Effects of oxygen (O2) tension in the gas atmosphere during in vitro maturation (IVM), in vitro fertilization (IVF) and in vitro culture (IVC) on the efficiency of in vitro production of mouse embryos were examined. Mouse oocytes recovered from large antral follicles were subjected to IVM in Waymouth medium for 15, 16 and 17 hr under 5 or 20% O2 and then subjected to IVF and IVC under 5 or 20% O2 tension. Lowering the O2 tension in the gas atmosphere for IVM from 20 to 5% improved the cleavage rate after IVF when the oocytes were subjected to IVM for 15 hr; however, no improvement in the cleavage rate was observed when the culture period for IVM was extended to 16 and 17 hr. Lowering the O2 tension to 5% for IVM and IVC improved the development of the cleaved oocytes to the blastocyst stage, regardless of the culture period for IVM. However, the O2 tension for IVF had no remarkable effect on the subsequent embryonic development. These results demonstrate that 5% O2 is superior to 20% O2 for IVM and IVC, and suggest that 20% O2 for IVM may delay oocyte maturation and/or the acquisition of fertilizability and impair the developmental competence of oocytes.

Although mammals produce either sperm or eggs depending on their sex, newborn MRL/MpJ male mice contain oocytes within their testes. In our previous study, the testicular oocyte appears as early as day 0 afterbirth and has morphological characteristics as an oocyte such as zona pellucida and follicular epithelial cells. Based on the observation of F1 between MRL/MpJ and C57BL/6, one of the genes causing the appearance of testicular oocyte exists on the Y chromosome. In the present study, we found testicular oocytes within newborn AKR mice. We have also analyzed the Sry genes from several inbred mouse strains and identified a shortened glutamine repeat near the C-terminal region that is unique to MRL and AKR. These results suggest that polymorphism of glutamine repeat within SRY correlates with the appearance of testicular oocyte and this phenotype is derived from AKR, one of the original strains of MRL mice.

Mouse metaphase two (M two) oocytes were exposed to 50 mug/ml etoposide (ETO) before and after parthenogenetic activation with 7% ethanol and they were washed with 0.75 M sucrose. The ETO treated parthenogenetically activated oocytes were cultured or fused to single blastomeres of late 2-cell stage mouse embryo to test their ability to support development in vitro. In parallel untreated parthenogenetically activated oocytes were cultured to serve as control. None of ETO treated oocytes developed beyond the 2-cell stage, whereas 4% of the reconstituted embryos and 35% of control developed to blastocysts. It is concluded that mouse M two oocytes can be functionally enucleated by ETO treatment and can be used for nuclear transfer experiments