Women may make new eggs throughout their reproductive yearschallenging a longstanding tenet that females are born with finite supplies, a new study says. The discovery may also lead to new avenues for improving women's health and fertility.

A woman has two ovaries, which release eggs during her monthly ovulation.(Learn more about the human body.)

Previous research had suggested that a woman is born with all the egg cells she will ever have in her lifetime.

But in recent experiments, scientists discovered a new type of stem cell in the ovaries thatwhen grown in the labgenerates immature egg cells.The same immature cells isolated from adult mouse ovaries canturn into fertile eggs.

Stem cells,found in embryos and certain adult body tissues, have the potential to grow into many different types of cells.

(See"Liposuction Fat Turned Into Stem Cells, Study Says.")

The finding reinforces the team's previous experiments in mice, which had identified a new type of ovarian stem cell that renews a female mouse's source of eggs throughout its fertile years.

That study, published in the journal Nature in 2004, was the "first to reach the conclusion that this long-held belief in our fieldthat young girls are given a bank account at birth that you can no longer deposit eggs to, just withdraw fromwas no longer true," said study leaderJonathan Tilly.

By reinforcing these earlier results in people, the new study is a "big step forward" from the mouse work, emphasized Tilly, director of the Vincent Center for Reproductive Biology at Massachusetts General Hospital in Boston.

From a purely biological perspective, the concept that a woman would continually generate new eggs during her reproductive years makes sensesince men constantly replenish their sperm, Tilly added. (Read how men produce 1,500 sperm a second.)

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Women Can Make New Eggs After All, Stem-Cell Study Hints

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