Structure of the subarachnoid lymphatic-like membrane as a meningeal selective barrier and immunological regulator

The subarachnoid lymphatic-like membrane (SLYM), identified through neuroimaging and molecular biology, is an extremely thin fourth subarachnoid meningeal layer that envelops blood vessels within the subarachnoid space. It performs critical functions including immune cell regulation, cerebrospinal fluid (CSF) flow modulation, brain protection, and lubrication. All components of the central nervous system (CNS) drainage system – interstitial fluid, perivascular spaces, CSF, meningeal lymphatic vessels, and cervical lymph nodes – operate in close coordination, providing not only waste and toxin removal from the brain, but also maintaining homeostasis essential for normal neuronal function. SLYM, as part of this complex system, plays a role in transporting and filtering cells and molecules, contributing to the regulation of the CNS immune response. It forms a sheath around the brain through which only very small molecules can pass, thereby separating "clean" CSF from "dirty" CSF. SLYM may be involved in processes associated with neuroinflammation and neurodegenerative diseases, such as Alzheimer’s disease and multiple sclerosis, potentially opening new avenues for treatment and understanding of the mechanisms underlying immune responses in the CNS. The revised understanding of the critical role of the meninges in brain function regulation, their interaction with other drainage system components, and the role of SLYM as a new selective barrier involved in compartmentalizing the subarachnoid space offer a new perspective on neurological disorders and create opportunities for regenerative therapies targeting neurodegenerative diseases.