We have consistently believed that a woman is born with all the eggs she will have during her lifetime. Over every cycle eggs are produced and either fertilised or lost but there is a finite number, or so we thought.
Now there is a new theory which suggests that the ovaries do continue to produce eggs during adulthood – something that has never been considered before and could offer new hope for women. This is based on a new genetic study tracing the origins of immature egg cells, or ‘oocytes’, from the embryonic period right on into adulthood. It has been a given up to now that oocytes cannot be renewed in mammals after birth – you have what you were born with and no more. This leads women to being very aware that they have a timeframe in which to conceive and give birth and the older you are then the louder that biological clock can be heard ticking.
This new theory is a joint effort by scientists from Massachusetts General Hospital and the University of Edinburgh who assessed data from a recent study published in PLoS Genetics. They controversially believe that the findings support formation of new eggs during adult life. Eggs are formed from progenitor germ cells that exit the mitotic cycle, thereby ending their ability to proliferate through cell division, and subsequently enter meiosis, a process unique to the formation of eggs and sperm which removes one half of the genetic material from each type of cell prior to fertilization.
While traditional thinking has held that female mammals are born with all of the eggs they will ever have, newer research by Shapiro has demonstrated that adult mouse and human ovaries contain a rare population of progenitor germ cells called oogonial stem cells capable of dividing and generating new oocytes. Using a powerful new genetic tool that traces the number of divisions a cell has undergone with age (its ‘depth’) Shapiro and colleagues counted the number of times progenitor germ cells divided before becoming oocytes.
If it were true that a woman is born with all her eggs then all divisions would have occurred prior to birth. This would mean that all oocytes would exhibit the same depth regardless of age. However, the opposite was found — eggs showed a progressive increase in depth as the female mice grew older.
Shapiro’s work (published recently in a PLoS Genetics Perspective article) was taken further by reproductive biologists Dori Woods, Evelyn Telfer and Jonathan Tilly. They have concluded that the most plausible explanation for these findings is that progenitor germ cells in ovaries continue to divide throughout reproductive life, resulting in production of new oocytes with greater depth as animals age.
Although these investigations were performed in mice, there is emerging evidence that oogonial stem cells are also present in the ovaries of reproductive-age women, and these cells possess the capacity, like their mouse counterparts, to generate new oocytes under certain experimental conditions. This does offer hope that postnatal oocyte renewal could be contributing to the reserve of ovarian follicles available for use in beyond those a woman is born with.
Clearly this is early stage research but does offer the future possibility that the biological clock could have a few more years than previously thought before it runs down. However, for women wanting to increase their chances of conception and a viable pregnancy the best option is still to ensure good progesterone levels as that is the most important element as its name suggests – pro meaning for and gest meaning life, it is about creating life.
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