Women with T1DM are known to be at a high risk for miscarriage and giving birth to infants with congenital malformations [31]. A murine genetic model of T1DM has significantly more apoptosis of granulosa and cumulus cells, both of which surround the maturing oocyte, than a non-diabetic control group [32]. Additionally, oocytes isolated from diabetic mice are smaller in size and have a delay in completion of meiosis I, a phenotype analogous to the murine diet-induced obesity model [26]. Although the hyperglycemia that characterizes obesity is less severe than that in T1DM, exposure to excess glucose could be responsible for aberrant follicular and oocyte development in both contexts.

The oocyte is not directly exposed to glucose; it instead receives nutrients, signaling molecules, and metabolic intermediates from the cumulus cells with which it is intimately coupled via gap junctions and paracrine signaling [33]. The cumulus cells in the cumulus–oocyte-complexes (COCs) are responsible for metabolizing available glucose and supplying the oocyte with its major source of energy, pyruvate [34, 35]. In fact, oocytes denuded of their surrounding cumulus cells cannot metabolize glucose efficiently and require pyruvate supplementation for survival [36]. Therefore, hyperglycemia may indirectly impair oocyte development by altering cumulus cell function. One study showed that cumulus cells from mice with T1DM had increased apoptosis rates and significant mitochondrial dysfunction [37]. The apoptotic cumulus cells also demonstrated a diffuse cytochrome c staining pattern and decreased caspase-3 activation, suggesting activation of the apoptosis pathway. Although the exact initiating event for mitochondrial dysfunction is unclear, glucose deprivation through downregulation of glucose transporters is a strong possibility [33]. In support of this idea, hyperglycemia has been shown to result in downregulation of the glucose transporter GLUT1, decreased glucose uptake, and increased apoptosis in murine preimplantation embryos and cumulus cells [37–40]. Decreased glucose uptake in cumulus cells from mice with chemically induced T1DM also correlates with lower ATP levels in the COCs [41]. Whether or not these are the mechanisms by which maternal obesity affects oocyte quality needs to be determined, but the above models of T1DM and hyperglycemia provide a good starting point for future research.

Unlike T1DM, obesity and T2DM are characterized by insulin resistance, which results in hyperinsulinemia in addition to hyperglycemia. Insulin signaling is mediated via the insulin receptor (IR) and insulin-like growth factor receptors (IGF1R and IGF2R), which are expressed in human, bovine, rat, and murine oocytes [42–46]. In vitro exposure of murine COCs to excess insulin produces morphologically normal oocytes but hinders subsequent embryo development from the two-cell to blastocyst stage [47]. However, this deleterious effect is only evident when follicle-stimulating hormone (FSH) is present in the culture media. Because insulin acts on granulosa cells in synergy with FSH to induce their differentiation and steroidogenesis [48], excess insulin may lead to aberrant differentiation of granulosa cells and an inability to support proper oocyte development [49, 50].

A recent study demonstrated that murine and human cumulus cells are capable of insulin-stimulated glucose uptake, whereas murine oocytes are not (human oocytes have not been assayed) [51]. This study also revealed that insulin activates the canonical phosphoinositide 3-kinase (PI3K) pathway, which is required for glucose transporter translocation to the cell surface, in cumulus cells. When mice were fed a HFD for four weeks, resulting in hyperinsulinemia without hyperglycemia, insulin-stimulated glucose uptake in the cumulus cells was significantly reduced. Impaired insulin sensitivity and subsequently decreased glucose uptake in cumulus cells is another potential mechanism for obesity’s detrimental effects on oocyte and early embryo development.

Another group recently suggested that the peroxisome proliferator-activated receptor-gamma (PPARγ) pathway may mediate the adverse effects of maternal obesity on oocyte and early embryo development [27]. Activation of the PPARγ pathway in adipocytes stimulates transcription of genes involved in lipid and glucose metabolism, resulting in improved insulin and lipid homeostasis [52]. PPARγ is also expressed in murine, ruminant, and human ovarian tissues, particularly in the granulosa cells [53]. Treatment of mice with the PPARγ agonist and the insulin-sensitizing agent rosiglitazone for 4 days before ovulation reverses the delays in early embryo progression observed in fertilized oocytes harvested from mice fed a HFD prior to conception [27]. The treatment also shows a trend towards reversing the cellular composition abnormalities in resulting blastocysts. In addition, rosiglitazone alters the expression of several PPARγ target genes within the ovary, suggesting a direct effect in this tissue [27]. However, its administration leads to significant weight loss and lower levels of serum glucose, serum triglycerides, and insulin in HFD-fed mice. Thus, it is still unclear whether the observed effects on oocytes and early embryo development are a consequence of whole-body improvements in insulin sensitivity and subsequent reversal of hyperinsulinemia and hyperglycemia or reflect direct action of rosiglitazone on the ovary. Although further studies are required, we can glean some additional answers from rats with a conditional knockout of PPARγ in the ovary. The resulting females are either infertile or severely subfertile [54]. They have normal follicular development, ovulation, and corpus luteum size, but the majority of the resulting embryos fail to implant. Because PPARγ was specifically knocked down in the ovary but not the uterus in these experiments, it is reasonable to hypothesize that PPARγ plays a significant role in ovarian function, either through regulation of oocyte competence or steroidogenesis after ovulation. Together, the above studies suggest that insulin sensitizers that specifically affect the PPARγ pathway may reverse some of the adverse effects of hyperinsulinemia on oocyte developmental competence by altering glucose metabolism in granulosa and cumulus cells.

