Both neurons and astrocytes share a common neuroepithelial origin and are born throughout embryogenesis in a temporally defined manner25,26. We hypothesized that because of the longer exposure to FGF2 and EGF, progenitors in the proliferative zones of hCSs would ultimately undergo a neurogenesis-to-gliogenesis switch. At early time points, dividing GFAP+ cells that coexpress PAX6 or TBR2 were localized only inside the VZ/SVZ-like zones (Fig. 2c–f). However, after ~7 weeks of differentiation in vitro, we observed astrocytes with thin GFAP+ processes intermingled with NEUN+ cells in the hCS parenchyma (Fig. 3a–d). As expected, we observed few GFAP+ cells over the first 35 d of differentiation (2.7% ± 0.7), but this number increased to ~8% by day 76 and almost 20% after 180 d. To better investigate the ultrastructure of the neuroglial architecture in hCSs, we used array tomography, a high-resolution imaging technique based on the use of ribbons of ultrathin serial sections (70 nm)27. We observed numerous GFAP+ branches traversing the dense neuropil and intricately intertwined with neuronal MAP2+ processes (Fig. 3b).
