To mimic tangential migration of interneuron progenitors, we fused hMGEOs and hCOs together to generate the fused MGE-cortical organoids (hfMCOs) (Figure 7A). To monitor migration, hMGEOs were infected with lentivirus expressing RFP under human synapsin I promoter. RFP-labeled neurons gradually migrated from MGE side towards hCOs and large number of RFP-labeled hMGEO cells were found at hCO side (Figure 7B). Because no NKX2-1-GFP+ cells were detected in hCOs after long-term culture (Figure 4C and Figure 5F), we performed further characterization of NKX2-1-GFP+ interneuron migration without RFP labeling. 3-D reconstitution of the fusion border between hMGEO and hCO revealed that at 3 days post fusion (dpf), NKX2-1-GFP+ progenitors already started to egress, and extensive progenitor migration was observed two weeks later (Figure 7C). Furthermore, immunostaining of hfMCOs cryosection confirmed that the NKX2-1-GFP+ progenitors migrated between organoids (Figure 7D). In total, all 42 hfMCOs that we generated showed migration of NKX2-1-GFP+ cells from hMGEO to hCO. We found that the majority of the migrating cells were confined to the superficial areas of hCOs. Histological analyses confirmed that migration routes of NKX2-1-GFP+ progenitors
