MEF-derived BAM-iN cells expressed the pan-neuronal markers MAP2 and synapsin (Fig. 5f). The BAM-pool was capable of efficiently generating iN cells from perinatal tail tip fibroblasts (Fig. 5e, Supplementary Fig. 8a–e). After infecting tail tip fibroblasts from adult mice with these three factors, we could detect neuronal cells expressing TauEGFP, Tuj1, NeuN and MAP2 (Supplementary Fig. 9). Importantly, when co-cultured with astrocytes, both MEF and perinatal tail-tip fibroblast-derived BAM-iN cells were capable of forming functional synapses as determined by the presence of both NMDA- and AMPA-receptor mediated EPSCs (Fig. 5g–h). Similar to 5F-iN cells, no IPSCs were detected in MEF-derived (n=16) or tail-derived (n=12) BAM-iN cells. This functional evidence suggests that a majority of BAM-iN cells are excitatory. Indeed, 53% (111/211 cells) of MEF BAM-iN cells expressed Tbr1, a marker of excitatory cortical neurons, whereas less than 1% (3/~500 cells) were GAD –positive (Supplementary Fig. 8f).
