, published Thursday (Jan. 5) in the journal Science, describes a thin sheet of tissue that measures only a few cells thick and splits an overarching compartment in the brain called the subarachnoid space into two halves horizontally. Several distinct layers of tissue sit between the inner surface of the skull and the outer surface of the brain, and the subarachnoid space lies between two of those tissue layers. The space itself isn't empty; it contains a spiderweb-like network of connective tissue that stretches between the neighboring tissue layers, major blood vessels, and a colorless fluid called cerebrospinal fluid (CSF), according to the online medical resource StatPearls.

The CSF surrounding the brain acts as a shock absorber, similar to the cushioning inside a bike helmet. However, this fluid doesn't hang out only in the subarachnoid space. Instead, it flows through various tubes and compartments in and around the brain, delivering nutrients to the organ while flushing its waste products out into the bloodstream. The newly discovered "shield" likely helps control these important functions of CSF, the study authors concluded.

"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, co-director of the Center for Translational Neuromedicine at University of Rochester and the University of Copenhagen, said in a statement.

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The shield, which the authors call the subarachnoid lymphatic-like membrane (SLYM), divides the subarachnoid space into an upper compartment, closer to the skull, and a lower compartment, closer to the brain. Experiments in mice suggested that the thin membrane blocks most proteins from crossing from one compartment into the other, although it allows very small molecules to pass through. (The team also found evidence of the SLYM in tissue samples from adult human brains.)

The newfound membrane may help separate fresh CSF from contaminated CSF containing waste and potentially harmful proteins, such as the 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 incurs damage "will require more detailed studies," they noted. 

The study also revealed that a large number and variety of immune cells can be embedded in the shield, and showed that these immune cells increase in number in response to inflammation and advanced aging in mice. This finding hints that the SLYM serves as a site of "immunological surveillance," from which immune cells monitor the CSF for signs of infection and inflammation and can summon additional defenses as needed, the authors concluded. 

However, if the SLYM ruptures, immune cells from the skull's bone marrow can then flood the surface of the brain, an area they normally wouldn't reach. This finding could help explain why traumatic brain injuries often trigger prolonged inflammation of the brain and disrupt the normal flow of CSF through and around the organ, the authors suggested, although these hypotheses will have to be tested. 

Traumatic brain injuries are also linked to an increased risk of developing Alzheimer's down the line, the authors added, and this increased risk may be partially explained by the trauma introducing new cracks in the brain's protective shield — the SLYM, the authors theorize.