Fig. 1. Counting parameters used for in vivo analysis. (A) Parasagittal view of an E12.5 wild-type forebrain showing the planes of section in which cell divisions were counted. Cells were counted in five sections at 25%, 50% and 75% through the telencephalon (broken lines). (B) Coronal section through an E12.5 wild-type embryo at a position 50% through the telencephalon (dorsal is upwards). Cell nuclei are stained with DAPI. Cell counts were made in the ventricular zone of the cortex between the two arrows. (C,D) Mitotic cells in (C) wild-type and (D) Pax6^Sey/Sey E15.5 cortex identified with an antibody against phosphorylated histone H3, which labels metaphase chromosomes. In the wild-type cortex the majority of dividing cells are located at the ventricular surface; in the Pax6^Sey/Sey cortex some dividing cells are located ectopically (away from the ventricular surface, arrows). (E) Late anaphase and telophase cells were classified by their plane of division relative to the ventricular surface (0°): –30° to 30°, asymmetrical division; 30° to 60°, intermediate division; 60° to 120°, symmetrical division. (F,G) DAPI-labelled chromosomes showing (F) symmetrical and (G) asymmetrical cell divisions. Broken line indicates plane of ventricular surface and arrow indicates plane of cell division. CP, cortical plate; IZ, intermediate zone; VZ, ventricular zone. Scale bars: 100 µm in B; 20 µm in C,D; 5 µm in F,G.