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.

Advanced age increases frequencies of de novo mitochondrial mutations in macaque oocytes and somatic tissues

“…This suggests limited replication of mtDNA in aging oocytes. Whereas replication of mtDNA and nuclear DNA is not necessarily linked ( 45 ), mammalian oocytes do not undergo mitotic cell divisions after birth. Indeed, compared with somatic tissues, oocytes exhibited lower fold differences in transition rates between old and young macaques, had slower increase in the transition-to-transversion rate ratio with age, and had a weaker C > T over G > A strand bias.…”

Mitochondria Transfer from Adipose Stem Cells Improves the Developmental Potential of Cryopreserved Oocytes

“…Moreover, adipose stem cells can transfer mitochondria in vitro and in vivo to exhausted cells to reduce ROS levels and increase the number of intact mitochondria [ 61 , 63 ]. Unlike most other cells, oocytes contain a higher number of mitochondria, showing few cristae spherical shapes [ 64 , 65 ]. Interestingly, ASC mitochondria have also demonstrated similar morphological phenotypes to oocytes, including spherical shape and limited cristae [ 29 ] ( Figure 3 D,E), and they are potentially active ones ( Figure 3 A–C).…”

Granulosa Cells Improved Mare Oocyte Cytoplasmic Maturation by Providing Collagens

“…One important criterion for oocyte cytoplasmic maturity is the distribution of cortical granules at the periphery of the cell, which is critical for the post-fertilization cortical response initiation to prevent polyspermy ( Carneiro et al, 2002 ; Cheeseman et al, 2016 ). Mitochondrial redistribution, involving migration toward the central zone after maturation and the homogeneous localization of mitochondria, is considered a marker of cytoplasmic maturation, while a marginal localization is more common in meiotically dysfunctional oocytes ( Kirillova et al, 2021 ). Additionally, mammalian oocytes and embryos are extremely susceptible to oxidative stress; elevated ROS levels will affect the oocyte maturation and embryonic development ( Ramalho-Santos et al, 2009 ).…”