Figure 2. Rank overexpression stimulates alveolar bone osteoclastogenesis.

TRAP activity assays were performed on frontal sections of the mandibles of 2 (a, b) and 3 (c–f) week-old mice to determine the effect of Rank overexpression on osteoclast numbers. At 2 weeks, the number of TRAP-positive cells was significantly increased around the first molar root in Msx2 ^−/− Rank^Tg mice (b, e). The root appeared longer but thinner in Msx2 ^−/− Rank^Tg than in Msx2 ^−/− mice, and advanced eruption was also clearly visible. At 3 weeks, no significant difference in the number of TRAP-expressing cells was observed (c, d, e). While the length of the first molar roots of Msx2 ^−/− mutants expressing or not expressing Rank was similar, it remained thinner in Msx2 ^−/− Rank^Tg mice (c, d). Asterisk in (d): Epithelial cyst on the lingual part of the root of a Msx2 ^−/− Rank^Tg mouse. M1, first molar; I, incisor. (e) Numbering of the TRAP positive cells in the alveolar bone surface performed on 7 µm thick sections (n>8) and presented as a table with statistical analyses.