The answer is in the chromosomes
Female fertility declines with age because of the way chromosomes are shared during cell division in the female sex cell, the egg, says a new study published in the journal Current Biology.
Looking into egg cell division
Age-related female infertility problems might spring from a specific defect in eggs, suggests the new study conducted by a team of researchers from the University of Montreal Hospital Research Center (CRCHUM). Using technologically-advanced microscopy techniques, the scientists looked into the cellular division in eggs of older mice. They were able to record a very particular error occurring in cell division in the sex cells (the eggs), resulting in mistakes in the sharing of chromosomes. Interestingly, the same defect can be found in the eggs of humans as well. Such a feat has been accomplished for the first time ever.
When a cell divides into two to form two new cells, the chromosomes of the first cell are shared equally between the latter. The chromosomes move in a fashion that allows for this to happen. The problem that was observed by the researchers entails this very process. It was found that the cell structure along which chromosomes are displaced, the microtubules, were behaving in an unusual manner in eggs that were older. Normally, the microtubules should form a spindle in a controlled symmetrical way. However, in older mice, the microtubules were moving in all directions. This apparently led to errors in chromosome sharing between the two new cells as the parent cell is dividing. Furthermore, spindles made from disorderly microtubules were found in around half of the eggs of older mice.
FitzHarris and his team add that spindle defects are not restricted to mice: rather, they extend to human females as well.
According to study author Greg FitzHarris, their findings constitute a new explanation for female infertility problems that arise with age. As the chromosomes are not moved in the proper manner, and thus not shared equally, some eggs have an abnormal number of chromosomes. These eggs are called aneuploid eggs, and they are increasingly being found in older women. This not only hinders the way for pregnancies but also increases the risk for miscarriage, and Down’s syndrome. FitzHarris says that this is a “key reason” for the troubles faced by older women to conceive, and to carry on with full-term pregnancies.
The authors conclude that their research might pave the way for new fertility treatments, hoping that egg defects might be reversed someday.