A Nature Neuroscience study identified a specialized population termed “base barrier cells” that compartmentalizes the choroid plexus, brain parenchyma and cerebrospinal fluid (CSF). The paper maps cellular architecture and suggests these cells help maintain molecular segregation between compartments previously considered more homogeneous. The discovery refines concepts of brain barrier physiology and could influence drug delivery strategies to the CSF and meninges, and reshape how researchers model CNS immune surveillance and toxin clearance.