Researchers reported discovery of specialized 'base barrier cells' that compartmentalize the choroid plexus, brain tissue and cerebrospinal fluid (CSF) in a Nature Neuroscience paper. The study maps a previously unrecognized cellular layer that influences fluid exchange and immune trafficking between the CSF and central nervous system. Authors used histology, single‑cell profiling and functional assays to characterize these cells’ molecular signatures and barrier functions. Findings may reshape models of neuroimmunology and drug access to the CSF in neurological disease. Clarification: The choroid plexus produces CSF and is a critical interface for immune signaling into the brain.