Speculation continues to swirl among Colts fans and doctors alike over Peyton Manning's possible stem cell treatment.Fox Sports reported over the weekend that the star quarterback, who has been sidelined by a nagging neck injury, flew to Europe to undergo stem cell therapy, a treatment that is not approved in the United States.Neither Manning's camp nor the Colts have confirmed the story."We have not discussed anything of that nature in terms of medical situations or whatever it may be," Colts head coach Jim Caldwell told reporters. "I think, also, in the release, we stated that were not going to discuss anything further, and that's where I'm going to end it."In a treatment practiced in Europe, stem cells are harvested from a healthy part of the body and then injected into the trouble spot in an effort to repair damaged cells, 6News' Stacia Matthews reported.Dr. Keith March with the Indiana University School of Medicine, who researches adult stem cells, said studies show they can decrease inflammation and regenerate nerve cells and bone growth."If the cells were used in the context of (Manning's) bone fusion procedure, the cells could very clearly enhance bone formation and enhance the rate of fusion," March said. "I think there's a good chance that it could have some clear biological effect."But other doctors are skeptical of the hype surrounding stem cells."The best way to repair the neck is to take the pressure off the nerve. There's zero data, none, that supports repairing a degenerative disc with stem cells," said Dr. Rick Sasso with the Indiana Spine Group. "What upsets me is people are going to think this is going to work."Manning missed Indy's first two games after having his third neck surgery in 19 months. He is expected to miss at least two months and possibly the entire season as he recovers.