The researchers wanted to turn those awakened stem cells into cartilage. The recipe that worked was to treat the stem cells with bone morphogenetic protein, which is used to help fuse bones.

The scientists also used a drug called Avastin, which prevents the stem cells from getting a blood supply. Unlike bone and bone marrow, cartilage has no blood supply, and the drug helped stimulate the stem cells to turn into cartilage.

The investigators provided the drugs directly to the ends of bones, putting them in a gel.

The cartilage that grew in the mice not only looked like normal but lasted for four months, a quarter of the animals lifetimes. Dr. Chan and Dr. Longaker envision a time when doctors will be able to resurface arthritic joints or, even better, to treat people who are just beginning to develop arthritis, perhaps staving off the sort of damage that even joint replacements cannot fix.

If the strategy works in humans, then early treatment may be the best approach, Dr. Marx said.

Arthritis deforms joints and changes bones, he said. By the time people have hips or knees replaced, irreversible damage may be done. Legs may be bowed, bones damaged.

You cannot totally turn back the clock, Dr. Marx said. At that point, he said, adding cartilage will not fix it.

He worries, though, that orthopedists may not wait for rigorous studies the method of awakening the dormant cells is relatively simple, and the drugs required are already on the market.

Faced with a patient with aching knees, orthopedists may be tempted to say, Lets try this. You dont have much to lose, Dr. Marx noted.

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Cartilage Is Grown in the Arthritic Joints of Mice - The New York Times