Fat harbors cells that could aid joints.

Fixing an injured joint could become a matter of commandeering fat. Researchers at Duke University and the tissue engineering company Artecel Sciences, both in Durham, N.C., have found a way to trick fat into generating cartilage, the flexible tissue that lines joints and is often damaged in sports injuries.

Cartilage is both tough and tough to replace. Once damaged by disease or injury, the tissue doesn't regenerate because it lacks the blood vessels for bringing in nutrients necessary for growth.

"There are not a lot of options right now for treating cartilage injuries," says Duke University bioengineer Farshid Guilak.

Sometimes, cartilage injuries require physicians to replace a patient's joint with a metal-and-plastic prosthesis. Another approach has been to replace damaged cartilage with adjacent healthy cartilage. In this technique, surgeons remove a piece of cartilage and send it to a lab, where the tissue is then coaxed to grow. But this is costly, can damage healthy cartilage, and requires a second operation for inserting the newly grown tissue, says Guilak.

He and his colleagues are investigating another tack in the laboratory and in animals. They discovered that under certain conditions, precursor fat cells called stromal cells can be reprogrammed to produce cartilage cells. The researchers reported their findings this week in San Francisco at the 47th annual meeting of the Orthopedic Research Society.

Similar precursor cells have been found in bone marrow. However, extracting bone marrow is painful and marrow contains few of the cells.

Besides, says Guilak, "everyone would like to lose some fat," and the procedure is fairly painless. Unlike cartilage cells, stromal cells grow quickly. Since the stem cells would come from a person's own body, there would be no problem with tissue-versus-host rejection.

Guilak points out that it's unlikely such cells will be useful for treating osteoarthritis, a cartilage-eroding disease, since the disease could destroy new cartilage as well.

The team stumbled upon fat's cartilage-rebuilding potential after learning about a rare disease that causes bone to form in fat. "Just observing the fact that some cells inside fat can actually turn into things other than fat is what gave us the idea that maybe we can turn these cells into cartilage as well," says Geoffrey Erickson, a graduate student at Duke and a coauthor of the study.

The researchers took fat collected by liposuction and isolated the stromal cells. They then grew the cells for 2 weeks and treated them with a biochemical cocktail of growth factors and vitamins. Within a day, the cells began producing the proteins and other molecules that cartilage cells make.

"Now that we have made cells that look, walk, and talk like cartilage cells, will they behave [in the body] like cartilage cells?" asks Laurence Higgins, an orthopedic surgeon at Duke University. So far, cells implanted under the skin of mice have retained their cartilage-like features, but the researchers have yet to assure themselves that these cells won't revert to their fatty ways, Guilak says.