Osteopetrotic mice have growth plate abnormalities with normal expression of MMP-9. Histological sections of metatarsal growth plates from c-Fos+/+ (A), c-Fos−/− (C), Csf1+/+ (E), Csf1op/op (G), PU.1+/+ (I), PU.1−/− (K), Rank+/+ (M) and Rank−/− (O) mice showing enlarged HC zones in the mutant growth plates. Immunostaining for MMP-9 showed similar expression of MMP-9 in the Csf1+/+ (F), Csf1op/op (H), PU.1+/+ (J), PU.1−/− (L), Rank+/+ (N) and Rank −/− (P) growth plates. The c-Fos−/− growth plate showed decreased immunostaining for MMP-9 (D) compared with the control c-Fos+/+ growth plate (B). Bars, 100 μm.