We found that vGAT and GABA, markers of GABAergic neurons, were widely expressed in hMGEOs as they matured (Figure 2G), suggesting the production of interneurons. Previous studies with mouse models showed that the dorsal and ventral subdivision of MGE preferentially produces interneurons expressing SST (Somatostatin) and PV (Pavalbumin), respectively (Tyson et al., 2015). To test whether human MGE development may have a similar dorsal-ventral fate determination in interneuron subtype production, we compared the effect of different doses of SHH on development of SST and PV expressing neurons (Figure 2H). High SHH dosage did not significantly enhance NKX2-1 expression at early stage, but rather maintained NKX2-1 expression at higher level during further development (Figure 2I). However, SST expression significantly increased under higher SHH dosage as hMGEOs matured together with the dramatic increase of SST+ neurons (Figure 2I and 2J). We found the expression of neuropeptide Y (NPY), marking an interneuron subtype closely related to SST+ interneuron development (Wonders and Anderson, 2006), also significantly increased under stronger SHH activation (Figure S1E). However, we did not observe obvious changes in PV or Calbindin
