This review provides a comprehensive overview of Complex I during mammalian oocyte maturation. Complex I (NADH:ubiquinone oxidoreductase) is a crucial member of the electron transport chain and serves two principal functions during oxidative phosphorylation: NADH oxidation and proton pumping. It is located at the inner mitochondrial membrane and consists of 14 core and 31 accessory subunits that are necessary for its function and assembly. Moreover, Complex I is the primary site of reactive oxygen species (ROS) production among the different tissues. In light of the literature, it has been demonstrated that ROS and oxidative stress are significantly important among the various factors that can affect oocyte maturation. Factors such as malnutrition, alcohol use, obesity, PCOS, aging, and smoking are some of the common causes of infertility. Each one of them causes disruption in the equilibrium of the body's redox system and related with oxidative stress. During oocyte maturation, excessive ROS levels are associated with chromosomal errors and developmental insufficiency. In addition, excess oxidative stress adversely affects embryo growth and development and may cause fetal embryopathies with damage to macromolecules in the cytoskeleton. At this particular juncture, Complex I plays a key role in determining ROS production and the success of the oocyte maturation. This review evaluates mitochondrial Complex I's function, structure, and its crucial role during oocyte maturation.

The success of both oogenesis and early embryo development relies heavily on dynamic epigenetic regulation in which gene activity changes without affecting the underlying DNA sequence. Epigenetics works through two main mechanisms: DNA methylation and histone modifications. DNA methylation typically leads to gene silencing, while histone modifications can either activate or repress genes depending on the specific modification, histone type, and targeted amino acid residue. Histone modifications affect important DNA regulatory processes in which the histone core area as well as the N-terminal tails that extend from the core region are vulnerable to a variety of posttranslational modifications (PTMs), including methylation, citrullination (deimination), acetylation, phosphorylation, ubiquitination, SUMOylation, ribosylation, and lactylation. This review article focuses on what is known about changes in the histone modifications and how these modifications and their responsible enzymes operate throughout mammalian oocyte maturation and early embryo development, highlighting their crucial roles in these processes.