Women who want to conceive but cannot often find it an emotionally wrenching experience. It has long been thought that women are born with a limited number of eggs. Imagine if they could produce new eggs throughout life just as men create sperm continually. A new study suggests there is a way to replenish this limited reserve.

Researchers have isolated stem cells in adult human ovaries that could possibly become eggs. These same scientists first isolated stem cells in rat ovaries in 2004, called oogonial stem cells or OSCs. OSCs have a protein on their surface called DDX4.

Using fluorescent antibodies that bind to this marker protein, they isolated OSCs from mouse ovaries using a cell sorting machine. These cells not only went on to become eggs, they were fertilized with sperm and produced embryos.

Recently the research team applied the same technology to donated human ovaries from Japanese women undergoing sex reassignment surgery. Just as with mice, the team was able to isolate OSCs which, when grown in a Petri dish, became immature eggs, called oocytes. When implanted in mice that had human ovarian tissue grafted inside, they matured into eggs. The eggs also expressed several surface markers that confirmed they were indeed eggs.

Since US laws forbid fertilization of human eggs for use in research, it wasn't possible to tell whether the eggs could actually develop into embryos. Even if they can, it will be some time before we'd know whether they'd produce healthy babies. What this study tells us is maybe this new technology can stimulate new egg production for infertile women.

What is most provocative about this study is the suggestion that perhaps women retain the potential to produce eggs later in life, which goes against what scientists have always believed - that egg production is lost soon after birth.