It might be worth seeing whether Venus and Serena Williams pay a visit to a certain doctor any time soon. The pair are probably feeling a little sore just now. Both crashed out of Wimbledon on the same day, losing against younger players.

In sport, it can hurt to get older. But don't count the Williams sisters out yet. They just need to have a word with the orthopaedic surgeon who treated the baseball player Bartolo Colón.

Thanks to stem-cell therapy, Colón, star pitcher of the New York Yankees, has grown a new tendon in his shoulder. After losing a whole year to injury -- a seemingly inevitable result of ageing -- the 38-year-old says that he now has the throwing arm of a man half his age. He has proved it: since he joined the Yankees this spring, he has been back pitching at 95 miles an hour.

Stem cells hold the promise of helping with a wide variety of medical problems. Embryonic stem cells are being trialled in treatments that should enable those who are nearly blind to regenerate photoreceptor cells in their retina. Though less versatile than stem cells harvested from embryos,
the adult stem cells central to Colón's therapy are remarkable nonetheless.

It took less than 45 minutes for Colón's doctor Joseph Purita, who is based in Boca Raton, Florida, to isolate the sportsman's stem cells from his fat deposits and bone marrow; he then injected these into the injured shoulder. There, they grew into the types of cell -- and thus tissue -- that make a shoulder work like new. Colón's recovery took just a few weeks.

Free for all

It's worth pointing out that stem-cell therapy is not a miracle cure-all. Though some clinics make grand claims for the efficacy of the therapy in treating Parkinson's disease and spinal cord injuries, for instance, these stories are still largely groundless. And it's a free-for-all out there: the International Society for Stem-Cell Research (ISSCR) has just given up trying to police the clinics.

A year ago, the ISSCR offered to help anyone interested in receiving stem-cell therapies check whether a clinic had an ethics committee or was overseen by a regulatory body such as the US Food and Drug Administration. But early inquiries met with replies from lawyers threatening legal action. By February, concerned that its limited funds might be drained in lawsuits, the ISSCR had shelved the plan.

One final warning: stem cells haven't solved all of Colón's problems. However good his shoulder, he is not the leanest of men and the triumphant return of the "portly pitcher" was marred by a strained hamstring when running to field a ball. Sometimes, science -- even stem-cell science -- is no substitute for a calorie-controlled diet and a proper warm-up.