Mouse embryonic stem cells turned spontaneously into eggs in an experiment that may point toward a new source of eggs for therapeutic cloning and perhaps remove a major obstacle from using stem cells to treat disease.

Without using any special chemicals or growth stimulants, researchers at the University of Pennsylvania said stem cells from mouse embryos will transform into oocytes, or eggs, and then into primitive embryos.

"Most scientists have thought it impossible to grow gametes from stem cells outside the body," said Hans R. Scholer of the school of veterinary medicine at the University of Pennsylvania. He said his team found that not only can the stem cells transform into eggs, but those cells then form embryos.

Scholer said the spontaneous embryos could not be used to reproduce mice because they contain an incomplete set of chromosomes, but the eggs probably could be used for cloning.

Embryonic stem cells can grow into virtually any cell in the body. Some researchers have suggested they could be used to grow new heart, liver, brain or pancreas cells which then could be used to revive or repair ailing organs.

To make these new organ cells compatible with a patient, researchers say they would have to clone an embryo using the nucleus from a cell of the patient. At an early stage of development, the new stem cells would be removed and then grown into the target cells.

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