The effect of maturation and aging on the quality of mammalian oocytes

The aim of this review was to summarise current knowledge regarding the effects of maturation, activation and aging on oocyte quality. Two main forms of aging were distinguished: (i) postovulatory aging, which occurs after ovulation due to a prolonged period before fertilisation and (ii) age-related aging, which is associated with a gradual decline in ovarian reserve and oocyte quality. Proper embryo development strongly depends on oocyte quality, which is most often assessed according to morphology. Numerous observations have demonstrated the influence of oocyte maturation, during which controlled cytoplasmic and nuclear modifications occur, and the influence of the timing of oocyte activation by the fertilising sperm. Post-ovulatory aging of an oocyte, which was not fertilised during the optimal period, manifests as impaired embryo development and results in fragmentation and embryonic mortality. Age-related oocyte aging leads to mitochondrial damage due to the long-term effects of reactive oxygen and nitrogen species (RONS), which play a key role in regulating physiological processes in the ovaries, including ovulation and oocyte maturation. Overall, it can be concluded that oocyte quality is the result of a complex interaction of meiotic, metabolic and redox mechanisms, which are significantly influenced by age, hormonal stimulation and the follicular microenvironment. Oocyte aging is associated with predominantly irreversible structural and functional changes leading to aneuploidy, mitochondrial dysfunction and failure of embryonic development.