Flexing Joints in the Lab (IMAGE) University of California - San Diego Caption Gayle E. Nugent-Derfus, now an engineer at Genentech, and UCSD bioengineering professor Robert Sah showed that shear forces on cartilage prompt chondrocyte cells to produce proteoglycan 4, a molecule that lubricates and protects joint surfaces. 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.