SAN FRANCISCO (AP) -- Japanese researchers have removed ovaries from fetal mice and matured the eggs in a test tube, a technique that someday could save the fertility of girls being treated for cancer.

The scientists removed genetic material from the immature eggs and transferred it into mature eggs. Those eggs were then fertilized and the embryos were inserted into the wombs of surrogate mothers.

Of the 64 embryos, 16 pups - or 25 percent - were produced by seven adult mice. None of the offspring displayed abnormalities, and all were fertile following development, the study found.

Details of the experiment appear in the current issue of the journal Nature.

While the technique has not been tried on humans, the study's author says it could work because female mammals of all kinds are born with a full complement of eggs, and the ability to freeze eggs for later use is already available.

"This is specifically effective for childhood cancer patients because they don't have any fully mature eggs," says author Issue Hatada of the Gene Research Center at Gunma University, Gunma, Japan.

Other scientists who did not participate in the experiment were cautiously optimistic, although they said the method raises some of the same concerns that surround cloning human tissue.

"It's promising research with clinical applications," said Jamie Grifo, director of reproductive endocrinology at New York University Medical School. He has performed similar work that involved transferring nuclear material from an older woman's eggs into a younger woman's eggs and producing an embryo.

Reproductive damage isn't as common in children as in adults with cancer. However, a higher occurrence does appear in children who undergo certain types of chemotherapy or radiation treatment for bone marrow transplants and ovarian cancer.

"There are subgroups where this sort of technology would be very important because there really aren't good alternatives for these young girls," said Charles Sklar, director of a program for survivors of childhood cancer at Memorial Sloan-Kettering Cancer Center.

But if it is tried on humans, the Japanese technique could run into some of the same ethical and legal complications that Grifo experienced. He said the U.S. Food and Drug Administration pulled the plug on his research in 1999 because the procedure was similar to controversial cloning techniques, even though the embryo would not have resulted in an exact copy of one parent.

Robert Lanza, medical director of Worcester, Massachusetts-based Advanced Cell Technology says the science involved may differ somewhat from cloning, but the same dangers exist.

"It's the same procedure as used for cloning, therefore it could pose many of the same risks, such as developmental abnormalities and genetic defects," he said. "I think the FDA and most medical scientists, including myself, would very strongly counsel against using this technology in humans."

Lanza said he would support using the research once such techniques are proven to produce more positive results.

"As soon as cloning technology is safe, this technology would be safe," he said. "You could use the cloning procedure to generate a child without it being an identical copy of one of the parents. It would be the natural mix just like it would occur through normal sexual reproduction."

The Japanese research involved removing the ovaries from mice fetuses and placing them in test tubes. The immature eggs were isolated and cultured for 28 days. The nuclear DNA material was then removed from the immature eggs and transferred into mature eggs.

The eggs were then fertilized and the embryos were inserted into the surrogate mothers to develop normally.

Hatada also said the research could help save endangered species by inserting eggs into animals in the same family. Similar techniques have already been used by Lanza's firm when it sponsored research in 2001 for cloning an endangered Asian ox called a gaur. A cow gave birth to the animal, which died two days later of dysentery.

Copyright 2002 The Associated Press. All rights reserved.