Figure 3. Dbx1-expressing progenitors at the PSB generate glutamatergic neurons at early stages of development. A, B, Immunostaining for Dbx1 on E12.5 coronal sections show that Dbx1-expressing progenitors at the PSB are pallial since they all express Ngn2 (A) but not Mash1 (B). C, Graphs represent the expression value ratios of the Dbx1 and Pax6 genes relative to that of the ribosomal protein rpS17 (see Materials and Methods) in RNA samples extracted from the entire pallium including the PSB (pallium PSB+) and from pallial territories dissected dorsally to the PSB (pallium PSB−). The values represent the mean of a triplicate (mean ± SD; **p = 0.0021 for Dbx1). The results show the absence of Dbx1 mRNA expression in the pallium PSB− sample, whereas Pax6 expression is slightly reduced because of its higher expression at the PSB. D–L′, Immunostaining for βGal (D–L′), Tbr2 (D–G), Tbr1 (H–L), and Reln (D–L′) on coronal sections of E12.5 Dbx1nlsLacZ/+ embryos. F, G, J, and K are high magnifications of the dashed white boxes in D, E, H, and I, respectively. L, L′, High magnification of green dashed box in H. More βGal+ cells are detected at the caudal than at the rostral PSB (compare F with G and J with K). The vast majority of βGal+ neurons express Tbr2 at the basal VZ/SVZ (D–G) and all express Tbr1 in the mantle zone (H–L, white and black arrowheads in J, K). βGal+Tbr1+Reln+ cells are detected in the lateral and dorsolateral rostral cortex (black arrowheads in J, and L, L′, respectively) but not in the lateral caudal cortex (K). However, the vast majority of βGal+ cells in the lateral cortex are Tbr1+Reln− (J–L′, white arrowheads) and few Tbr1+ Reln− cells are observed in the dorsal pallium (L, L′, white arrowheads) at this stage. **p < 0.01, t test. Scale bars: A, D, F, J, 50 μm; L, 20 μm.