Osteopontin expression and localization of Ca++ deposits in early stages of osteoarthritis in a rat model.

Calcium deposits have been related to articular cartilage (AC) degeneration and have been observed in late stages of osteoarthritis (OA). However, the role of those deposits, whether they induce the OA pathogenesis or they appear as a consequence of such process, is still unknown. In this work, we present the kinetics of expression and tissue localisation of osteopontin (OPN), a mineralisation biomarker, and calcium deposits in samples from (normal, sham) and osteoarthritic cartilage (in a rat model). Immunohistochemical and Western blot assays for OPN, as well as Alizarin red staining for calcium deposits were performed; superficial, middle, and deep zones of AC were analysed. An increased expression of OPN and calcium deposits was found in the osteoarthritic cartilage compared with that of control groups, particularly in the superficial zone of AC in early stages of OA. In addition, the expression and localisation of OPN and calcium deposits during the OA pathogenesis suggest that the pathological AC mineralisation starts in the superficial zone during OA pathogenesis.