The human oocyte. Genetic aspects.

Since the beginning of in vitro fertilization (IVF), basic research has enlightened the field of human reproduction, especially in genetics. Indeed, the contribution of chromosomal anomalies to oocyte disorders and impaired developmental capacities of the embryos is now well known. Among oocytes that failed to fertilize after in vitro insemination, 26.5% were found to be abnormal comprising 13.3% hypohaploidy, 8.1% hyperhaploidy, 1.6% structural anomalies and 3.5% diploidy. The total incidence of abnormalities seems to be correlated to the female status, and was found to be higher in oocytes from women with tubal or unexplained infertility than in those from women whose husband was infertile as a sole cause of couple infertility. Although few oocytes recovered during a natural cycle were studied, gonadotropins widely used to stimulate follicle growth and ovulation do not increase the risk of anomalies. The effect of maternal age on fetal aneuploidy, well documented at birth, has not yet be found unambiguously to be a consequence of an increased rate of aneuploid oocytes. Intra- and extrafollicular influences (perifollicular microvasculature, oxygenation, the presence of residues from cigarette smoke) are able to disturb maturation leading to immaturity and aneuploidy. To conclude, oocyte meiosis is very sensitive to endogenous or exogenous factors, which could lead to chromosomally abnormal oocytes and as a consequence, to abnormal zygotes.