Aneuploidy in Human Oocytes: Some Facts and Questions

Studies on the origin of aneuploid conceptions have indicated that many chromosomal abnormalities arise during maternal meiosis. Therefore, the direct cytogenetic analysis of female gametes is of interest to evaluate underlying mechanisms. Human oocytes failing to fertilize after the application of assisted reproductive technologies have predominantly been used for this purpose. In addition, indirect data can be obtained from the examination of the first polar body that reflects the chromosomal constitution of the oocyte. Both approaches have been conducted with different methods and provided substantial information on the incidence of aneuploidy originating from errors during first meiotic division. However, the results reported so far display great variability, most probably due to technical problems and heterogeneity of the study population. Notwithstanding these limitations, it was unequivocally shown that two aneuploidy-causing mechanisms exist: non-disjunction, resulting in the loss or gain of whole chromosomes, and predivision, resulting in the loss or gain of single chromatids in mature oocytes. All chromosome groups are affected by aneuploidy but abnormalities in groups D, E, and G exceed the expected values. Considering only abnormal complements with one extra or missing chromosome/chromatid reveals a slight increase in predivision compared with non-disjunction. Chromosome and chromatid abnormalities are positively correlated to maternal age. The simultaneous analysis of oocytes and first polar bodies suggests that some cases of aneuploidy are caused by gonadal mosaicism instead of meiotic errors.