The Role of Mitochondria in Oocyte Maturation

Abstract

With the nucleus as an exception, mitochondria are the only animal cell organelles containing their own genetic information, called mitochondrial DNA (mtDNA). During oocyte maturation, the mtDNA copy number dramatically increases and the distribution of mitochondria changes significantly. As oocyte maturation requires a large amount of ATP for continuous transcription and translation, the availability of the right number of functional mitochondria is crucial. There is a correlation between the quality of oocytes and both the amount of mtDNA and the amount of ATP. Suboptimal conditions of in vitro maturation (IVM) might lead to changes in the mitochondrial morphology as well as alternations in the expression of genes encoding proteins associated with mitochondrial function. Dysfunctional mitochondria have a lower ability to counteract reactive oxygen species (ROS) production which leads to oxidative stress. The mitochondrial function might be improved with the application of antioxidants and significant expectations are laid on the development of new IVM systems supplemented with mitochondria-targeted reagents. Different types of antioxidants have been tested already on animal models and human rescue IVM oocytes, showing promising results. This review focuses on the recent observations on oocytes' intracellular mitochondrial distribution and on mitochondrial genomes during their maturation, both in vivo and in vitro. Recent mitochondrial supplementation studies, aiming to improve oocyte developmental potential, are summarized.

Keywords: IVM; mitochondria distribution; mitochondrial supplementation reagents; mtDNA copy number; oocyte maturation.

Figure 1

The regulation of mitochondrial DNA (mtDNA) copy number and mitochondria distribution during oocyte maturation. In primordial germ cells, the amount of mtDNA is very low. As oogenesis progresses, mtDNA copies increase significantly and then reach their maximum at the MII stage. There are two main points at which mtDNA copies go up very quickly: the first one is during the transition from the primordial to the primary follicle stage, and the second one is during the transition from GV to MII oocytes at the late antral follicle stage. In parallel as oogenesis progresses, ATP level also increases significantly as oocyte maturation requires a large amount of ATP for the subsequent transcription and translation. During the transition from GV to MII oocytes at the late antral follicle stage, the distribution of mitochondria changes significantly. They migrate from the center of the ooplasm to the pericortical region, distributing evenly through the whole ooplasm.