Dlxi1/2b::eGFP+ cells in hSS and in fused hSS-hCS. We found that hSS-derived cells that moved into hCS increased the complexity of their branching (Fig. 4e; Extended Data Fig. 10a, b). We then measured their electrical properties in hSS before and after assembly. We found that Dlxi1/2b::eGFP+ cells that had migrated into hCS had double the maximum action potential generation rate as compared to Dlxi1/2b::eGFP+ cells in unfused hSS or to non-migrated cells in fused hSS-hCS (Fig. 4f; Extended Data Fig. 10c). We then assessed the integration of migrated neurons by using AT and observed the presence of gephyrin (GPHN), a postsynaptic protein localized to GABAergic synapses, in hCS fused to hSS but not in unfused hCS (Fig. 4d; Extended Data Fig. 10d). We constructed ‘synaptograms’ consisting of a series of high-resolution sections through a single synapse, and found colocalization of eGFP from Dlxi1/2b-labeled cells with the presynaptic proteins SYN1 and VGAT and adjacent to the postsynaptic protein GPHN (Extended Data Fig. 10e). To investigate the presence of functional synapses in migrated Dlxi1/2b::eGFP+ neurons, we performed whole-cell voltage clamp recordings in sliced hSS-hCS. We found that Dlxi1/2b::eGFP+ that migrated into hCS display both excitatory postsynaptic currents (EPSCs, downward deflecting) and IPSCs
