Mice could be used as factories to produce eggs of other species. The eggs could then be fertilised and used to impregnate the endangered animals.

Purdue University researcher John Critser led a team that transplanted ovarian tissue from African elephants into lab mice that had been bred so that their bodies wouldn't reject foreign tissue.

Several of the mice developed egg-producing follicles and one contained a mature -- though misshapen -- egg. Critser's team did not use it to try to impregnate an elephant, figuring the egg was not healthy enough to produce a successful pregnancy.

The elephant egg posed no size problem for the mice because the eggs of both species are microscopic. ``It's a very important step but it's clearly going to be a long time before anyone is going to make an elephant out of this,'' said Randall Prather, aprofessor of animal science at the University of Missouri at Columbia.

Scientists previously managed to grow egg follicles in lab mice using ovarian tissue from sheep and a few other species but they didn't look to see whether the mice had produced eggs, Critser said.

Critser said he took the ovarian tissue from the carcasses of three freshly killed elephants he came upon during a visit to South Africa. The tissue was frozen, then thawed a year later and transplanted into the mice, which are commonly used in biomedical research.

The findings were reported in the October issue of the journal Animal Reproduction Science.

The experiment shows that rather than stocking animal tissue banks with hard-to-obtain eggs and embryos, it would be far easier to collect ovary tissue and use it to grow eggs in other species, Critser said.

Critser said the technique could also help cancer patients who want to get pregnant someday but are about to undergo fertility-destroying radiation and chemotherapy. Doctorscould remove their ovary tissue, freeze it and then return it to their bodies if they beat the disease.

He is now working to fine-tune the freezing, thawing and transplant process to ensure that the mice produce a steady supply of healthy eggs.

The experiment shows that such material can remain on ice for a long time and still be usable, said Betsy Dresser, senior vice president of the Audubon institute in New Orleans, which maintains a bank of about 500 samples of sperm, eggs and embryos from endangered species such as gorillas, rhinos and antelopes.