Purpose of review Mitochondria are cellular organelles that are required for energy production. Emerging evidence demonstrates their role in oocyte development and reproduction. In this review, we examine recent animal and clinical studies on the role of mitochondria in fertility. We also analyse the impact of assisted reproductive techniques (ARTs) on mitochondrial function and discuss the future clinical implications of mitochondrial nutrients and mitochondrial replacement. Recent findings Mitochondria affect all aspects of mammalian reproduction. They are essential for optimal oocyte maturation, fertilization and embryonic development. Mitochondrial dysfunction causes a decrease in oocyte quality and interferes with embryonic development. ART procedures affect mitochondrial function, while mitochondrial nutrients may increase mitochondrial performance in oocytes. New mitochondrial replacement procedures using mitochondria obtained from polar bodies or from the patient’s own oogonial stem cells are promising and may address concerns related to the induction of high-levels of heteroplasmy, which could potentially result in negative long-term health effects. Summary Optimal energy production is required for oocyte and embryo development, and mitochondrial abnormalities have devastating reproductive consequences. Improvement of oocyte mitochondrial function via intake of compounds that boost mitochondrial activity may have clinical benefits, and mitochondrial replacement could potentially be used for the prevention of mitochondrial diseases.

H.O.D.C. has clinical research support for laboratory consumables and staff from Bayer AG and provides consultancy advice (but with no personal remuneration) for Bayer AG, PregLem SA, Gedeon Richter, Vifor Pharma UK Ltd, AbbVie Inc, and Myovant Sciences GmbH. H.O.D.C. receives royalties from UpToDate for article on abnormal uterine bleeding. A.K. receives royalties from UpToDate, Wolters Kluwer for work on the topic hysterosalpingography. E.E.M. consults for Myovant Sciences. K.A.M. is coinvestigator for Bayer Essure longitudinal research study and clinical trial (all funds for this research go to the site of research [W.I.H.] e no personal compensation). K.A.M. is scientific advisor for Myovant (advises on patient questionnaires related to AUB-compensation goes to employer [C.N.E.M.G.]-no personal compensation). K.A.M. has received honoraria from ABOG, ACOG, and NIH for participating in working groups and meetings. K.A.M. is HHS Office of Population Affairs Title X Grant Reviewer (received honorarium). I.M. is employee of Igenomix R&D. C.S. is Head of the Igenomix Scientific Advisory Board. The other authors report no conflict of interest.

SummaryCaseinolytic peptidase P mediates degradation of unfolded mitochondrial proteins and activates mitochondrial unfolded protein response (mtUPR) to maintain protein homeostasis. Clpp −/− female mice generate a lower number of mature oocytes and two‐cell embryos, and no blastocysts. Clpp −/− oocytes have smaller mitochondria, with lower aspect ratio (length/width), and decreased expression of genes that promote fusion. A 4‐fold increase in atretic follicles at 3 months, and reduced number of primordial follicles at 6–12 months are observed in Clpp −/− ovaries. This is associated with upregulation of p‐S6, p‐S6K, p‐4EBP1 and p‐AKT473, p‐mTOR2481 consistent with mTORC1 and mTORC2 activation, respectively, and Clpp −/− oocyte competence is partially rescued by mTOR inhibitor rapamycin. Our findings demonstrate that CLPP is required for oocyte and embryo development and oocyte mitochondrial function and dynamics. Absence of CLPP results in mTOR pathway activation, and accelerated depletion of ovarian follicular reserve.

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.

Objective Micro RNAs (miRNAs) are a large family of short (∼21-nucleotide) non-coding mRNAs that repress gene expression through degradation of target mRNAs and/or inhibition of their translation. In the mouse, miRNAs play a key role in cumulus cell function and miR-21 is implicated in the regulation of cumulus cell viability. In this study, we asked whether miRNA expression is associated with the number of oocytes retrieved in women undergoing IVF and aimed to identify candidate miRNAs that may play a role in human cumulus cell function. Design Experimental study. Materials and Methods Pooled cumulus cells were collected from 189 consecutive women undergoing in vitro fertilization-intracytoplasmic sperm injection (IVF-ICSI). Poor responders were identified as patients who produced fewer oocytes than the 25th percentile of their respective age group. MiRNAs were extracted from cumulus cells and miRNA microarray was performed comparing normo-responders (n=3) to poor responders (n=3). Data were analyzed using Partek Genomics Suite software and MATLAB. Expression of miR-21-5p (active strand of miR-21) and miR-21-3p was tested in poor responders (n=21) and non-poor responders (n=29) using quantitative real time polymerase chain reaction (qRT-PCR). Regulation of miR-21-5p and miR-21-3p in KGN cells by estradiol was tested in vitro. Results MiRNA microarray analysis showed up-regulation of 16 miRNAs and down-regulation of 88 miRNAs in poor responders compared to the non-poor responders. Notably, miR-21 was significantly up-regulated 5-fold in poor responder samples (p=0.03). qRT-PCR analysis confirmed that miR-21-5p expression was significantly upregulated in poor responder patients (p=0.04), while miR-21-3p expression was significantly lower (p=0.003), suggesting that elevated miR-21-5p expression in cumulus cells is not regulated at the pre-miR-21 level in poor responders. Lastly, we found that both miR-21-5p and miR-21-3p are increased in KGN cells in response to higher doses of estradiol (p<0.05), while their expression is not affected at lower estradiol concentrations. Conclusion We found that poor response to IVF is associated with altered miRNA expression in cumulus cells, specifically with elevated expression of miR-21-5p, and that this elevated expression is independent of lower serum estradiol levels in poor responders. Whether miR-21 plays a role in human cumulus cell function and whether miRNA expression in cumulus cells may be used as a biomarker for oocyte or follicular viability remains to be investigated.

