New insights into the structure and function of the brain are consistently revealed by advances in imaging technologies. Both whole new bits of tissue and rare varieties of nerve cells have been uncovered.
Researchers at the Universities of Copenhagen and Rochester have found a new layer of tissue that helps protect our gray and white matter. This layer of tissue is one that hasn’t been distinguished earlier.
This membrane is only a few cells thick, but it appears to have a role in aiding the passage of dissolved chemicals and small molecules from one part of the brain to another. In addition to aiding in waste removal through the glymphatic system, it also seems to serve as the brain’s immune cells’ home base.
Kjeld Mllgrd, a molecular biologist at the University of Copenhagen, and his colleagues have coined the term “Subarachnoid LYmphatic-like Membrane” to describe a new structure they have discovered in the brain (SLYM). Most of the research on this structure has been done in mice, but the team has used two-photon imaging and dissection to show that the SLYM is also present in the brains of adult humans.
The SLYM is sandwiched by two more membranes that are important for the brain’s protection. It achieves this by splitting the fluid space in our brain in half, bringing the total number of membranes that are known to surround our brain up to four. It appears to operate as a barrier for molecules in our brain fluid that are larger than roughly 3 kilodaltons; this is akin to a very small protein.
Since it lacks lymphatic (immune) pathways, our central nervous system enjoys a special status in terms of our immunological defenses. This is a term that refers to places in our body where immune responses are highly controlled, such as our eyes and testes.
Thus, the team hypothesizes that CSF has a role in the brain analogous to that of the immune system. The SLYM’s presence may provide insight on the workings of this mechanism.
In the SLYM, Mllgard and his team found a variety of immune cells, including myeloid cells and macrophages. It was shown that these cells provide a vigilance function over the brain. Changes in cell types due to inflammation and aging in mice indicate a potential involvement for this area in the pathophysiology of disease.
Similar molecular markers are present in the SLYM and the mesothelial membrane that lines the remainder of our organs, protecting their blood vessels and providing a home for immune cells. Between these two membranes is where the SLYM resides. Since the brain’s mesothelium lines the blood arteries in the space between the brain and the skull, this is what the researchers have concluded to be the SLYM.
Mesothelium also acts as a lubricant between neighboring organs, allowing them to glide over one another with ease.
Mllgard and his group suggest that severe brain injury may have long-term consequences if the SLYM is torn. A breach in this barrier would allow immune cells circulating in the skull free access to the brain. The immune cells in question have not evolved to deal with the unique conditions of the brain. This might be the reason why the irritation persists.
Long after a brain injury has occurred, it may still be difficult for waste products to leave the brain. This might be because the cerebrospinal fluid flows in different directions once the membrane bursts.
There is still a lot to figure out with the recent discovery of this extra layer of brain protection. It is of interest to the researchers to learn whether this tissue is involved in a broader kind of central nervous system inflammation and hence contributes to associated illnesses like multiple sclerosis.
