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The only researchers to publicly show that they have cloned a human embryo said on Tuesday that they had successfully repeated the experiment, growing an embryo to the 16-cell stage.

Researchers at Advanced Cell Technology of Worcester, Massachusetts, have also repeatedly created embryos using a process called parthenogenesis -- using only a human egg cell and no sperm, and without cloning.

The company says the experiments, reported in the January issue of Wired magazine, are not breakthroughs but a natural progression of its efforts to create human embryonic stem cells to use for medical treatments.

"It's not a scientific advance," ACT medical director Dr. Robert Lanza said in a telephone interview.

But he said the researchers had managed to replicate experiments reported in late 2001, in which they used cloning technology to create a human embryo that grew to the six-cell stage. They also created more advanced embryos, called blastocysts, using parthenogenesis.

Sheep, cattle and pigs have all been cloned using an egg cell and an adult cell from another animal.

But critics had questioned whether the cloning process known as nuclear transfer has ever worked with humans. Lanza said the new experiments, which began last June, suggest they were successful.

PARTHENOGENESIS MEANS "VIRGIN BIRTH"

Even more successful were the parthenogenesis experiments -- in which five of eight human eggs were coaxed into growing into blastocysts. At this stage -- approximately 100 cells -- an embryo can be mined for its stem cells.

The stem cells -- nature's template for all cells -- can become any sort of cell or tissue in the body.

Scientists hope the cells may one day allow custom-made tissue transplants to heal damaged hearts or cure diabetes by replacing dead pancreatic cells.

Opponents raise moral objections, saying that cloning technology involved in making embryonic stem cells creates a living human being.

The U.S. federal government refuses to fund embryonic stem cell research beyond a few limited cultures, while Congress and some members of the United Nations have made several failed attempts to ban so-called therapeutic cloning outright.

Lanza says parthenogenesis can bypass these objections. Parthenogenesis is not known to lead to the development of a fetus in mammals.

"If implanted into a woman's uterus, we don't think it would develop into a child," he said.

But the resulting blastocyst, called a parthenote, can be a source of embryonic stem cells. Tissue from such cells would be easier to match with patients and less likely to be rejected, Lanza said, because they contain only one person's DNA.

It also would be more readily available than tissue from a patient's cloned cells, which would take months to prepare.

"For many therapies you don't have time to start from scratch," he said.

It would take just 40 batches, or lines, of parthenote-generated stem cells to create tissue matches for 70 percent of the U.S. public, Lanza said. Embryonic stem cells are immortal, so it would not take many human eggs to create several dozen lines.

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