A procedure that encourages an egg to begin embryo development without being fertilized could suggest a new way to produce stem cells, at least for certain patients.

Researchers were able to get stem cells using unfertilized mouse eggs, they report in Thursday's online issue of the journal Science.

They are now turning their attention to eggs from people, but several problems remain to be worked out.

Stem cells can develop into many different types of tissue, making them basic building blocks of the developing body. Finding ways to produce them has become a major research focus, as these cells hold the promise to one day help repair damage from nerve-destroying illnesses such as Lou Gehrig's disease, or from spinal cord injuries or other illnesses.

But the work also poses an ethical dilemma, since current methods of producing stem cells require a fertilized embryo, which is killed by removal of the cells. President Bush has imposed strict limits on federal funding for such research.

The new study, led by Dr. George Q. Daley of the Children's Hospital of Boston, uses a procedure known as parthenogenesis in which a series of chemical treatments are used to encourage an unfertilized egg to begin embryonic development.

The report was welcomed by Dr. Ann Kiessling, director of the Bedford Research Foundation that also studies stem cells.

The idea of starting embryonic development without fertilization isn't new, she said, but these researchers have taken it a step further by generating stem cells. Kiessling, who was not part of the research team, said this report should generate interest in trying to do the same with human cells.

The stem cells produced in the process are a genetic match for the egg donor, and thus won't be rejected by the immune system. But using eggs also means the cells are limited to use in females.

There are different procedures to produce stem cells from male sperm, but the researchers said that is technically challenging and inefficient.

Stem cells developed by parthenogenesis have what is called altered expression of certain genes. Imprinted genes act differently depending on whether they are passed along via the egg or sperm. These new stem cells have two copies of the imprinted genes from the egg instead of one from each, a situation that has been associated with cancer and poor growth.

Also, such stem cells may have duplicate copies of some mutant genes that have been linked to cancer and abnormal tissue growth.

"We'll have to demonstrate the safety and durability of cells derived from parthenogenetic embryonic stem cells before we could imagine any clinical use," Daley said.

Daley said his group is also continuing research on cloning, which should produce higher-quality stem cells than the egg-only process, but it's a very inefficient method.