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Introduction
In vitro maturation (IVM) of oocytes is increasingly demanded in both human-assisted reproductive technology and animal in vitro embryo production (IVP). However, in vitro-matured oocytes have been shown to have lower fertilization and blastocyst formation rates than their in vivo-matured counterparts (Banwell & Thompson, 2008). This low IVP efficiency using in vitro-matured oocytes may be related to a heterogeneous oocyte population at the beginning of IVM (Sirard et al., 2006; Fair, 2009). Thus, preselection of the immature oocytes that are most likely to develop is crucial for improving the IVP efficiency when using IVM oocytes.
Many morphological, cellular, and molecular predictors of oocyte developmental competence have been reported (Wang & Sun, 2006; Bukowska et al., 2012). Several intrinsic markers of oocyte quality are also known to be closely related to the onset of the nuclear maturation (germinal vesicle breakdown, GVBD), such as maturation-promoting factor (MPF) activity (Naito et al., 1992; Christmann et al., 1994; Chesnel & Eppig, 1995; Ledda et al., 2001; Catalá et al., 2011). However, although some reports refer to the relationship between the timing of completion of nuclear maturation (first polar body extrusion or reaching metaphase II: MII stage) and subsequent developmental competence of in vitro-matured oocytes in human and bovine (Dominko & First, 1997; Son et al., 2005), a direct comparison between GVBD timing and immature oocyte quality has never been reported.
Most procedures used to evaluate intrinsic predictors can have adverse effects on oocyte viability and/or subsequent developmental competence with varying degrees. Therefore, the objective of this study was to establish a non-invasive oocyte evaluation method based on GVBD timing for preselecting more developmental competent oocytes in a commonly used mouse oocyte model. First, we investigated the time course of changes in nuclear maturation and preimplantation developmental competence of in vitro-matured oocytes to estimate GVBD timing in high developmental competent oocytes. Second, we compared the preimplantation developmental competence of early and late GVBD oocytes. Because the O2 concentration during IVM may affect the nuclear kinetics (Zeilmaker et al., 1972; Mingoti et al., 2011), all experiments were performed under two distinct oxygen concentrations generally used...