Political opposition and limited federal funding have delayed efforts to learn how the cells can be used, but work is beginning to progress, James Thomson and John Gearhart said at a briefing at the National Press Club.
The session marked the fifth anniversary of their publication of pioneering research papers on stem cells, the basic cells of early development that can grow into any body part.
Thomson, of the University of Wisconsin-Madison, was the first to isolate and cultivate human embryonic stem cells. Gearhart, of Johns Hopkins University in Baltimore, was first to isolate and culture human germ cells.
Asked to predict the research status if invited back for the 10th anniversary of their findings, Gearhart said: "I am confident we will be in clinical trials with several of these cells." He did not, however, specify which cells or what types of treatment he expected to be undergoing tests five years in the future.
Because stem cells can develop into any body part, scientists say they may hold out hope for one day being able to treat a variety of diseases from Parkinson's to Alzheimer's and others.
However, the work is controversial because embryos discarded from fertility clinics are a major source of embryonic stem cells. Opponents decry killing embryos to obtain the cells and President Bush has restricted federal funding to those cell lines that already were being used for research two years ago.
Actor Christopher Reeve, immobilized by a spinal cord injury suffered in a horse-riding accident, joined the two researchers and pleaded for more federal funding for their research.
"It is painful to contemplate where we might be today if embryonic stem cell research had been allowed to go forward with full support of the government," Reeve said.
Thomson agreed that "if embryonic stem cell research had not been involved in politics we would be far ahead of where we are today." But, he added, research is picking up with private money and some federal funds.
Gearhart said research at his university also is expanding and that he was encouraged by some of the findings in safety studies.
There had been concerns that transplanted stem cells could grow unchecked and cause cancer, but he said "we haven't seen a tumor yet" in thousands of studies in animals.
By Randolph E. Schmid