As many as 60% of a woman's eggs are thought to be defective by errors in chromosomal division. These errors represent the number-one cause of infertility. When conception does occur, these same errors in the chromosomes can lead to Down syndrome and early spontaneous abortion (miscarriage).

The mechanism causing these chromosome errors has remained a mystery but a team of researchers has identified a glitch in a gatekeeper type of action that seems to play a key role.

Dr. Simon Lane and Professor Keith Jones, of the University of Southampton's Center for Biological Sciences in England, worked with affiliates at the School of Biomedical Sciences & Pharmacy at Australia's University of Newcastle, Callaghan, in New South Wales, in a study funded by the Australian Research Council. The Southampton study involved mouse oocytes. Oocytes are ovarian cells that will divide to produce an ovum (egg).

The research team focused on a process called the Spindle Assembly Checkpoint, or SAC, to determine how the process works in developing female sex cells. All the cells of the body divide but the researchers isolated their study to just the division of the cells that would become an egg.

The SAC process is the gatekeeper. When working properly, it allows only cells with healthy chromosomes to pass through to the process of division. By allowing division of only cells with healthy chromosomes, the process preserves and forwards cellular function and identity. It stops faulty or defective cells from dividing.

If a cell is allowed to divide, though it is missing the SAC or the SAC is defective in some way, the ensuing cellular division can produce cells that carry the wrong number of chromosomes or chromosomes without SAC.

Lane and Jones discovered that when the SAC gatekeeper doesn't go completely from a closed gate to an open gate, chromosome errors can occur. When the SAC gate is left ajar, it doesn't prevent division of defective cells. In this scenario, oocyte division can result in a cell called aneuploid (having too few or too many chromosomes).

Aneuploid cells continue dividing until early miscarriage occurs or a child with chromosome-related disorders is born.

Jones says the discovery doesn't provide an immediate cure for chromosomal error but it does add to the body of knowledge surrounding oocyte chromosome division. He expresses hope that the study will contribute to the development of control mechanisms to eliminate chromosomal errors or to better egg-screening technologies. Improved screening could be highly desirable during IVF but of little relevance to spontaneous pregnancies.

Source: "News release: New understanding of why chromosome errors are high in women's eggs." University of Southampton. University of Southampton. Mar 18, 2014. Web. Mar 23, 2014.