Female fertility is highly dependent on successful regulation of energy metabolism. Central processes in the hypothalamus monitor the metabolic state of the organism and, together with metabolic hormones, drive the peripheral availability of energy for cellular functions. In the ovary, the oocyte and neighboring somatic cells of the follicle work in unison to achieve successful metabolism of carbohydrates, amino acids, and lipids. Metabolic disturbances such as anorexia nervosa, obesity, and diabetes mellitus have clinically important consequences on human reproduction. In this article, we review the metabolic determinants of female reproduction and their role in infertility.

The ovary is the first organ to age in humans with functional decline evident already in women in their early thirties. Reproductive aging is characterized by a decrease in oocyte quantity and quality which is associated with an increase in infertility, spontaneous abortions, and birth defects. Reproductive aging also has implications for overall health due to decreased endocrinological output. Understanding the mechanisms underlying reproductive aging has significant societal implications as women globally are delaying childbearing and medical interventions have greatly increased the interval between menopause and total lifespan. Age-related changes inherent to the female gamete are well-characterized and include defects in chromosome and mitochondria structure, function, and regulation. More recently, it has been appreciated that the extra-follicular ovarian environment may have important direct or indirect impacts on the developing gamete, and age-dependent changes include increased fibrosis, inflammation, stiffness, and oxidative damage. The cumulus cells and follicular fluid which directly surround the oocyte during its final growth phase within the antral follicle represent additional critical local microenvironments. Here we systematically review the literature and evaluate the studies that investigated the age-related changes in cumulus cells and follicular fluid. Our findings demonstrate unique genetic, epigenetic, transcriptomic, and proteomic changes with associated metabolomic alterations, redox status imbalance, and increased apoptosis in the local oocyte microenvironment. We propose a model of how these changes interact, which may explain the rapid decline in gamete quality with age. We also review the limitations of published studies and highlight future research frontiers.

Meiotic maturation and cumulus expansion are essential for the generation of a developmentally competent gamete, and both processes can be recapitulated in vitro. We used a closed time-lapse incubator (EmbryoScope+™) to establish morphokinetic parameters of meiotic progression and cumulus expansion in mice and correlated these outcomes with egg ploidy. The average time to germinal vesicle breakdown (GVBD), time to first polar body extrusion (PBE), and duration of meiosis I were 0.91 ± 0.01 hr, 8.82 ± 0.06 hr, and 7.93 ± 0.06 hr, respectively. The overall rate of cumulus layer expansion was 0.091 ± 0.002 μm/min, and the velocity of expansion peaked during the first 8 hours of in vitro maturation (IVM) and then slowed. IVM of oocytes exposed to Nocodazole, a microtubule disrupting agent, and cumulus oocyte complexes (COCs) to 4-methylumbelliferone, a hyaluronan synthesis inhibitor, resulted in a dose-dependent perturbation of morphokinetics, thereby validating the system. The incidence of euploidy following IVM was &gt;90% for both denuded oocytes and intact COCs. No differences were observed between euploid and aneuploid eggs with respect to time to GVBD (0.90 ± 0.22 vs. 0.97 ± 0.19 hr), time to PBE (8.89 ± 0.98 vs. 9.10 ± 1.42 hr), duration of meiosis I (8.01 ± 0.91 vs. 8.13 ± 1.38 hr), and overall rate and kinetics of cumulus expansion (0.089 ± 0.02 vs 0.088 ± 0.03 μm/min) (p &gt; 0.05). These morphokinetic parameters provide novel quantitative and non-invasive metrics for the evaluation of meiotic maturation and cumulus expansion and will enable screening compounds that modulate these processes.