A new study has revealed an interesting novel structure in the brain – a fourth meningeal layer. The Subarachnoid Lymphatic-like Membrane (SLYM) is located in the subarachnoid space and divides it into an outer, superficial compartment and an inner, deeper area surrounding the brain.
SLYM is immunophenotypically distinct from the other meningeal layers in the human and mouse brain, and it acts as a barrier to particles with a molecular weight greater than 3kDa. SLYM encases blood vessels and harbours immune cells (mainly a large population of myeloid cells, the number of which increases in response to inflammation and ageing). The membrane is closely apposed to meningeal venous sinuses and permits direct exchange of small solutes between cerebrospinal fluid and venous blood.
It is similar in structure to the mesothelium in other parts of the body and could therefore play an important role against movement-related friction, in immune protection and fluid balance in the brain.
SLYM impermeability to any molecule ≥3kDa has important consequences since the membrane can avoid the leakage of brain waste as Amyloid-β (4kDa), and suggests that CSF transport is likely more organised and specialised than our current knowledge admits.
The discovery of the SLYM represents an important addition to existing anatomical knowledge and may contribute to new insights into brain function and potential therapeutic approaches. However, further studies are needed to better understand the exact function and potential clinical relevance of the SLYM.
