updated 12/10/2003 8:13:24 PM ET 2003-12-11T01:13:24

Scientists say they have turned mouse embryonic stem cells into primitive sperm cells — and then used the sperm cells to fertilize eggs. The scientific team's work could offer insights into male infertility and boost human stem cell research.

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The sperm cells were not fully developed sperm, but rather their tailless precursors. When they were injected into eggs, the eggs developed into embryos. Scientists are now studying whether such embryos can develop into live-born mice.

The team was led by Dr. George Q. Daley of Harvard Medical School. Its findings were published online Wednesday by the journal Nature.

Embryonic stem cells can develop into virtually any kind of cell of the body, and scientists hope to use them someday to create replacement parts to treat illnesses such as Parkinson's disease and diabetes. The new work also involved embryonic germ cells, which appear in early embryos and mature into either sperm or egg cells.

Recently, other scientists reported that they had turned mouse stem cells into egg cells. Hans R. Schoeler of the University of Pennsylvania, who led the team that created those mouse egg cells, said the new work is a "very careful and beautiful study" that complements his team's findings.

While Japanese researchers announced in September that they had also created sperm from stem cells, Schoeler said their work was conducted largely inside living mice, something he called cumbersome and labor-intensive.

"But here we have the example where everything has been done in the dish and you can really study it in this way because it's right in front of you," Schoeler said.

Dr. John Gearhart, a Johns Hopkins University stem cell researcher, said that the new research is intriguing but that it is not clear whether the fertilized mouse eggs would have developed into normal embryos.

Daley said the laboratory methods his team developed as part of the research will allow scientists to study closely the development of germ cells.

"Germ cells are in some sense the immortal cells of our species. They're really endowed and given the responsibility for perpetuating the species," Daley said.

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