 This is the VOA Special English Health Report. New research could raise hope for women who are having trouble getting pregnant. For years the thinking has been that women are born with all the eggs their bodies will ever produce. In fact, this has been the thinking not just about humans, but all mammals. Then, in 2004, a researcher at Massachusetts General Hospital reported that adult female mice could produce new eggs. Now Jonathan Tilley has extended his work and reported another discovery this time about human females. We've isolated essentially the female equivalent of the stem cells that we know exist in men that actively make new sperm. He says having these cells now isolated opens up a lot of opportunities to consider. Stem cells are special kinds of cells. Starting in embryos just a few days old, they produce the many different cell types and tissues that form the body. In some adult tissues, stem cells produce replacements for damaged or worn out cells. In his new study, Jonathan Tilley and other researchers removed cells from ovaries. Ovaries are the female reproductive organs that produce eggs. The scientists did laboratory tests to make sure they had the right cells that they were looking for. Then they used what are known as culturing methods to grow the cells so they would divide and produce more cells. And we could start with perhaps 100 cells and over several months' time take those 100 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 then placed some of the cells into human ovary tissue and implanted the tissue under the skin of mice. The studies found that these human cells were, in Jonathan Tilley'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 were 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 Tilley says his work could also lead to fertility treatments that might improve the chances for more traditional fertilization methods. The journal Nature Medicine published the findings. Some experts raised questions about his earlier work with mice. Some are not so sure about his study either until other researchers can reproduce the findings. For VOA Special English, I'm Mario Ritter.