ITHACA, N.Y.  —  Researchers at Cornell University have grown human ovaries complete with human eggs in laboratory mice.

It's part of research aimed at discovering why humans have such a high rate of birth defects and miscarriage compared to other mammals.

Researchers can't explore the issue by working directly on humans, and lab animals are of limited use unless they take on human qualities.

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The mice are 20 days old when a piece of human ovary from a fetus is placed under their skin. By last fall, the lab was able to show the ovaries could grow in the mice for up to 24 weeks and produce eggs similar to those found in adult women.

Geneticist Paula Cohen noted that it would not be ethical to fertilize the human eggs produced in the Cornell mice.

"Humanized" mice have become common in labs. At Upstate Medical Center, a researcher is studying leukemia treatment in mice that have human immune systems.