The chondro-osseous junction of articular cartilage

In the synovial joints the transition between the soft articular cartilage and the subchondral bone is mediated by a layer of calcified cartilage of structural and mechanical characteristics closer to those of bone. This layer, buried in the depth of articular cartilage, is not directly accessible and is mostly visualized in histological sections of decalcified tissue, where it appears as a darker strip in contact with the subchondral bone. In this study conventional histology and scanning electron microscopy (SEM) with secondary electron imaging (SE) or backscattered electron imaging (BSE) were used to discriminate the calcified and the uncalcified cartilage in high resolution on native, untreated tissue as well as in deproteinated or demineralized tissue. This approach evidenced a high heterogeneity of the calcified layer of articular cartilage. High resolution pictures revealed that the mineralization process originates by progressive accretion and confluence of individual, small mineral clusters, in a very different way from other hard tissues such as bone, dentin and mineralized tendons. Finally, selective removal of the soft matrix by thermal treatment allowed for the first time a face-on, unrestricted 3D view of the mineralization front.