The subarachnoid lymphatic-like membrane (SLYM) is a possible fourth
meningeal layer that was proposed in 2023 in the
brain
The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
of
human
Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
s and
mice
A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
.
Its existence is still very controversial.
The SLYM would be located in the
subarachnoid space, the space between the middle reticular meninges and the innermost tender meninges that lie close to the brain.
It divides the subarachnoid space into an outer, superficial compartment and an inner, deeper area surrounding the brain.
Structure
The SLYM is reported to be a thin monolayer of cells and contains its own immune cells.
The SLYM may inhibit larger molecules, such as
peptide
Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...
s and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s, from passing into the interior of the brain and could thus assume a barrier function.
Discovery and interpretation
SLYM was first reported as a possible novel anatomical structure in the human brain.
SLYM is impermeable to any molecule larger than 3,000
daltons. It may have an immunological role, in which immune cell numbers change with aging or inflammation.
In February 2023, research groups from Germany, Finland, Switzerland, South Korea and USA submitted comments to the eLetters section of the paper challenging the paper's conclusions for its methodology and conclusions as misinterpreted or by stating the structure was already known.
The SLYM investigators from Denmark, Germany, Brazil, Poland, Japan, Spain and US argued that the textbook description of the meningeal layers is based on histology only. The subarachnoid space as well as the meningeal membranes are exposed to significant deformation during preparations of histological sections. This was further probed by in vivo MRI and fresh dissection. Only in vivo studies of brain fluid transport can reveal the dynamics of cerebrospinal fluid transport and its barriers. Pietilä et al., 2023 also described the existence of a Prox1+ cell layer located within the arachnoid beneath and separate from E-cadherin+ arachnoid barrier cells.
References
{{Reflist
Brain
Meninges