Abstract
Infertility affects 15% of people of reproductive age globally. With more individuals suffering from infertility, assisted reproductive technologies (ARTs) such as in vitro maturation and in vitro fertilisation are used to achieve clinical pregnancies. However, success rates for these technologies are disappointingly low. In New Zealand, of the total cycles of ARTs in 2018, 22.7% resulted in live births.
The viability of embryonic development is dependent on the quality of the oocyte. Oocyte quality relies on the correct distribution, morphology, and number of cytoplasmic organelles to achieve successful maturation, fertilisation, and embryogenesis. In animals, adult oocytes are more developmentally competent than juvenile oocytes. Past literature showed that mitochondria in adult sheep and cattle oocytes alter their morphology from round to ‘hooded’ during maturation. In a paper analysing the cytoplasmic differences between adult and prepubertal oocytes in sheep, adult oocytes showed an increase in hooded mitochondria over maturation, beginning with a mean percentage of 58.3% and increasing to 75.7% by the end of maturation. Conversely, prepubertal oocytes showed a decrease in the mean percentage of hooded mitochondria over maturation, beginning at 77.7% and decreasing to 63%. We hypothesise that these differences in adult and prepubertal oocyte competency are due to mitochondrial morphological differences throughout maturation. Therefore, inducing these changes in mitochondria using
mitochondrial inhibitors may be beneficial. This project aimed to determine if altering mitochondrial function could result in changes to morphology and, therefore, oocyte quality in hopes of improving in vitro maturation.
Sheep oocytes were treated with mitochondrial inhibitors FCCP and oligomycin during in vitro maturation. Changes to the ultrastructure and activity of mitochondria were observed to understand their effects on oocyte competency. Transmission electron microscopy was used to detect differences in mitochondrial morphology, and confocal microscopy paired with the fluorescent stain TMRM was used to measure mitochondrial membrane potential (MMP) or activity.
Oligomycin decreased the proportion of hooded mitochondria (p=0.04) and increased MMP (p<0.001), whereas FCCP decreased MMP (p<0.001). However, FCCP did not alter the proportion of hooded mitochondria (p=0.25). Neither inhibitor affected cleavage rates in adult oocytes.
Collectively, the results from this study suggest that a reduction in the proportion of hooded mitochondria correlates with increased mitochondrial activity. However, the cleavage rates were not altered. Future experiments need to be conducted to verify the function of changes in mitochondrial morphology and their effect on oocyte quality to develop a method to improve in vitro maturation.