Abstract
An oocyte undergoes several stages of development to achieve a state of viability. This happens over a prolonged period, starting during folliculogenesis and completing shortly after ovulation. The timing, progression and quality of the development are all determinants of its capabilities of further developing into a viable embryo. As such, identifying a component which measures the capability of an oocyte to form an embryo, is an important mystery to be solved. In particular, in vitro fertilisation (IVF) could benefit from identifying the best oocytes early on, to ensure that success rates are as high as possible. Techniques such as superovulation, the production of more oocytes through exogenous hormones, produces lower quality oocytes. The ‘Quiet Embryo Hypothesis’, created by Henry Leese, suggests that lower quality embryos would have a higher metabolic output, with mitochondria working harder to generate more energy. Knowledge of how this applies to oocytes is limited as metabolism between oogenesis and embryogenesis is rarely studied. To try and draw a conclusion between the two stages of development, quantification of the metabolic output of both superovulated and naturally ovulated oocytes was undertaken. This project aimed to identify if there is a relationship between oocyte metabolism and the chances of it fulfilling the quiet embryo hypothesis, as well as determining if there are metabolic differences between the two treatments.
The project utilised mice that had exogenous gonadotrophin stimulation, the superovulated group, and mice that had their oestrus cycle tracked to obtain oocytes at ovulation, the naturally ovulated group. The oocytes were stained with vital dyes, MitoTracker Green, TMRM and Hoechst Blue, to identify the mitochondria and nuclear DNA. Quantification of the image fluorescence was conducted via ImageJ software. This project aimed to identify the mitochondrial DNA number through quantitative PCR (qPCR), but issues in reliability led to abandonment of qPCR. Instead, MitoTracker Green was used to measure mitochondrial number. The metabolism between naturally ovulated and superovulated oocytes was significantly different, with naturally ovulated oocytes having a higher overall metabolism compared to the superovulated oocytes. This suggests that having a quieter metabolism does not correlate to a higher viability. Five different metabolic phenotypes were identified, and there were fewer superovulated oocytes within the normal phenotype. This goes against the hypothesis that there is a linkage between the quiet embryo hypothesis and oocyte metabolism, suggesting that the link between embryogenesis and oogenesis is not as similar as believed.