Every egg a woman will produce begins to form before she is born. However, meiosis then pauses in prophase I, sometimes for several decades, before it restarts. What happens during that wait?

Slowly, things start to go wrong, and it becomes more likely that eggs end up with the wrong number of chromosomes, a problem called aneuploidy. It turns out that it is more common in human egg cells (oocytes) than in those of mice, pigs or cows and is one of the reasons why fertility declines with age. Prof Melina Schuh’s team at the Max Planck Institute in Göttingen has now worked out some of the molecular reasons, and new biotechnology companies are hoping to develop treatments for age-related infertility.

Meiosis halves the chromosome number from 46 to 23, so that fertilisation restores it to 46. In meiosis I the homologous chromosomes are pulled apart by spindle fibres, and in meiosis II the two chromatids of each chromosome are separated. If either step fails, this is non-disjunction: the egg has 22 or 24 chromosomes and the embryo ends up with 45 or 47.

A pair of chromatids is held together at the centromere by cohesin, a ring-shaped protein complex that works like a microscopic cable tie. There is another protein called Shugoshin 1 that is named after the Japanese for 'guardian spirit' because it protects the cohesin until the cell is ready to cut it and allow the chromatids to separate. But the problem is time: during those decades of waiting, Shugoshin 1 and cohesin levels gradually decline, so chromatids can drift apart too early. Once separated, they can be shared out unevenly when the cell divides, leaving the egg with too many or too few chromosomes.

A 2026 preprint article (Saha et al., bioRxiv) showed that older mouse and human eggs had less of the Shugoshin 1. When extra Shugoshin 1 mRNA (with instructions for building the protein) was injected into donated human eggs, the proportion of eggs showing premature separation of chromatids fell from about 53% to 29%. The effect was clear in women under 35. In women aged 35 and over it fell from about 65% to 44%, but only 9 eggs were tested, which is too few to be statistically significant, meaning the change could be due to chance.

It’s an exciting news story, but read everything with a critical eye. This study is a preprint, so other scientists have not yet reviewed it, and it used a small number of donated eggs and measured chromatid separation, not healthy babies. It is a promising first step, but still some way from a treatment.

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