Pig oocytes and embryos differ from those of other species in having a large quantity of endogenous lipid, a potential role for which has yet to be identified. In the present study, the hypothesis that endogenous triglyceride acts as a metabolic substrate during in vitro maturation and early embryo development was tested. Embryos were produced by in vitro fertilization (IVF) of in vitro-matured, abattoir-derived immature oocytes, cultured in medium NCSU23 up to the blastocyst stage. The triglyceride content of single oocytes and embryos was measured throughout development. Oxygen and glucose consumption and the formation of lactate were measured non-invasively over the same period, enabling total ATP production to be calculated. The triglyceride content of oocytes before maturation (135+/-4.9 ng) decreased by 13 ng (P<0.05) during in vitro maturation, but there was no apparent change in triglyceride content during embryo development (117.68 ng). Oxygen consumption was low throughout embryo cleavage before reaching a peak at the blastocyst stage (P<0.01), a pattern similar to that seen in other mammals studied. Glucose consumption and lactate production were also at a maximum at the blastocyst stage (P<0.05). These data indicate that pig oocytes may use endogenous triglyceride as an energy source during in vitro maturation and that most (91-97%) of the ATP produced during embryo development comes from oxidative phosphorylation. The high exogenous glucose concentration in NCSU23 (5.5 mmol l(-1)) may be needed to form pyruvate, which in turn, produces oxaloacetate, which is required to prime the tricarboxylic acid cycle. However, the reason for the high lipid content in early pig embryos remains to be elucidated.

This review examines the ‘Quiet Embryo Hypothesis’ which proposes that viable preimplantation embryos operate at metabolite or nutrient turnover rates distributed within lower ranges than those of their less viable counterparts. The ‘quieter’ metabolism consistent with this hypothesis is considered in terms of (i) ‘functional’ quietness; the contrasting levels of intrinsic metabolic activity in different cell types as a consequence of their specialized functions, (ii) inter-individual embryo/cell differences in metabolism and (iii) loss of quietness in response to environmental stress. Data are reviewed which indicate that gametes and early embryos function in vivo at a lower temperature than core body temperature, which could encourage the expression of a quiet metabolism. We call for research to determine the optimum temperature for mammalian gamete/embryo culture. The review concludes by examining the key role of reactive oxygen species, which can induce molecular damage, trigger a cellular stress response and lead to a loss of quietness.

Elevated concentrations of serum non-esterified fatty acids (NEFA), associated with maternal disorders such as obesity and type II diabetes, alter the ovarian follicular micro-environment and have been associated with subfertility arising from reduced oocyte developmental competence. We have asked whether elevated NEFA concentrations during oocyte maturation affect the development and physiology of zygotes formed from such oocytes, using the cow as a model. The zygotes were grown to blastocysts, which were evaluated for their quality in terms of cell number, apoptosis, expression of key genes, amino acid turnover and oxidative metabolism. Oocyte maturation under elevated NEFA concentrations resulted in blastocysts with significantly lower cell number, increased apoptotic cell ratio and altered mRNA abundance of DNMT3A, IGF2R and SLC2A1. In addition, the blastocysts displayed reduced oxygen, pyruvate and glucose consumption, up-regulated lactate consumption and higher amino acid metabolism. These data indicate that exposure of maturing oocytes to elevated NEFA concentrations has a negative impact on fertility not only through a reduction in oocyte developmental capacity but through compromised early embryo quality, viability and metabolism.

Is the developmental timing and metabolic regulation disrupted in embryos from overweight or obese women? Oocytes from overweight or obese women are smaller than those from women of healthy weight, yet post-fertilization they reach the morula stage faster and, as blastocysts, show reduced glucose consumption and elevated endogenous triglyceride levels. Female overweight and obesity is associated with infertility. Moreover, being overweight or obese around conception may have significant consequences for the unborn child, since there are widely acknowledged links between events occurring during early development and the incidence of a number of adult disorders. We have performed a retrospective, observational analysis of oocyte size and the subsequent developmental kinetics of 218 oocytes from 29 consecutive women attending for ICSI treatment and have related time to reach key developmental stages to maternal bodyweight. In addition, we have measured non-invasively the metabolic activity of 150 IVF/ICSI embryos from a further 29 consecutive women who donated their surplus embryos to research, and have related the data retrospectively to their body mass index (BMI). In a clinical IVF setting, we compared oocyte morphology and developmental kinetics of supernumerary embryos collected from overweight and obese women, with a BMI in excess of 25 kg/m² to those from women of healthy weight. A Primovision Time-Lapse system was used to measure developmental kinetics and the non-invasive COnsumption/RElese of glucose, pyruvate, amino acids and lactate were measured on spent droplets of culture medium. Total triglyceride levels within individual embryos were also determined. Human oocytes from women presenting for fertility treatment with a BMI exceeding 25 kg/m² are smaller (R² = −0.45; P = 0.001) and therefore less likely to complete development post-fertilization (P < 0.001). Those embryos that do develop reach the morula stage faster than embryos from women of a BMI < 25 kg/m² (<0.001) and the resulting blastocysts contain fewer cells notably in the trophectoderm (P = 0.01). The resulting blastocysts also have reduced glucose consumption (R² = −0.61; P = 0.001), modified amino acid metabolism and increased levels of endogenous triglyceride (t = 4.11, P < 0.001). Our data further indicate that these differences are independent of male BMI. Although statistical power has been achieved, this is a retrospective study and relatively small due to the scarcity of human embryos available for research. Consequently, subanalysis of overweight and obese was not possible based on the sample size. The analysis has been performed on supernumerary embryos, originating from a single IVF unit and not selected for use in treatment. Thus, it was not possible to speculate how representative the findings would be of the better quality embryos transferred or frozen for each patient. The data indicate that a high BMI of women at conception is associated with distinct phenotypic changes in the embryo during the preimplantation period, highlighting the importance of prepregnancy body weight in optimizing the chances of fertility and safeguarding maternal and offspring health. These changes to the metabolic fingerprint of human embryos which are most likely a legacy of the ovarian conditions under which the oocyte has matured may reduce the chances of conception for overweight women and provide good evidence that the metabolic profile of the early embryo is set by sub-optimal conditions around the time of conception. The observed changes could indicate long-term implications for the health of the offspring of overweight and obese women. This study was funded by the Hull IVF Unit Charitable Trust and the Hull York Medical School. There are no conflict of interests.

Growing evidence suggests that endogenous and exogenous fatty acids play diverse roles in developing mammalian oocytes and early embryos. In this review, we describe some of the regulatory roles of fatty acids in early development, in addition to their metabolic functions. We focus initially on the provision of individual fatty acids, and then discuss how these might affect metabolism, oxidative stress, membrane composition, cell signalling events and gene expression. We propose that ongoing research should focus on physiologically relevant ratios and combinations of fatty acids, rather than isolated individual fatty acids, as their combined roles are both subtle and complex. Changing the ratio of specific fatty acids in the diet of animal models, and in vitro culture medium can cause significant dysregulation of cellular processes and development, an issue that extends to human fertility.