However, hope remains as the Chinese team detected for the first time an oocyte stem cell in the ovary of a living adult mammal. An oocyte is an immature ovum, or female reproductive cell that can divide and develop into an embryo after it is fertilised by a male cell.

Prior to this, women were believed to have a fixed number of oocytes - about 750,000 - at birth.

As they require so many activated oocytes to produce an egg during their monthly menstrual cycle, scientists assumed that women’s lifetime reserves were so badly depleted by the time they reach 50 that it resulted in them becoming infertile.

But the research team led by Zheng Ping, a professor at the Chinese Academy of Sciences’ Kunming Institute of Zoology in southwest China’s Yunnan province, is now challenging that assumption.

Zheng used transgenic techniques - the introduction of foreign DNA into a host organism’s genome - to label the stem cells responsible for producing the oocytes with a yellow florescent protein so they could later be traced in mice.

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She found that the same stem cells were still present in the mice’s ovaries deep into adulthood. Moreover, some had grown into developed oocytes to replenish the declining reserve.

What marks this discovery out as important is the fact that the stem cells were detected in a living mammal. In contrast, previous studies only identified them in ovarian tissue cultured in test tubes.

WATCH: What happened when similar stem cells were cultured in test tubes in 2012 (Nature Video)

“This is a huge discovery,” said Professor Wang Yanling, who researches pregnancy related disorders at the State Key Laboratory of Stem Cell and Reproductive Biology in Beijing. Wang was not involved in Zheng’s research.

One implication of the finding by Zheng’s team is that the egg-making stem cells found in the ovaries of the mice could be “revived” by new drugs or therapies to produce enough new oocytes to restore the reproduction capability of an ageing woman, said Wang.

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The stem cells were not detected earlier because their numbers are so small, Wang added.

The finding offers “a fresh look at dogma”, according to professors Richard Anderson and Evelyn Telfer, two reproductive biologists at the University of Edinburgh in Scotland.

“It seems increasingly likely that we may now be seeing the dawn of a new understanding of the basis of female fertility,” they wrote in a commentary to the study, which was published on the same issue of the internationally peer-reviewed journal.

Clarifying the existence of active egg-making stem cells at such an early stage - as well as their functional significance in adult mammalian ovaries - could provide new insights about how any why the ovaries stop working as women age, Zheng’s team wrote in the paper.

But the researchers stressed that more studies are needed as their experiment was restricted to mice hosts.

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“Whether or not the results are applicable to humans remains unclear,” they wrote.

Wang said that determining this would likely be difficult because the transgenic tracing technology used in the experiment on the mice may be off-limits for humans on ethical or other grounds.

Researchers also need to evaluate the potential risks after the stem cells are revived, she said, as they could develop the wrong way and possibly even mutate into harmful cells like tumours.

Other risks include a greater chance of birth defects occurring as the quality of the eggs produced naturally declines with age. Other studies show that older women are more likely to encounter complications during labour and delivery.