Oocyte Meiotic Maturation

Oocyte maturation stands uniquely at the center and culmination of female reproductive biology. This process is driven by both the oocyte intrinsic developmental programs and the gonadotropins secreted by pituitary. This chapter summarizes features and regulations of three interconnected four aspects of oocyte maturation: (1) nuclear maturation, (2) cytoplasmic molecular maturation, (3) cytoplasmic organelle maturation, and (4) epigenetic maturation. Nuclear maturation refers to resumption of meiosis I [also called germinal vesicle breakdown (GVBD)], polar body emission, and cell cycle progression to metaphase of meiosis II (MII). Cytoplasmic maturation includes cytoplasmic processes that prepare the oocyte for fertilization and developmental competence and occur throughout oocyte growth and concurrently with nuclear maturation. This process can be further divided into cytoplasmic molecular changes and organelle changes. Epigenetic maturation involves de novo DNA methylation that silences maternal imprinted genes, exchanges of histone variants, and histone modification that support genome remodeling and gene transcription during oogenesis and preimplantation development. Clinically, in human assisted reproduction, nuclear maturation is easy to be judged by the appearance of first polar body under a microscope, but the degrees of cytoplasmic and epigenetic maturation are difficult to judge. For successful embryo development, synchronous nuclear, cytoplasmic, and epigenetic maturation are required.