Transplantation reveals birthdate-dependent post-mitotic competence in cortical neurons

Cortical glutamatergic neurons are generated through temporally ordered developmental programs that link neuronal birthdate to laminar position and identity. Yet, because differentiation continues after cell-cycle exit, it remains unclear how far post-mitotic identity can be reshaped by the environment and whether this residual competence varies across neuronal types. Here, we used transplantation to uncouple birthdate from final laminar position by placing newborn neurons in ectopic laminae in the postnatal mouse cortex. E13-born (i.e. deep layer-destined) and E15-born (i.e. superficial layer-destined) donor neurons displayed distinct birthdate-associated molecular programs before transplantation. After grafting into P0 hosts, both populations migrated and settled across cortical layers, enabling comparison across laminar environments. We simultaneously profiled transcriptomic, morphological and electrophysiological neuronal identities using Patch-seq. Transplanted neurons developed pyramidal morphologies and active membrane properties, but their final identity was not imposed by their laminar position. Thus, the postnatal cortex supports integration and maturation but does not impose canonical laminar identity. Critically, neuronal competence was asymmetric across birthdates. E15-born neurons remained committed toward a robust superficial layer neuron-like profile independently of cortical position, whereas E13-born neurons retained a broad, non-canonical differentiation rather than either converging on a deep- or superficial-layer reference state. This is consistent with the wider intrinsic repertoire of early-born neurons, which the postnatal environment does not resolve. Thus, post-mitotic identity is constrained by developmental history in a birthdate-dependent manner, with early- and late-born neurons retaining fundamentally different differentiation potentials after cell-cycle exit.