Animals and tissue preparation
129SV strain mice were used. The day on which the vaginal plug was confirmed was defined as E0. All the animal experiments were performed in accordance with the Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23). Pregnant females were sacrificed by cervical dislocation at various intervals between E5 and E17. Before E9, embryos were embedded within the maternal uterus. After E9, embryos were removed from their membranes and embedded separately. Experiments were performed with four to six animals at each developmental stage. The mature brain used was derived from mice of three to four month of age. The embryos and tissues were fixed in freshly prepared 4% paraformaldehyde solution for 2–12 h depending on the tissue thickness [35]. They were dehydrated with ethanol and embedded in paraffin according to standard procedures. Microtome sections were cut at 7–10 μm, placed on slides and stored until further processing.