Biochemical alterations in the oocyte in support of early embryonic development

Abstract

Notwithstanding the enormous reproductive potential encapsulated within a mature mammalian oocyte, these cells present only a limited window for fertilization before defaulting to an apoptotic cascade known as post-ovulatory oocyte aging. The only cell with the capacity to rescue this potential is the fertilizing spermatozoon. Indeed, the union of these cells sets in train a remarkable series of events that endows the oocyte with the capacity to divide and differentiate into the trillions of cells that comprise a new individual. Traditional paradigms hold that, beyond the initial stimulation of fluctuating calcium (Ca²⁺) required for oocyte activation, the fertilizing spermatozoon plays limited additional roles in the early embryo. While this model has now been drawn into question in view of the recent discovery that spermatozoa deliver developmentally important classes of small noncoding RNAs and other epigenetic modulators to oocytes during fertilization, it is nevertheless apparent that the primary responsibility for oocyte activation rests with a modest store of maternally derived proteins and mRNA accumulated during oogenesis. It is, therefore, not surprising that widespread post-translational modifications, in particular phosphorylation, hold a central role in endowing these proteins with sufficient functional diversity to initiate embryonic development. Indeed, proteins targeted for such modifications have been linked to oocyte activation, recruitment of maternal mRNAs, DNA repair and resumption of the cell cycle. This review, therefore, seeks to explore the intimate relationship between Ca²⁺ release and the suite of molecular modifications that sweep through the oocyte to ensure the successful union of the parental germlines and ensure embryogenic fidelity.