Via e! Science News: "loss of the protein HMGB2, found in the surface layer of joint cartilage, leads to the progressive deterioration of the cartilage that is the hallmark of osteoarthritis. ... the protein HMGB2 is uniquely expressed on the surface layer of cartilage in joints, where it supports the survival of chondrocytes, the cells that produce and maintain cartilage. Aging is associated with the loss of HMGB2 and an accompanying reduction or total elimination of chondrocytes in the superficial zone. The scientists provided further links between HMGB2 and osteoarthritis by breeding mice to be genetically deficient in HMGB2; these mice had an earlier and more severe onset of osteoarthritis. ... If small molecules can be found to prevent or stop the loss of HMGB2, or conversely, to stimulate the production of this protein, then it is possible that osteoarthritis may one day either be prevented or reversed ... Because cartilage is unable to heal itself, scientists have been searching for ways to use stem cells to grow replacement cartilage in the lab that could be used to surgically replace damaged or non-existent cartilage. With the discovery of the link between HMGB2 and surface layer protein, scientists now have a clue about how they might be able to engineer the surface layer cartilage."

Link: http://esciencenews.com/articles/2009/01/12/scripps.research.scientists.find.cause.cartilage.degeneration.osteoarthritis