Study shows eggs survive in older ovaries

In research that could have broad implications for women's fertility treatments, scientists have found that despite their age, female mice have a renewable egg supply in their ovaries.

The discovery, by Associate Professor Jeff Kerr from Monash University's Department of Anatomy and Cell Biology and Professor Jock Findlay from Prince Henry's Institute of Medical Research, has sparked controversy among biologists and challenged the theory, held for more than 50 years, that female mammals are born with a finite number of oocytes(eggs).

Two years ago, international researchers speculated that mice could continue to produce eggs throughout puberty and adulthood. Although their speculation caused debate throughout the scientific community, the scientists could not produce evidence to confirm their idea. However, Dr Kerr and Professor Findlay's research gives support to the theory. Their findings have been published in the July issue of Reproduction.

In the mammalian ovary, reproductive cells called oocytes (eggs) develop within ovarian follicles. In humans, the eggs are believed to die off from late in foetal life, after birth and into adult life. When egg numbers decline towards zero females can no longer reproduce -- resulting in the condition we know as menopause.

Dr Kerr, Professor Findlay and their colleagues have found that the total number of eggs in young and normal healthy adult female mice do not decline over time and that overall egg number is maintained for longer than previously thought. Their research suggests that mice have a source of renewable oocytes, Dr Kerr said.

"The mechanism behind renewable oocytes is still unknown," he said. "Although other scientists have suggested that the new eggs come from stem cells in the bone marrow or the ovary, we really don't know and further experimentation is needed to find out."

Professor Findlay said the phenomenon of egg regeneration in mice did not necessarily mean the same happened in humans. "But the mechanism could provide direction for ovarian stem cell research and help women with fertility conditions," he said.

One in six Australian couples faces some form of infertility. Due to the limitations and sensitivity of human ovaries, few studies have been conducted into the factors that influence egg survival, growth or death in relation to fertility.