Genetic analysis of the oocyte--a review.

Chromosome number abnormalities are remarkably common in human reproduction. Most are caused by chromosomal non-disjunction and premature chromatid separation in oocyte meiosis I. Pooled data from previous studies showed that one in five oocytes that failed to fertilize after in vitro insemination was abnormal when analysed by conventional cytogenetics. Preconception genetic diagnosis, carried out on the first and second polar bodies by FISH, using 5 chromosome-specific probes (13, 16, 18, 21 and 22), showed that the rate of aneuploidy is higher in women aged 35 or over (52.1 per cent). Oocyte dysmorphy seems to have little effect on the rate of aneuploidy except for giant oocytes, which are usually diploid and may cause triploidy after fertilization. Intra- and extrafollicular influences (perifollicular microvasculature, oxygenation, the presence of residues from cigarette smoke) may disturb maturation, leading to immaturity and aneuploidy. Thus, oocyte meiosis is very sensitive to endogenous and exogenous factors that may cause the production of oocytes with chromosomal abnormalities and therefore, of abnormal zygotes.