Reproductive aging is associated with changes in oocyte mitochondrial dynamics, function, and mtDNA quantity

Mitochondria affect numerous aspects of mammalian reproduction. We investigated whether the decrease in oocyte quality associated with aging is related to altered mitochondria. Oocytes from old (12 months) and young (9 weeks) C57BL/6J mice were compared in relation to: mitochondria morphology and dynamics (mitochondria density, coverage, size and shape) throughout folliculogenesis; levels of mitochondrial DNA (mtDNA); mitochondrial stress reflected in the expression of mitochondrial unfolded protein response (mt-UPR) genes; and levels of reactive oxygen species (ROS) under baseline conditions and following H₂O₂ treatment. In old mice, mitochondria of primary follicle-enclosed oocytes were smaller, with lower mitochondria coverage (total mitochondria μm²/μm² cytosol area) (p<0.05). Other follicular stages showed a similar trend, but the changes were not significant. Mature oocytes (Metaphase II-MII) from old mice had significantly less mtDNA (p<0.01), and elevated mt-UPR gene Hspd1 expression (p<0.05), compared with those from young mice. ROS levels in aged MII oocytes were also higher following pretreatment with H₂O₂ (p<0.05). Aging is associated with altered mitochondrial morphological parameters and decreased mtDNA levels in oocytes, as well as an increase in ROS under stressful conditions and elevated expression of mitochondrial stress response gene Hspd1. Delineation of the mechanisms underlying mitochondrial changes associated with ageing may help in the development of diagnostic and therapeutic tools in reproductive medicine.