Abstract
The quality of an oocyte determines its ability to correctly undergo cytoplasmic and nuclear maturation, and this contributes to the success of the oocyte and future embryo. Advancements in our understanding of oocyte quality are key to improve in vitro maturation, an assisted reproduction technique, currently showing limited success.
It is known that in adult oocytes following in vitro maturation, the proportion of mitochondria taking on a hooded morphology increases and that mitochondria redistribute from a peripheral to a more even distribution throughout the cytoplasm. The mechanism behind this change is currently not defined within the literature, however, research in other cell types have shown similar morphological changes following treatment with inhibitors of mitochondrial function.
Ovine oocytes were examined regarding mitochondrial ultrastructure and how this is related to mitochondrial metabolic activity. Immature adult oocytes were treated with mitochondrial activity inhibitors antimycin-A, oligomycin and FCCP, with each targeting a different region of the electron transport chain. Confocal microscopy was used to analyse changes in oocyte mitochondrial activity following inhibitor treatment, using the fluorescent dye TMRM, a marker of mitochondrial membrane potential (MMP). Transmission electron microscopy was also used to analyse the oocyte organelle ultrastructure and determine if the same inhibitors caused morphological changes matching what is seen following normal oocyte maturation. These changes included an increase in the proportion of hooded mitochondria as well as the redistribution of mitochondria and cortical granules.
FCCP was the only inhibitor that successfully decreased MMP (p = 0.0046). This inhibitor also caused a trend towards a significant increase in the proportion of hooded mitochondria (p = 0.0626). Contrastingly, oligomycin revealed no significant change in MMP and a significant decrease in the proportion of hooded mitochondria (p = 0.0287). Antimycin-A had no significant effect on any parameter tested. Further, both FCCP and oligomycin caused an increase in the density of mitochondria both overall and in the edge regions of the oocyte (p < 0.05) and oligomycin alone, displayed a delay in maturation regarding cortical granule distribution (p = 0.0299).
These results demonstrate that there is a relationship between decreased mitochondrial activity and increased hooded mitochondria in oocytes. It suggests mitochondria uncouple the electron transport chain during oocyte maturation and that this may affect the overall density of mitochondria and cytoplasmic maturation rate. This warrants further investigation using the same mitochondrial inhibitors to determine if these effects are beneficial regarding oocyte/embryo quality. Such research may provide novel insight into ways of improving oocyte mitochondrial function during in vitro maturation and lead to advances in overcoming infertility.