A team at Washington State University has found that immature human egg cells, or oocytes, contain an unbalanced number of chromosomes around seven percent of the time. This suggests that meiosis, the process of division that produces oocytes with half the chromosomes, goes wrong more often than previously thought. This challenges the idea that maternally advancing age is a key cause of chromosome abnormality in eggs.
'We have known for a long time that advancing maternal age increases the likelihood of chromosomally abnormal eggs, but this observation demonstrates that many chromosome errors have nothing to do with maternal age,' says team leader Professor Terry Hassold. 'They are, instead, errors that are extremely common in our species, for reasons that are unclear.'
The research shows just how often meiotic recombination, which precedes cell division in meiosis, goes wrong. At this stage, genetic material from the paternal and maternal copies of each chromosome is exchanged. Recombination failure is a leading cause of aneuploidy, but until now, there had been no attempt to directly measure the incidence of exchangeless chromosomes in a large series of human oocytes.
In the study, published in the American Journal of Human Genetics, Professor Hassold and his collaborators conducted a large population-based analysis of exchangeless chromosomes in the fetal ovary. In total, they examined 7396 oocytes from 160 tissue samples. To determine the overall proportion of human oocytes containing one or more exchangeless chromosomes, they counted chromosome pairs that lacked a particular crossover-associated protein.
'Probably the most surprising observation was simply the high proportion of eggs that contained exchangeless chromosomes,' Professor Hassold said. 'We had known from previous preliminary studies and from trisomic pregnancies that the value would be high, but seeing it directly in human eggs was still a little jarring.'
Moving forward, the researchers will search for genetic variants that may affect the likelihood of having exchangeless chromosomes.