(A) Schematic summary of published postnatal developmental events in murine hearts that coincide with loss of cardiac regenerative capacity at P7. (B) Scheme summarizing our results, where postnatal pig hearts display discordant timing of cardiomyocyte maturational milestones compared to loss of regenerative capacity at P3. Transcriptomic changes in extracellular matrix, immune signaling, sympathetic neuron remodeling, and ROS response are conserved during transition from a regenerative to a non-regenerative postnatal heart in both mice and pigs. *Pig cardiac regenerative window is as described in previous reports [14, 15].