In what could be a major breakthrough for fertility treatment, unlimited egg supply for women may be a step closer following new stem cell research.
The study, published in the journal Nature Medicine, challenges the long-held belief that women are born with a finite supply of eggs that will be exhausted by menopause.
Scientists isolated stem cells in the ovaries of reproductive age women and found they can produce immature eggs (oocytes) in laboratory conditions – and further tests on mice revealed the eggs could be fertilised.
The study is not the first to dispute the notion that a woman’s egg supply is gradually depleted, running out by middle age.
Lead researcher Dr Jonathan Tilly, from Massachusetts General Hospital, first questioned this theory in 2004 in a study that revealed mice have some egg-producing stem cells.
Dr Tilly said: "The primary objective of the current study was to prove that oocyte-producing stem cells do in fact exist in the ovaries of women during reproductive life, which we feel this study demonstrates very clearly."
Researchers found the stem cells, which go on to produce oocytes, by searching for a protein that is unique to the surface of the stem cells.
Oocytes are immature female sex cells that mature to give rise to a fully mature egg cell.
The researchers grafted human ovarian tissue around the stem cells inside mice and found they “spontaneously generated” egg cells that looked and acted like oocytes.
Tilly added: "The discovery of oocyte precursor cells in adult human ovaries, coupled with the fact that these cells share the same characteristic features of their mouse counterparts that produce fully functional eggs, opens the door for development of unprecedented technologies to overcome infertility in women and perhaps even delay the timing of ovarian failure."
Rachel Cutting, Chairman of the Association of Clinical Embryologists, told The Huffington Post:
"The study is very exciting as it shows that eggs can be made from stem cells in the ovary which completely changes our understanding that women are born with a finite number of eggs.
“The study not only shows that cells form with oocyte properties but in mice these cells could be fertilised as well. If developed further, the technique could go a long way to help infertile women overcome problems such as early menopause and preserving fertility before cancer treatment.
“Whilst ACE supports all research which helps overcome infertility, further studies are needed to look at how the fertilised eggs from the research develop as embryos and to check the safety of the technology."
Dr Allan Pacey, a fertility expert at the University of Sheffield, told the BBC: "This is a nice study which shows quite convincingly that women's ovaries contain stem cells that can divide and make eggs.
"Not only does this re-write the rule book, it opens up a number of exciting possibilities for preserving the fertility of women undergoing treatment for cancer, or just maybe for women who are suffering infertility by extracting these cells and making her new eggs in the lab."