Does the brain really have a fourth meningeal membrane?*

The glymphatic system, a pathway for flow of cerebrospinal fluid (CSF) in the brain, is crucial for clearance of toxic cellular waste from the brain's extracellular space. Growing evidence suggests that reduction in glymphatic transport contributes to etiology of neurodegenerative diseases, such as Alzheimer's. A recent breakthrough discovery (Møllgård et al, Science 2023) reported a newly-identified 4th layer of the meninges, which they named the "subarachnoid lymphatic-like membrane" (SLYM). However, this discovery has ignited considerable controversy among neuroanatomists and neuroscientists. We will present in vivo recordings of CSF flow at the dorsal surface of the mouse brain, imaged through a cranial window using two-photon microscopy. By injecting fluorescent microspheres (1 µm diameter), we visualize CSF flow and obtain evidence that any rupture in the SLYM layer (which is only 1-3 cells thick in some locations) drastically alters CSF flow dynamics. We perform particle tracking velocimetry to quantify the impact of SLYM rupture on glymphatic flow speed. Our measurements support the notion that such a membrane does exist, separating the subarachnoid space into different compartments. We will discuss technical challenges associated with invasive surgical procedures required to obtain brain optical access which may lead to SLYM rupture. We will also argue how SLYM rupture may explain why traumatic brain injury is linked to increased risk of neurodegenerative disease.