Researchers have identified a type of cell that provides a barrier function in the brain, specifically at the base of the choroid plexus, which separates it from the cerebrospinal fluid and the rest of the brain. This discovery sheds light on a previously unknown mechanism of compartmentalization in the brain and cerebrospinal fluid system. The findings suggest the existence of a specialized barrier or "gatekeeper" cell that plays a role in shielding the brain.