Flexed and Immobile Joint Cartilage (IMAGE) University of California - San Diego Caption When the knee joint is not subjected to continuous passive motion (left), less proteoglycan 4 is produced by chondrocyte cells than when the knee joint is continuously moved (right). The dark purple stain is a specific visualization tool to identify chondrocyte cells in cartilage that are producing proteoglycan 4. Credit UC San Diego Usage Restrictions Credit: UC San Diego License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.