Mitochondrial abnormalities are another potential mechanism to explain poor oocyte quality and impaired embryo development in obese women. Like all other cells, oocytes rely on mitochondria for energy production in the form of adenosine triphosphate (ATP). As mentioned above, it is thought that the surrounding cumulus cells provide the oocyte with pyruvate [35], which is metabolized through the mitochondrial oxidative pathways to synthesize ATP. The importance of pyruvate metabolism was illustrated by a study examining the effects of conditional inactivation of one of the subunits of the pyruvate dehydrogenase complex, Pdha1 [55]. The resulting oocytes appeared to progress through the growth phase but were unable to support embryo development after fertilization. As expected, the Pdha1-deficient oocytes also had decreased ATP levels. Interestingly, the developmental defect was partially rescued in oocytes that were matured within intact COCs, suggesting that cumulus cells can provide the oocyte with ATP or other metabolites through gap junctions [55].

Mitochondrial distribution and morphology are altered in oocytes harvested from diet-induced obese mice [28]. Whereas mitochondria in control-derived oocytes are distributed diffusely throughout the ooplasm, mitochondria in obese-derived oocytes are perinuclear and in cortical clusters. Transmission electron microscopy of oocytes and the surrounding cumulus cells from obese mice reveal mitochondria that have fewer and disarrayed cristae, increased swelling, more vacuoles, and decreased electron density of the matrix [30]. Similar aberrations in mitochondrial morphology have been found in oocytes and cumulus cells from T1DM mouse models and in embryos undergoing cleavage arrest [37, 56, 57].

Maternal obesity also appears to alter the function of mitochondria in oocytes. Oxidative phosphorylation in mitochondria produces reactive oxygen species (ROS), which can damage the cell when present in excess. Mitochondria tightly regulate the redox status of cells by regenerating antioxidant systems and maintaining the NADPH:NADP+ ratio in the cytosol [58]. Using a low-toxicity potentiometric fluorescent dye, one group observed that oocytes from obese mice had a significantly higher inner mitochondrial membrane potential than those from control mice, and this difference persisted in the zygotes [28]. This was likely due to an increase in mitochondrial respiration because these oocytes and zygotes also had a shift of the redox status towards oxidation and increased rates of ROS production. One proposed mechanism for these findings is that the increased availability of energy substrates, such as carbohydrates and fatty acids, found in an obesogenic state causes mitochondrial hyperactivity and increased ROS production and ultimately leads to mitochondrial dysfunction and oocyte damage. A growing body of evidence suggests that abnormal energy balance in the oocyte due to mitochondrial abnormalities leads to abnormal spindle and chromosome alignments, oocyte maturation failure, and early embryo developmental defects [59].

Mitochondrial dysfunction induced by excess ROS is thought to cause compensatory upregulation of mitochondrial biogenesis in numerous cell types [60], and there is evidence that this may be the case in oocytes as well [57]. Oocytes derived from mice fed a HFD have significantly higher mitochondrial DNA copy number than those from control-fed mice [28, 30]. These oocytes also have increased expression of genes involved in mitochondrial biogenesis, including PGC-1α, Drp-1, TFAM, and NRF1. Interestingly, oocytes form obese mice have lower levels of citrate but unchanged levels of ATP, suggesting that although mitochondrial function is disturbed, overall oocyte metabolism is not, perhaps as a result of compensatory increases in mitochondrial number [30]. However, zygotes isolated from obese mice 24 h after mating show no evidence of increased mitochondrial biogenesis [28], nor do the number of zygotes recovered from the HFD and control groups differ. This is surprising given that signs of ROS-induced damage persist in obese-derived zygotes. This finding might be explained by the fact that a period of mitochondrial DNA turnover and maternal RNA destruction occurs shortly after fertilization [61]. These initial studies using the HFD-fed murine model are still only correlative, with no proof that ROS-induced mitochondrial damage is responsible for the inability of the oocytes to support further embryo development. Further studies will also be needed to determine whether obesity’s effects on ROS homeostasis contribute to the long-term developmental deficiencies observed in the offspring of obese mice, such as small fetal size and congenital abnormalities.

