From the Wall Street Journal: "Scientists looking for ways to repair damaged cartilage - a leading cause of osteoarthritis - are employing horses to test a new method of tissue regeneration that uses concentrated stem cells. ... but there isn't any good way of detecting cartilage breakdown until the cartilage is broken or lost or osteoarthritis develops. And though cartilage-repair surgery has improved over the years, the repaired scar tissue isn't as strong as real cartilage and isn't able to bear the same weight. ... For about two decades, [researchers] have been trying to improve treatment by regenerating cartilage tissue. While many scientists have been successful at creating new tissue in the lab, they haven't been able to grow cartilage in humans. The main challenge is that the structure of cartilage, which is critical to its supporting weight, is hard to mimic ... It's easy to generate a piece of tissue, but it's not so easy to generate a tissue that works ... [researchers] hypothesize that the more stem cells that are used to repair cartilage at the site of the damage, the better the regeneration of the tissue should be. They first create a dime-sized cartilage injury in the horse's stifle joints, which resemble the human knee, and then test two different ways of concentrating the stem cells after taking them from the horse's own bone marrow."