Scientists discover why it's harder for women in their 40s to get pregnant - egg defects

Fertility in women declines with age, and scientists have found a new explanation for why this happens. They identified a specific defect in the eggs of older female mice, which resulted in these eggs having an abnormal number of chromosomes – and they think women may be going through something similar.

When women reach their late thirties or forties, the likelihood of getting pregnant starts decreasing. This has been linked to the fact that their risk of having eggs with the wrong number of chromosomes is higher – a phenomenon known as aneuploidy.

"For years, we have known that the likelihood of the eggs having the wrong number of chromosomes increases as women get older, and that this increases the risk of infertility and miscarriage. In fact it is a leading reason for infertility in older women. Our paper is about is about understanding why these defects happen", Greg FitzHarris from Université de Montréal professor, lead author of the new study published in Current Biology, told IBTimes UK.

For many years, scientists had tried to find out why the eggs of older women are more frequently aneuploid, without success. But a few years ago, the chromosome cohesion hypothesis emerged. The idea is that as women become older, they progressively lose the cohesin protein which maintains chromosomes together in the dividing egg.

However, the authors of this paper thought other factors may also be responsible, especially since some forms of aneuploidy were not well explained by the chromosome cohesion hypothesis.

Microtubule behaviour

In their study, the scientists worked with older mice, using live imaging techniques and manipulation under the microscope to study their eggs and chromosome segregation during cell division.

They showed that the mice's eggs presented altered microtubule dynamics. Microtubules are tiny cylindrical structures in the egg that organise themselves to form a spindle, to gather the chromosomes together and to sort them at the time of cell division.

The scientists discovered that in the older mice, the microtubules behaved abnormally in about 50% of the eggs. Instead of forming a spindle in a controlled symmetrical fashion, the microtubules went in all directions. This led to subsequent errors in chromosome segregation.

As such, abnormal microtubule behaviours in older mice represent a novel explanation for aneuploidy and age-related infertility – one that is distinct to the chromosome cohesion hypothesis.

Although this research has so far only been conducted in mice, there is a real possibility the findings could also apply women. These are still early days, but in the future, they may help improve the way older women are treated for infertility.

"Maybe one day in the very distant future our findings could help women who suffer from infertility, perhaps by allowing us to prevent aneuploidy in eggs from older females", FitzHarris concluded.