(MENAFN- The Conversation) A woman's fertility usually begins to decline in her mid-30s. This means the chances of becoming pregnant decreases drastically each month.

For a long time, scientists thought that the key culprit behind this rapid decline in fertility was egg quality. This is understandable, since women are born with all of the eggs they will ever have – and as they get closer to the menopause, the number of eggs in the ovary decreases. So too does the quality of these eggs.

But a recent study shows that the ovary's cells and tissues play a bigger role than previously thought in how fertility wanes. This finding could have major implications for how we understand the fundamental process of reproductive ageing and how fertility might be preserved.

Studying fertility has long been difficult to do. Not only is women's health research historically underfunded, it's also difficult to study because the ovaries and ovarian tissues are hard to access.

In such cases, scientists typically use laboratory animals whose biology closely resembles a human's. But it's again tough to do this, given humans are one of only a handful of species that go through menopause. The only other animal species that go through the menopause are certain types of whales - including orcas and belugas.

But while only a few animal species actually go through the menopause, many animals share a similar ovarian biology as humans. This is why the research team began their investigation into reproductive ageing by using mice.

The research team took ovary tissue from young and old mice, and compared it to ovaries from women in their 20s, 30s and 50s. They then used 3D-imaging and compared the gene profiles of the cells in the ovaries to generate detailed maps of the different cell types and their functions across the lifespan.

They found both similarities and differences in ovarian function and ageing between mice and humans. These initial findings were important in confirming which instances mice might be useful as a model when studying human fertility.

They found that older eggs were more similar in humans and mice than younger eggs were. Human and mouse ovaries also contain similar cell types which support the growth of the egg.

In humans, granulosa cells surround the egg and produce oestrogen. Mice appear to have a similar type of cell, which performs a similar function.

However, theca cells, which in humans produce testosterone and stimulate the granulosa cells, appear to operate differently in mice.

The ovary's tissues and cells contribute to the decline in fertility that occurs after 30. Shot4Sell/ Shutterstock

The researchers found evidence that a particular nerve support cell, called a glial cell, is present within both mouse and human ovaries – and that this cell develops early in foetal life. In both humans and mice, the glial cells appear to stimulate the ovary to produce eggs.

They also genetically manipulated the development of glial cells in mice and found the ovaries mimicked what is seen in polycystic ovarian syndrome (PCOS). Doing this led to more early-stage eggs developing in the ovary – but these failed to mature properly. The result provides hope that mouse models could help develop new, much-needed treatments for PCOS.

By comparing ovaries in the old and young, researchers found that the tissue structure in humans and mice differed. Humans develop gaps between the eggs – and the ovary becomes stiffer as more fibrous tissue is laid down, probably due to ovulation and repair of the tissue over a woman's reproductive life.

These changes in the ovary's cells and tissues could explain why human ovaries age relatively earlier than other species. This study's findings also show that it isn't just the eggs, but rather the broader ecosystem of the ovary, that contributes to the decline in fertility that occurs after 30.

Being able to use animal models for research into women's health will advance our understanding of conditions such as PCOS and infertility, which have been historically underfunded and under-researched. It will allow researchers to better study the reproductive diseases which affect women and develop new drugs that can treat these debilitating conditions.

This knowledge improves understandings of the fundamental processes of ovarian ageing which will enable better diagnosis and treatments of infertility.

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