A new study looking at the regulation of egg production in mice ovaries has identified a defect in 'quality control', which may help scientists understand why some women have a miscarriage during pregnancy.
Researchers at Washington State University in Pullman in the USA observed cell division, called meiosis, in immature eggs from mice. They found some eggs have an incorrect number of chromosomes and believe this phenomenon is also likely to be seen in humans.
Sperm or egg cells are formed through meiosis from a parent cell. During early meiosis, pairs of chromosomes align at the middle of the cell, called the meiotic spindle. The spindle assembly checkpoint (SAC) is a chemical signal that prevents cell division until all of the chromosomes are aligned.
The researchers found that in mice ovaries, the SAC trigger waits for a critical mass of chromosomes to be aligned rather than requiring that all the chromosomes align. This causes some eggs to have too many or too few chromosomes.
An abnormal number of chromosomes is a major risk factor for age-related miscarriages and birth defects, such as Down's syndrome. Dr Patricia Hunt, who led the study, said: 'We think that by the time a woman is in her 40s, about half the eggs she's ovulating are probably chromosomally abnormal. And for women in their 20s, it’s probably about ten percent. So it's a huge change'.
Dr Hunt believes the human body may allow defective eggs to divide because humans are evolutionarily programmed to prevent the loss of these precious cells. 'It is better to try and fail than to simply give up on an egg before it is even fertilised', she said.
Iha Nagaoka, co-author of the study, believes a screening method could be included in IVF treatment to check for abnormal eggs and help reduce the risk of miscarriage.
One in five pregnancies ends in miscarriage and the risk increases with mother's age.