Retrosplenial cortex maturation aligns with the transition from infant forgetting to persistent memory

Memories formed early in life are rapidly forgotten, whereas those formed later persist. Recent work revealed that physical traces of infant memories endure in the brain but are inaccessible to natural recall. However, the developmental processes regulating this transition remain unclear. Here, we performed a brain-wide developmental screen, revealing age-dependent shifts in regional activation patterns that parallel the emergence of stable memory expression. We focused on the retrosplenial cortex (RSP)-a critical region for adult memory consolidation with poorly understood developmental roles-as a candidate whose refinement may influence memory recall. Persistent memory aligned with increased reactivation of RSP ensembles and dendritic spine refinement. Reactivation of RSP infant learning ensembles a month, but not a week, after learning reinstated forgotten memories, coinciding with dendritic spine refinement on those cells. These findings reveal the developmental trajectory of the RSP, uncovering key maturation features aligning with the transition from forgetting to remembering.