Oocytes from mice on a HFD have a higher incidence of metaphase II chromosome misalignment, ectopic microtubule organizing centers, and malformed spindles than oocytes from mice fed a control diet [30]. Similar chromosomal abnormalities are found in oocytes from T1DM mouse models [41, 57]. It is thought that proper chromosome alignment and spindle formation relies on mitochondrial function and distribution within the cell [57, 62]. The Pdha1-deficient mice described above provide evidence for this; 98.4 % of the oocytes from these mice have gross abnormalities in meiotic spindle formation, chromosome alignment, and meiotic maturation [55]. Studies also link lower ATP levels in oocytes with abnormal spindle formation, suggesting that meiotic spindle formation requires adequate energy supplies [55, 63]. Alternatively, a biochemical study showed that microtubule assembly during spindle formation requires adequate NADPH, most likely for maintaining a proper redox state [64]. Although the pentose phosphate pathway was the provider of NADPH in this study, we know that in the oocyte, where glucose is poorly metabolized, mitochondrial metabolism of pyruvate regulates NADPH availability [58].

Oocytes with significant spindle or chromosomal alignment abnormalities are likely to fail to fertilize, generate embryos with aneuploidy, or produce embryos incapable of normal developmental progression. These abnormalities could contribute to the higher rates of infertility and spontaneous pregnancy loss observed in obese women. In support of this idea, a recent study reported that oocytes from obese women undergoing IVF often have gross morphological abnormalities, spindle anomalies, and non-aligned chromosomes [65, 66].

Although the full function of adipose tissue is poorly understood, it is known that this tissue stores excess nutrients in the form of triglycerides and that it also plays an endocrine role as a source of adipokines [67, 68]. Accumulating evidence now suggests that both of these functions can become dysregulated in the context of obesity, thus damaging other tissues, including the reproductive tract [69]. As the storage capacity of adipose tissue becomes overwhelmed, triglycerides begin to accumulate in non-adipose tissues. The resulting high levels of fatty acids cause a lipid-induced apoptotic cascade-termed lipotoxicity [70]. Although this process is not yet fully understood, the resulting increased oxidative stress is thought to induce endoplasmic reticulum (ER) stress, an unfolded-protein response, and apoptosis. Prime examples of lipotoxicity include lipid accumulation in the liver, skeletal muscle, heart, and pancreas; such accumulation contributes to the development of obesity, diabetes, and heart failure.

Mammalian oocytes contain lipid droplets, which are thought to be necessary for oocyte and preimplantation embryo development [71, 72]. For example, fatty acid oxidation is required for oocyte germinal vesicle breakdown and early embryo development through the blastocyst stage in mice [73, 74]. Mice fed a HFD for 4 weeks have higher levels of lipids within their oocytes and surrounding cumulus cells than mice fed a control diet [29]. These COCs also have increased expression of the ER stress marker genes ATF4 and GRP78 and decreased fertilization rates. Additionally, Wu et al. found that ATF4 expression is elevated in granulosa cells from obese women, suggesting that ER stress contributes to lower conception rates in obese women [29]. The source of the excess lipid within oocytes and cumulus cells remains an unanswered question. Possibilities include de novo lipogenesis in the oocyte and cumulus cells or diffusion from the maternal serum or follicular fluid. Consistent with the latter possibility, increased triglyceride levels have been observed in the follicular fluid of obese women [75]. Another study also found that elevated follicular free fatty acid (FFA) levels are associated with poor COC morphology in women undergoing IVF [76]. Interestingly, this study found that serum FFA levels do not correlate with follicular FFA levels, suggesting that the follicular environment is the source for the excess lipids accumulating in the COCs and granulosa cells.

As adipose tissue accumulates to accommodate the excess energy, adipocytes grow in quantity and size and alter their secretory profiles [67]. One important secreted adipokine is leptin (the product of the Obesity [Ob] gene in mice), which primarily acts on the hypothalamus to regulate satiety. Levels of leptin in the serum and leptin mRNA in adipocytes both directly correlate with BMI in humans [77]. In addition to its stimulatory effects on the hypothalamic–pituitary–gonadal axis [78], leptin appears to directly regulate ovarian function. In fact, the leptin receptor is expressed in human and rodent oocytes, theca cells, and granulosa cells [79, 80]. Additionally, follicular fluid from women undergoing IVF contains leptin, the levels of leptin positively correlate with BMI [81], and increased follicular leptin levels are associated with poor ovarian response and decreased IVF success rates [82–84]. Numerous studies using human tissue and various animal models have shown that excess leptin has deleterious effects on oocyte and embryo development [83]. One such study reported that although there were no differences in number of oocytes retrieved, fertilized, or developed past the cleavage stage, the embryos generated from oocytes retrieved from women with high leptin:BMI ratios were of reduced quality at day three post-retrieval [84]. The subsequent implantation rates were also significantly lower in the high vs. low leptin:BMI groups (13.2 % vs. 26.7 %). These findings support a hypothesis that excess leptin has deleterious effects on the oocyte, which in turn affect embryo quality and lead to increased pregnancy loss. It is also possible that developmental defects resulting from elevated leptin at the preconception stage persist in the surviving fetuses and contribute to some of the congenital defects reported in children of obese mothers.