Bridging veins govern discrepancy of CSF efflux into dura mater in mice

Meningeal lymphatic vessels within the dura provide a route for cerebrospinal fluid (CSF) outflow, but how CSF crosses the arachnoid barrier, particularly toward basal dural compartments, remains unclear. Using in vivo CSF tracer infusion and ultrastructural analysis in mice, we show that bridging vein-associated arachnoid cuff exit (ACE) domains provide specialized routes for CSF movement from the SAS into the dura. These ACE-associated structures are abundant in the basal than dorsal meninges, supporting preferential CSF efflux toward basal meningeal lymphatics. In contrast, spinal meninges lack comparable ACE structures, and spinal CSF exits along the nerve root arachnoid sleeves rather than entering the spinal dura. Depletion of parenchymal border macrophages stalls tracer movement, leading to retention within the subarachnoid and dural compartments, revealing their regulatory role in arachnoid-bypassing CSF drainage. These findings establish a compartmentalized CSF outflow model, in which basal bridging vein-associated ACE structures serve as major dural exit portals, with dorsal limitation and spinal exclusion.