Assessment of Mitochondrial Number and Function in Oocytes and Embryos

Abstract

Introduction: Infertility is an increasingly common condition that affects many people around the world. The advent of in vitro fertilisation (IVF) has aided many couples in conceiving children, but its success rates remain low. A better understanding of the mechanisms behind the natural variability in female fertility, specifically in the oocyte, is required to improve IVF outcomes. Furthermore, there is a need for reliable and conclusive markers of embryo quality in order to improve success rates with IVF. Currently, there has been interest in the role of the energyproducing organelle, the mitochondria, in in oocyte and embryo quality. Aims: The aim of the current study was to investigate various aspects of mitochondrial biology in oocytes and blastocyst-stage embryos from mice and humans, respectively. This included (i) investigation of the relationship between mitochondrial DNA (mtDNA) content of oocytes and natural fertility levels in mice, (ii) investigation of whether mtDNA content is evenly distributed throughout human blastocysts, (iii) optimisation of a method to directly assess mitochondrial function in mouse oocytes and cumulus cells using the Seahorse XFp analyser, and (iv) determining whether the stress of superovulation affected the expression of genes involved in mitochondrial maintenance, stress, and general competence, in different mouse strains with varying fertility.Results: The mitochondrial content of mouse oocytes was not indicative of their natural fertility. Mitochondrial content in human aneuploid blastocysts was not evenly distributed among fragments of the trophectoderm; however, a positive relationship was observed between mtDNA content in the trophectoderm and blastocyst grade after freezing and thawing. The baseline oxygen consumption rates of mouse oocytes and cumulus cells were too low to accurately utilise the Seahorse XFp analyser for mitochondrial functional assessment in these samples. There was considerable variability in the gene expression profile of cumulus cells from mice of different strains. Conclusion: Variability in mitochondrial content could still be part of the reason for the natural variability in fertility in the human setting. Tests claiming to aid in selection of embryos based on mitochondrial content may be invalid due to a sampling effect impacting on the reliability of their results. Devices with greater sensitivity are required before there can be reliable direct assessment of mitochondrial function in cumulus cell samples. Gene expression profiles of cumulus cells hold promise as a non-invasive bio-marker of oocyte and embryo quality.