Update on the biology of the chondrocyte and new approaches to treating cartilage diseases.

Osteoarthritis (OA) is a joint disease that involves degeneration of articular cartilage, limited intraarticular inflammation manifested by synovitis and changes in the subchondral bone. The aetiology of OA is largely unknown, but since it may involve multiple factors, including mechanical, biochemical and genetic factors, it has been difficult to identify unique targets for therapy. Chondrocytes, which are the unique cellular component of adult articular cartilage, are capable of responding to structural changes in the surrounding cartilage matrix. Since the initial stages of OA involve increased cell proliferation and synthesis of matrix proteins, proteinases and cytokines in the cartilage, laboratory investigations have focused on the chondrocyte as a target for therapeutic intervention. The capacity of the adult articular chondrocyte to regenerate the normal cartilage matrix architecture is limited, however, and the damage becomes irreversible unless the destructive process is interrupted. Current pharmacological interventions that address chronic pain are insufficient and no proven disease-modifying therapy is available. Identification of methods for early diagnosis is of key importance, since therapeutic interventions aimed at blocking or reversing structural damage will be more effective when there is the possibility of preserving normal homeostasis. At later stages, cartilage tissue engineering with or without gene therapy with anabolic factors will also require therapy to inhibit inflammation and block damage to newly repaired cartilage. This review will focus on experimental approaches currently under study that may lead to elucidation of effective strategies for therapy in OA, with emphasis on mediators that affect the function of chondrocytes and interactions with surrounding tissues.