Hunting the source of renewable oocytes

In research that could have broad implications for women's fertility treatments, Melbourne 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's Department of Anatomy and Cell Biology and Professor Jock Findlay from Prince Henry's Institute of Medical Research, has sparked controversy by challenging the long-standing theory that female mammals are born with a finite number of oocytes.

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, Kerr and Findlay's research gives support to the theory.

In the mammalian ovary, oocytes 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 are in menopause.

Kerr, 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, 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."

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.

Their findings have been published in the July issue of Reproduction.