The experimental stem cell procedure that Texas Gov. Rick Perry underwent this summer could be restricted or even blocked under new rules being considered Friday by the state's Medical Board.

Some top scientists are questioning the safety and wisdom of the procedure, and doctors say it may run up against federal rules. It also carries potential health threats, ranging from blood clots to increased cancer risk.

The Republican presidential candidate had stem cells taken from fat in his own body, which were then grown in a lab. They were injected into Perry's back and his bloodstream during an operation in July to fuse part of his spine.

Adult stem cells have long been used to treat leukemia, lymphoma and other cancers. While the cells are being studied to treat other ailments, from heart disease to diabetes, experts say it's too soon to know if the approaches are safe or effective. The Food and Drug Administration hasn't approved using adult stem cells to help people heal from surgery _ but experimentation is common.

The medical board was meeting to discuss rules that would require an accredited body to review any procedures involving stem cells before they're carried out, to access research trials and ensure patient safety. The rules also would require that such therapies be done by physicians and in adherence to Texas and federal laws.

The 19 volunteer board members _ all appointed by Perry, including a dozen physicians _ could approve the proposed rules, make or seek changes, or scrap them altogether, said board spokeswoman Leigh Hopper.

Perry opposes greater oversight in his state, and he sent a letter to the board urging members to recognize the "revolutionary potential" of adult stem cell research and therapies.

"Texas is a leader in innovation in many fields," Perry wrote after his surgery. "It is critical that we continue to foster an environment that encourages technological advancement in the health care arena."

Adult stem cell therapy is different from using embryonic cells, a controversial technology that the governor opposes.

If the board vote goes against him, the matter could become a campaign issue as Perry struggles to reinvigorate his White House bid. His polling numbers have tumbled in recent weeks. Following his surgery, Perry has worn a back brace but maintained his hectic campaign schedule _ though he has begun wearing orthopedic shoes to ease the pain, foregoing his trademark cowboy boots.

The impetus for the proposed rules did not grow out of Perry's procedure directly, but did involve the physician who performed it, Dr. Stanley Jones. Perry's doctor and friend, the Houston-based orthopedist Jones touted the benefits of stem cell therapies at the board's June 3 meeting, even saying he had benefited from it following a procedure overseas.

Hopper said Jones told the board that many Americans pay thousands of dollars to undergo treatment abroad and that it is a shame that Texans can't access the treatment in their home state.

Some orthopedic surgeons are experimenting with stem cells to help bones heal, with the cells being taken from bone marrow and injected or implanted in the trouble spot. The theory is that such "master cells" will follow cues from cells around them and form bone or cartilage, though researchers worry they also might spur unwanted growth and cancer.

Perry's treatment, which involved using stem cells from fat, was even more experimental. He said in a letter to board chairman Irvin E. Zeitler on July 25 that he understood the need to protect patients, but added, "we need to ensure that physicians in this state can continue to pursue new technologies and treatments that will benefit all Texans."

Perry also helped push an amendment to a larger health care bill through the Texas Legislature in June creating a state bank to store and cultivate adult stem cells for treatment purposes. NBC reported in September that the first bank approved by the state _ Celltex Therapeutics Corp. of Houston _ is co-owned by Jones and David G. Eller, the former chairman of the board of Texas A&M University and a top Perry donor.