A woman's 'biological clock' refers to the fact that a woman's oocytes, i.e. immature egg cells progressively decline with age. For decades, researchers have believed that oocytes cannot be renewed in mammals after birth, a view that has created controversy in recent years.

PLoS Genetics reports on an interesting new genetic study that traces the origins of immature egg cells from the embryonic period throughout adulthood, which now adds new information to the growing controversy. Researchers from Massachusetts General Hospital and Edinburgh University conducted a comprehensive assessment of data from a recent study, which supports that new eggs are formed during adult life, adding to the rising debate over the topic in recent years.

Progenitor germ cells that exit the mitotic cycle are responsible for the formation of eggs. The mitotic cycle ends their ability to proliferate through cell division, and subsequently enter meiosis, which is a unique process in the formation of eggs and sperm, whereby one half of the genetic material from each type of cell is removed prior to fertilization.

According to traditional beliefs, female mammals are born with the amount of the eggs they will have during their entire life, but recent research has provided evidence that the ovaries of adult mice and humans contain a rare population of progenitor germ cells (oogonial stem cells) that are capable of dividing and producing new oocytes. Shapiro and his team counted the number of times progenitor germ cells divided before becoming oocytes by using a powerful new genetic tool that traces the number of divisions a cell has undergone with age, i.e. its depths in a study published in PLoS Genetics in this year's February issue.

Also, all divisions would have occurred prior to birth, and therefore all oocytes would have the same depth irrespective of age, but researchers found the opposite to be true. They discovered that the eggs in female mice progressively increased in depth with age.

Reproductive biologists Dori Woods, Evelyn Telfer and Jonathan Tilly concluded on Shapiro and his team's study, which was recently published in a PLOS Genetics Perspective article, that the most plausible explanation for these findings is that progenitor germ cells in ovaries continue to divide throughout reproductive life, and produce new oocytes with greater depth as the animals age.

Even though the findings were based on an animal study, evidence is emerging that oogonial stem cells are also present in the ovaries of reproductive-age women, and that these cells are able under certain experimental conditions, just like those in their mouse counterparts, of producing new oocytes.

Woods and his team state that further studies are required to settle the debate over the question whether oocytes are renewable in adult mammals, pointing out that, "the recent work of Shapiro and colleagues is one of the first reports to offer experimental data consistent with a role for postnatal oocyte renewal in contributing to the reserve of ovarian follicles available for use in adult females as they age."

Written by Grace Rattue