"In this unique study, we were able to use the new technology of array CGH to examine the copy number of all 23 pairs of chromosomes, in all three products of female meiosis in over 100 oocytes with abnormal numbers of chromosomes," says Professor Handyside. "What happens in female meiosis is that the 23 pairs of chromosomes duplicate and each pair of duplicated chromosomes comes together and the four single chromosomes, or 'chromatids', become 'glued' together along the whole length of each chromosome. This actually occurs before the woman is born and is the stage at which DNA is swapped between the grandparents' chromosomes.
"Sometimes, decades later, just before ovulation, the glue 'dissolves' first between the two duplicated chromosomes and finally after fertilisation between the two individual chromosomes. This enables pairs of chromosomes to segregate in the first meiotic division producing the first polar body. In the second meiotic division the second polar body is produced, resulting in a single set of chromosomes in the fertilised oocyte or 'zygote', which, when combined with the single set in the fertilising sperm, restores the 23 pairs," he says.
The researchers believe that ovarian stimulation may be disturbing this process in older women because the chromosomes are becoming unglued prematurely, particularly the smaller ones like chromosome 21. Ovarian stimulation uses hormonal medication to stimulate the ovaries to release a larger number of oocytes than normal, in order to provide enough good quality oocytes for fertilisation in vitro.
Following natural conception in older mothers, Down's pregnancies are predominantly caused by errors in the first female meiotic division. "Our evidence demonstrates that, following IVF, there are multiple ch
|Contact: Mary Rice|
European Society of Human Reproduction and Embryology