Gcps in the emerging fissures have a shorter mitotic index than other gcps. (a, b) Medial sagittal sections of E17.5 (a) and E18.5 (b) embryos treated for approximately 20 minutes with BrdU. Anti-BrdU (green) immunostaining shows uniform BrdU incorporation throughout the EGL (asterisks depict locations where fissures will form). (c) Quantification of the number of pH3 positive gcps in the region where the EGL is thicker (blue bars) versus the intervening thinner regions (red bars). Bar height indicates the mean value of each data set, and error bars indicate standard error. An asterisk indicates statistically significant differences between the two regions (p < 0.001 for E16.5, p < 0.0001 for E17.5). (d) Anti-pH3 immunostaining at E16.5 (data not shown) and E17.5 indicates that there are more gcps in mitosis in the regions where future fissures will form (areas between dotted outline) than in other regions. (e) By E18.5 pH3-positive cells are distributed equally throughout the EGL (between dotted lines). Anti-Calbindin was used to mark the Pc layer. Arrows indicate pH3 positive cells in the EGL. Scale bars: (a, b) 100 μm; (d, e) 50 μm.