(A–D) Gross examination of whole mount WT (Bloc1s2+/+) and Bloc1s2−/− embryos at E12.5, E14.5, and newborns (P0). Note that the (A) impaired hematopoiesis, (B) loss of eye pigmentation, (C) craniofacial malformation, and (D) smaller brain size of Bloc1s2−/− mice are indicated by the arrows compared with WT littermates at E12.5 and E14.5 and in newborns. (E) Coronal sections of E14.5 brains with Nissl staining of WT and Bloc1s2−/− embryos. The black boxes indicate the areas shown at higher magnification in (G). The enlarged lateral ventricles (LV) are shown. (F) Volumetric analysis of the LV in WT and Bloc1s2−/− brains at E14.5. The values for LV volume in Bloc1s2−/− mice are indicated as percentages of the corresponding WT values. The ratios of the volume of LV to brain are also indicated as percentages of the corresponding WT values. ***p<0.001 (Student’s t-test); n=6 mice. (G) The cortical plate (CP), intermediate zone (IZ), and ventricular zone (VZ) are labeled and compared. In Bloc1s2−/− brain, the thinner CP and IZ are shown, but VZ is almost normal. (H) Quantification of the thickness of the neocortex, CP, IZ and VZ in WT and Bloc1s2−/− brains at E14.5. 212.85 ± 10.39 (neocortex), 44.48 ± 3.01 (CP), 72.12 ± 6.46 (IZ), 94.66 ± 3.78 (VZ) for WT; 162.61 ± 1.99 (neocortex), 26.12 ± 1.27 (CP), 48.00 ± 1.26 (IZ), 87.17 ± 2.95 (VZ) for Bloc1s2−/−. **p<0.01 (IZ); ***p<0.001 (neocortex and CP); n.s., not significant, p=0.137 (VZ); n=6 mice. (I) Immunohistochemistry (Ctip2 and Tbr1) on sections of WT and Bloc1s2−/− brains at E12.5 and E14.5. The numbers of Ctip2+ and Tbr1+ cells were decreased in the Bloc1s2−/− brain at E12.5 and E14.5. (J and K) Quantification of neuronal number at E12.5 and E14.5. 24.00 ± 0.62 (Ctip2+), 22.08 ± 0.33 (Tbr1+) for WT at E12.5, 12.22 ± 0.17 (Ctip2+), 12.00 ± 0.68 (Tbr1+) for Bloc1s2−/− at E12.5; 48.40 ± 1.73 (Ctip2+), 27.78 ± 1.66 (Tbr1+) for WT at E14.5, 30.1 ± 1.41 (Ctip2+), 14.55 ± 0.66 (Tbr1+) for Bloc1s2−/− at E14.5. ***p<0.001; n=9. All graphs are mean ± s.e.m. Scale bars: 1 mm in (D); 100 µm in (E); 50 µm in (I).