Skulls were fixed in formaldehyde–formic acid and divided coronally at the bregma to separate the PF and SAG sutures. Samples were then embedded to obtain paraffin or frozen sections, which were stained with hematoxylin-eosin for histology, AB for chondrogenesis, alkaline phosphatase for osteoblastogenesis, or antibodies for immunological staining, which was detected with avidin–biotinylated enzyme complex as described (14 (link), 15 (link), 53 (link)–54 (link)). The immunological staining was visualized by enzymatic color reaction or fluorescence according to the manufacturer’s specification (Vector Laboratories). Images were taken with a Zeiss Axio Observer microscope (Carl Zeiss). Mouse monoclonal antibodies against collagen II (Thermo Fisher; 1:50) and activated β-catenin (ABC) (Millipore; 1:200); rabbit polyclonal antibodies against FGFR1 (Santa Cruz; 1:200), laminin (Sigma; 1:50), phosphorylated histone H3 (Cell Signaling; 1:100), pSMAD1/5/8 (Cell Signaling; 1:50), and SOX9 (Santa Cruz; 1:200); rabbit monoclonal antibody against Ki67 (Thermo Fisher; 1:200) were used in these analyses. Details for β-Gal staining in whole mounts or sections were performed as described previously (33 (link), 48 (link)). TUNEL (terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate nick end labeling) staining was performed with ApopTag (Millipore) as described (33 (link)). Whole-mount GFP analysis was performed with fluorescence stereomicroscopy to visualize the skull (33 (link)). For analysis in sections, skulls were further fixed in 4% paraformaldehyde–phosphate-buffered saline, decalcified in 14% EDTA at 4°C for 3 days, processed for frozen section, and evaluated with a Zeiss Axio Observer microscope.