The efficient induction of NKX2.1 largely independent of the timing of SHH exposure raised the question of whether timing impacts the subtype of NKX2.1+ neural progenitors generated. Analogous to rodents, NKX2.1 is expressed during early neural human development in both the FOXG1+ telencephalon as well as in the FOXG1-negative ventral diencephalon (Figure 2A, (Kerwin et al., 2010; Rakic and Zecevic, 2003). During embryonic mouse development NKX2.1 expression in the telencephalon (FOXG1+) is restricted to the ventral (subcortical) domain. In contrast, reports in human fetal tissue suggested that NKX2.1 may not be restricted to the ventral forebrain, instead extending into the dorsal forebrain and cortical anlage (Rakic and Zecevic, 2003; Yu and Zecevic, 2011). However, those studies were largely based on the analysis of second trimester human fetuses making it difficult to distinguish whether NKX2.1+ cells were induced in the dorsal forebrain or migrated dorsally after induction in the ventral domain (Fertuzinhos et al., 2009). By performing immunocytochemical analyses in early stage human embryos (Carnegie Stage 15, ~ 38 days post conception), we observed restriction of NKX2.1 expression to the ventral
