The formation of an SVZ layer containing intermediate progenitor cells outside of the VZ is a distinctive feature of human neocortical development (Lancaster et al., 2013). We therefore examined hCOs for the presence of SOX2+ progenitors outside of VZ-like area (Figure 3H). Staining for the IPs marker TBR2 and the oRGs marker FAM107A confirmed the presence of SVZ-like area in hCOs (Figure 3I and 3J). Furthermore, horizontal Reelin+ cells were present in the surface area of hCOs, indicating the production of Cajal–Retzius cells (Figure 3K). With further development of the hCOs, separation of early-born CTIP2+ and late-born SATB2+ neurons became prominent in CP-like area (Figure 3L), indicating the specification of deep and upper cortical layers.
