35 - Polar body screening for aneuploidy in human oocytes

Summary

Introduction

Aneuploidy is the most common chromosome abnormality in humans, and is the main genetic cause of miscarriage and congenital birth defects following both natural conception and in vitro fertilization (IVF). It is now known that the majority of chromosome errors originate in maternal meiosis I (MI), maternal age is a risk factor for most human aneuploidies, and defects in recombination are strictly related to meiotic non-disjunction [1].

The possible reason for the increased predisposition to aneuploidy in oocytes when compared with sperm [2] resides in the profound dissimilarity between male and female gametogenesis. While male meiosis is a continuous process, females are born with a complete set of immature oocytes in a quiescent state, and terminate the maturation process in adulthood.

In both genders, the irst step of meiosis is DNA replication followed by two rounds of cell division that lead to the formation of haploid gametes. In prophase I, homologous chromosomes align, form chiasmata, and recombination occurs.At the end ofMI, homologs segregate, whereas sister chromatids separate in meiosis II (MII), the second meiotic division. In spermatogenesis, this process initiates after puberty and continues throughout male lifetime, whilst in the female, meiosis begins during fetal development, arrests at prophase I before birth, and resumes prior to ovulation at the luteinizing hormone (LH) surge. For some oocytes, this last phase may never happen, whereas for others it will only occur ater several years, or even decades later. It is postulated that the long time interval of oocyte quiescence, elapsing between meiotic arrest in the fetus and each ovulation cycle, could be responsible for the increased incidence of aneuploidy.