Synovial joints provide skeletons with flexibility but what controls where these fluid-filled structures form during bone development? On , Sohaskey and colleagues report that the novel protein JAWS (joints abnormal with splitting)coordinates cartilage formation and synovial joint positioning in mice. Jaws-/- mice, they report, have many skeletal defects(including stunted limbs and short, round faces) that indicate that the development of the endochondral skeleton (the part of the skeleton in which a cartilage template directs bone formation) requires JAWS. Most strikingly,they note, Jaws-/- mice have ectopic joints within their digits. Furthermore, JAWS deficiency delays chondrocyte maturation and impairs the metabolism of chondroitin sulphate and the proteoglycan aggrecan, two components of cartilage. Thus, the researchers suggest, JAWS limits joint formation to specific locations in the embryonic skeleton by acting as a key regulator of chondrogenesis and synovial joint positioning. Future studies of Jaws-/- mice might, therefore, provide insights into what causes joint degeneration in osteoarthritis and other debilitating joint conditions.
