A leading cause of infertility is a defect in the woman’s egg (oocyte) that causes it to have the wrong number of chromosomes. These so-called ‘aneuploid’ oocytes cause developmental conditions such as Down’s, and can lead to pregnancy loss. These errors become increasingly common with advancing maternal age, and are therefore a major reason why older women experience difficulties establishing a healthy pregnancy.
The part of the cell responsible for maintaining the correct chromosome number is the spindle – a complex biological machine that gathers and sorts chromosomes at the time of cell division, and dispatches them to the two daughter cells in a process termed chromosome segregation. Why chromosome segregation often goes wrong in oocytes is poorly understood. Once the oocyte is fertilized the first several cell divisions in the newly forming embryo are also highly prone to chromosome miss-segregation. These embryonic errors are also associated with decreased fertility, but their causes and consequences remain obscure.