Concentric lamellae – novel microanatomical structures in the articular calcified cartilage of mice

The structure, ultrastructure and function of hyaline articular cartilage (HAC) and subchondral bone (SCB), and their involvement in the pathogenesis of osteoarthritis (OA) have been extensively researched. However, much less attention has been focused on the intervening tissue, articular calcified cartilage (ACC) and its role in the initiation and progression of OA. Using both light microscopy (LM) and transmission electron microscopy (TEM), a study of ACC in wild type (WT) mice, and mice with genetic osteoarthropathies (AKU) was undertaken to further understand the role played by ACC in the early stages of OA.Tibio-femoral joints were obtained from BALB/c WT and BALB/c AKU mice aged between 7 and 69 weeks. One joint was processed for routine histological analysis. The tip of the medial femoral condyle (MFC), which contained HAC, ACC, and SCB, was dissected from the contra-lateral joint and processed for TEM.In WT and AKU mice novel microanatomical structures, designated concentric lamellae, were identified surrounding chondrocytes in the ACC. The lamellae appeared to be laid down in association with advancement of the tidemark indicating they may be formed during calcification of cartilage matrix. The lamellae were associated with hypertrophic chondrocytes throughout the ACC.Novel microanatomical structures, termed concentric lamellae, which were present around hypertrophic chondrocytes in the ACC are described for the first time. Their apparent association with mineralisation, advancement of the tidemark, and greater abundance in a model of osteoarthropathy indicate their formation could be important in the pathogenesis of OA and AKU.