Getting into and out of oocyte maturation

Abstract

Introduction The developmental journey from primordial germ cell to a mature oocyte (or egg) is unusually protracted, beginning during fetal life and concluding during postnatal life. In humans the entirety of this process can last a staggering four to five decades for those oocytes that are ovulated towards the end of a woman's reproductive lifetime. Two major components of this journey characteristic of most mammals, including humans, are firstly, an extended phase of oocyte growth leading to a 100- to 200-fold increase in oocyte volume and secondly, meiotic maturation which occurs after the fully grown oocyte re-awakens from a protracted arrest at prophase I (equivalent to G2-phase of the cell cycle).Prophase I-arrested oocytes are characterized by the presence of an intact nucleus, the enlarged nucleus in oocytes with diffuse and weakly staining chromosomes being referred to as the germinal vesicle (GV). After prophase I arrest is lifted, oocytes re-enter and complete meiosis I (MI) before progressing uninterruptedly into meiosis II (MII) where a second arrest is imposed at metaphase II. Re-entry into MI is marked by GV breakdown (GVBD) after which the oocyte undergoes the first (or reductional) nuclear division when recombined homologous chromosomes (or bivalents) segregate followed shortly thereafter by exit from MI, evidenced by first polar body extrusion (PBE). The metaphase II arrest state that follows PBE is only broken if fertilization occurs or if oocytes are artificially activated by a parthenogenetic stimulus. Ultimately, this results in the completion of MII and, given the absence of DNA replication between MI and MII, the halving of the DNA complement, thereby allowing the diploid state to be restored in the zygote post-fertilization.