However, the journal Nature, which published the study, is standing behind it, and Advanced Cell says the criticisms are stemming from a misunderstanding of their research.

Advanced Cell, which published the results in the Aug. 24 online version of Nature, has seen their stock rise and fall after the study came out. Shares were up more than 400 percent immediately after the study was released but have come down since and were up 35 percent in late Wednesday trade. The company also lined up $13.5 million in additional funding from investors, but that does not appear to be tied to the results of the study.

Although the technique was initially hailed as providing a way around moral objections to stem cell research, the embryos used in the study actually were destroyed, leading some scientists and critics to suggest that the study does not show the process is safe for embryos.

In the study, Advanced Cell showed a technique called preimplantation genetic diagnosis, routinely used at fertilization clinics, could be used to pluck out or biopsy a single cell from an early-stage embryo and give rise to an embryonic stem cell line.

The PGD technique, which is used to screen for genetic disorders, has been used thousands of times and apparently does not harm the embryo, which can then be implanted in the womb where it develops normally.

Andy LaBarbera, a professor in the University of Cincinnati's obstetrics and gynecology department, said the study was promising, but there were still some unanswered questions about whether it would actually spare the embryo and the efficiency of the technique.

"It's touted as, 'Look, you can make human embryonic stem cell lines without destroying embryos,' but, point in fact, they did destroy the embryos," LaBarbera, who also serves as scientific director for the American Society for Reproductive Medicine, told United Press International.

"The big question here is, 'If you were to go and biopsy a blastomere the same way you do in PGD, would that single cell develop into a stem cell line?'" LaBarbera said. "The answer is that has not been demonstrated," he said.

But at the same time, LaBarbera acknowledged, "Our impressions are that it would not harm the embryo." He noted that PGD has led to about 2,000 live births, indicating the technique leaves the embryo unharmed. In addition, some fertilization clinics already extract two cells, instead of only one, from the embryo for PGD, suggesting that extracting one cell for genetic screening and one cell to give rise to an embryonic stem cell line is theoretically possible.

LaBarbera questioned the efficiency of the technique, saying the study does not give a clear indication of how many embryos or biopsied cells were required to generate a stem cell line.

But he added that the technique could most certainly be made more efficient with some tweaking in the lab.

"I presume with practice and refining conditions, that will become a highly efficient process," he said. "I don't have any doubt about that."

Other scientists have echoed similar criticisms, and Sen. Arlen Specter, R-Pa., during a hearing Wednesday, chastised Advanced Cell for exaggerating their finding and making it more difficult to get legislation passed that would ease restrictions on federal funding for stem cell research.

But Advanced Cell has not backed down in refuting the critiques.

Robert Lanza, the company's vice president of research and scientific development, said the science was sound and the biopsy technique they used was precisely the same method used for PGD.

"The whole point was to address a medical and scientific question, which is if you have a cell biopsied for PGD, does it have the potential to make stem cells?" Lanza told UPI. "The answer is yes," he said.

"Everybody is now saying it's a big leap of faith ... but that's ridiculous, that's absurd," Lanza said.

The reason for the confusion, Lanza said, is his research team was short on embryos, which made it necessary to go in and biopsy embryos multiple times to have enough data to show the biopsied cells had the potential to give rise to embryonic stem cell lines.

The leftover embryos were discarded because it was already known that embryos survive this procedure, via the experience with PGD-biopsied embryos that have resulted in successful pregnancies, so it was not necessary to demonstrate that finding again for this study, he said. Multiple biopsying would not be done if this technique were used in a real-world setting, he noted.

As far as the efficiency of the technique, Lanza said that with the better grade embryos it was roughly equivalent to the efficiency of conventional techniques for obtaining embryonic stem cells.

Nature also defended the validity and importance of the study and its decision to publish it.

"The paper was rigorously peer reviewed by three reviewers, whom supported publication after revisions were made," Natalie DeWitt, the Nature editor who worked on the Advanced Cell study, told UPI.

"Why did Nature publish it?" DeWitt said. "As a potentially important new technique for isolating human (embryonic stem) cells, and the first demonstration that a single blastomere could form human (embryonic stem) cells, the findings were innovative and important enough to publish in Nature."

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