A mesothelium divides the subarachnoid space into functional compartments

The central nervous system is lined by meninges, classically known as dura, arachnoid, and pia mater. We show the existence of a fourth meningeal layer that compartmentalizes the subarachnoid space in the mouse and human brain, designated the subarachnoid lymphatic-like membrane (SLYM). SLYM is morpho-and immunophenotypically similar to the mesothelial membrane lining of peripheral organs and body cavities, and it encases blood vessels and harbors immune cells. Functionally, the close apposition of SLYM with the endothelial lining of the meningeal venous sinus permits direct exchange of small solutes between cerebrospinal fluid and venous blood, thus representing the mouse equivalent of the arachnoid granulations. The functional characterization of SLYM provides fundamental insights into brain immune barriers and fluid transport.

Automated summary

The central nervous system is lined by four meningeal layers, including a newly discovered layer called subarachnoid lymphatic-like membrane (SLYM), which resembles mesothelial membrane found in peripheral organs. SLYM encloses blood vessels and contains immune cells. It allows direct exchange of small solutes between cerebrospinal fluid and venous blood, similar to arachnoid granulations in mice. The functional characterization of SLYM provides important insights into brain immune barriers and fluid transport.