A study overturning long-held views from mouse work reports that bone marrow-derived cells infiltrate the aging human brain and replenish microglia. The finding reframes what drives microglial maintenance over the lifespan and points to new avenues for brain-targeted cell therapies. Researchers reported the human data as evidence that circulating or marrow-derived sources contribute to microglial replenishment during aging. (In this context, microglia are the brain’s resident immune cells, central to neuroinflammation and synaptic remodeling.) For developers of cellular and immune-modulating interventions, the result provides an actionable hypothesis: if microglial replenishment can be influenced through bone marrow-derived cell trafficking or conditioning, therapeutic design could move beyond local delivery to include systemic or preconditioning strategies.