Hematoxylin and eosin stain and immunohistochemistry were performed as previously described (7 (link)). Tissue sections were used for TRAP staining according to the standard protocol. Tissues were fixed in 4% paraformaldehyde for 48 hours and incubated in 15% DEPC (diethyl pyrocarbonate)–EDTA (pH 7.8) for decalcification. Then, specimens were embedded in paraffin and sectioned at 7 μm. Immunofluorescence was performed as previously described (33 (link)). Sections were blocked in PBS with 10% horse serum and 0.1% Triton for 1 hour and then stained overnight with anti-PCNA antibody (SC-56). Donkey anti-rabbit Alexa Fluor 488 (1:1000; Molecular Probes, A21206) was used as secondary antibodies. DAPI (4′,6-diamidino-2-phenylindole) (Sigma-Aldrich, D8417) was used for counterstaining. Slides were mounted with anti-fluorescence mounting medium (Dako, S3023), and images were acquired with a Leica SP5 and SP8 confocal microscope. For embryonic mice, 5-mm tissue sections were used for immunohistochemistry staining, DIG-labeled in situ hybridization (Roche), and immunohistochemical staining (Dako).