Recently, a number of protocols for MGE induction from hPSCs was published [4,18,19,21,37], as summarized in Table 6. Most of these protocols use inhibition of BMP (or dual SMAD inhibition [26]) to inhibit non-neural lineage induction, with the exception of the protocol of Liu et al., which employs low density culture for neural induction without any added inhibitor. Though a side-by-side comparison will be needed, there appears to be a tendency for sphere cultures to generate MGE cells more efficiently than adherent cultures (Table 6). These protocols commonly employ a high concentration of SHH (or SHH pathway activator), sometimes along with an inhibitor of the dorsalizing Wnt pathway, resulting in ventralization of the neuroectoderm (Table 6). Strong SHH signaling used to ventralize telencephalic tissues can induce mutually inhibitory downstream signaling of FGF8 and FGF19, which generate MGE cells vs. CGE cells, respectively [21]. In our protocol, by employing exogenous FGF8 and FGF19 signaling after initial ventralization of neuroepithelium, we can efficiently induce either MGE cells or CGE cells, allowing us to avoid any stochastic events during ventral telencephalic induction.
