The biochemistry of oocyte maturation.

Abstract

Oocyte maturation is a complex process that encompasses growth and differentiation of the oocyte, acquisition of meiotic competence, initiation and completion of nuclear maturation, and cytoplasmic changes related to fertilizability and post-fertilization developmental potential. The oocyte develops within a syncytial environment, being intimately coupled to the entire somatic compartment of the follicle via granulosaoocyte and granulosa—granulosa gap junctions (Larsen and Wert 1988). This condition provides an efficient delivery system through which small molecular weight nutritional and regulatory signals can reach the developing oocyte. As the oocyte grows, it continuously stockpiles macromolecules needed for later development, and near the end of its growth phase, the oocyte completes the biochemical changes required to achieve meiotic competence, a condition that supports the resumption of meiosis beyond prophase I. Once oocytes achieve meiotic competence, they are maintained in a prophase I-arrested, germinal vesicle stage until stimulated to resume maturation, either by the preovulatory gonadotropin surge or by follicular atresia. The former stimulus produces developmentally competent ova while the latter condition leads to degenerating oocytes trapped within anovulatory follicles. Meiotic progression beyond the germinal vesicle stage requires the activation of maturation, or M phase, promoting factor (MPF), a heterodimeric protein complex composed of a 34-kDa kinase and 45-kDa cyclin regulatory subunit. Control of the activity of MPF is an essential component of the system regulating both meiotic arrest and meiotic resumption, since this kinase is the driving force behind cell division. Following meiotic resumption (initially manifested by germinal vesicle breakdown), maturation proceeds through extrusion of the first polar body and formation of the second metaphase spindle (MII), where the oocyte arrests for a second time until activated by fertilization. For the purpose of this review, maturation will be defined as the process of meiosis reinitiation; the focus will be on regulation of the immature, germinal vesicle stage, with particular emphasis on how the oocyte is stimulated to resume meiotic maturation.