JONATHAN TILLY: "And we could start with perhaps one hundred cells, and over several months' time take those one hundred cells and make hundreds of thousands of cells. And interestingly, we noticed that these cells would spontaneously generate immature eggs, all on their own, in these cultures."

The researchers did not stop there. They wanted to further confirm the ability of these cells to make eggs. They placed some of the cells into human ovary tissue. Then they implanted the tissue under the skin of laboratory mice.

The studies found that these human cells were -- in Jonathan Tilly's words -- "more than happy to create brand-new human egg cells." He says growing eggs in the lab could improve the chances for women who are having the process known as in vitro fertilization. With IVF, an egg is fertilized with sperm in a laboratory, then placed in the woman's uterus to develop.

Jonathan Tilly says his work could also lead to fertility treatments that might improve the chances for more traditional fertilization methods.

JONATHAN TILLY: "We can also think about, perhaps, targeting these cells in the ovaries with hormones that these cells like, and thereby increasing their activity and perhaps increasing the size of the egg cell reserve in the ovary, when it would be desirable to do so."

His paper on making new human eggs from stem cells was published online this week by the journal Nature Medicine. Some experts raised questions about his earlier work with mice. Some are not so sure about this study either, until other researchers can reproduce the findings.