New challenges in the analysis of gene transcription in bovine blastocysts

Vitrification and in-straw warming do not affect pregnancy rates of biopsied bovine embryos

In the cattle industry, in vivo or in vitro embryo production combined with genotyping and cryopreservation technologies allows the selection and conservation of embryos carrying genes for desirable traits. This study aimed to assess the efficiency of a vitrification method suitable for in-straw warming of biopsied in vivo derived (IVD) bovine embryos. Three experiments were carried out using two methodologies: the Cryotop®, the gold standard vitrification and 3-step warming methodology, or the VitTrans, a vitrification/in-straw 1-step warming method that enables direct embryo transfer to the uterus. In experiment 1, intact and biopsied in vitro produced (IVP) day 7 expanded blastocysts were vitrified using the Cryotop® and warmed in 1- or 3-steps. No differences in survival rates were recorded at 24 h after warming for intact or biopsied IVP blastocysts irrespective of the warming procedure. In experiment 2, the effect of the time from trophectoderm (TE) biopsy to vitrification/in-straw warming on post-warming survival rate was assessed. No significant differences in survival were observed when blastocysts were vitrified/in-straw warmed immediately after biopsy or after 3 h in culture when compared to intact blastocysts. In experiment 3, IVD embryos were vitrified 3 h after biopsy using the Cryotop® or the VitTrans method and pregnancy rates were assessed at day 60 after transfer. Fresh, biopsied embryos served as control. Similar pregnancy rates were observed when IVD biopsied embryos were transferred fresh or vitrified/warmed by the Cryotop® or VitTrans method. No significant effect of the embryo quality or developmental stage was detected on the percentage of pregnant recipients when IVD biopsied embryos were transferred fresh or after vitrification. While fresh female IVD embryos produced significantly higher pregnancy rates than male embryos, there were no differences in pregnancy rates when male or female vitrified/warmed embryos were transferred. About 81% from the biopsies analyzed successfully determined the embryo sex, confirming that DNA was there, and it was efficiently amplified. To conclude, our findings indicate that both vitrification methodologies produced similar post-warming outcomes for both intact and biopsied IVP embryos. Besides, vitrification/in-straw warming of biopsied IVD bovine embryos did not affect the viability to originate pregnancy, being a useful option for their direct transfer in field conditions.

Sex-specific embryonic origin of postnatal phenotypic variability

Preimplantation developmental plasticity has evolved in order to offer the best chances of survival under changing environments. Conversely, environmental conditions experienced in early life can dramatically influence neonatal and adult biology, which may result in detrimental long-term effects. Several studies have shown that small size at birth, which is associated with a greater risk of metabolic syndrome, is largely determined before the formation of the blastocysts because 70%–80% of variation in bodyweight at birth has neither a genetic nor environmental component. In addition, it has been reported that adult bodyweight is programmed by energy-dependent process during the pronuclear stage in the mouse. Although the early embryo has a high developmental plasticity and adapts and survives to adverse environmental conditions, this adaptation may have adverse consequences and there is strong evidence that in vitro culture can be a risk factor for abnormal fetal outcomes in animals systems, with growing data suggesting that a similar link may be apparent for humans. In this context, male and female preimplantation embryos display sex-specific transcriptional and epigenetic regulation, which, in the case of bovine blastocysts, expands to one-third of the transcripts detected through microarray analysis. This sex-specific bias may convert the otherwise buffered stochastic variability in developmental networks in a sex-determined response to the environmental hazard. It has been widely reported that environment can affect preimplantation development in a sex-specific manner, resulting in either a short-term sex ratio adjustment or in long-term sex-specific effects on adult health. The present article reviews current knowledge about the natural phenotypic variation caused by epigenetic mechanisms and the mechanisms modulating sex-specific changes in phenotype during early embryo development resulting in sex ratio adjustments or detrimental sex-specific consequences for adult health. Understanding the natural embryo sexual dimorphism for programming trajectories will help understand the early mechanisms of response to environmental insults.

Effect of maternal obesity on estrous cyclicity, embryo development and blastocyst gene expression in a mouse model

STUDY QUESTION

Does maternal obesity affect estrous cyclicity, embryo development and blastocyst gene expression in mice?

SUMMARY ANSWER

Maternal obesity alters estrous cyclicity and causes the down-regulation of two key metabolite receptors (Slc2a1 and Ldlr) in blastocysts recovered from diet-induced obese females, but embryo development is not affected.

WHAT IS KNOWN ALREADY

Maternal obesity reduces fertility because of effects in the periconception period, but its negative influence is on estrous cyclicity, oocyte quality or embryo development.

STUDY DESIGN, SIZE AND DURATION

This was a randomized study based on a mouse model for obesity. Twenty-one outbred NIH Swiss mice were used and obesity was induced by a diet high in fat administered for 12 weeks prior to breeding to control males.

MATERIAL, SETTING AND METHODS

Females were fed either a control diet (C, n = 9) or a diet high in fat [diet-induced obesity (DiO), n = 12] for 12 weeks, and were then co-housed with fertile males. Mice that failed to breed during 20 consecutive days were considered infertile. Control and diet-induced obese females that demonstrated vaginal plugs were euthanized 3.5 days after mating, blood was sampled for glucose and hormone measurements, corpora lutea counted and embryos recovered; the relative mRNA abundance of 11 candidate genes was determined in blastocysts by qPCR.

MAIN RESULTS AND THE ROLE OF CHANCE

Five DiO females failed to breed and displayed anovulatory ovaries (DiOI), whereas the other seven DiO females (DiOF) could breed, albeit over an extended period compared with controls. DiOF weighed significantly less than DiOI. Both groups had elevated serum insulin compared with C, although blood glucose level was only significantly higher than that in controls in the infertile DiOI group. Adiponectin was lower in the DiOI and leptin higher in both the DiOI and DiOF mice than in C. DiOF ovulated the same number of oocytes as C, and embryo development to blastocyst was normal. The expression of genes encoding metabolic hormone receptors (Insr, Igf1r, Igf2r, Adipor1, Adipor2 and Lepr) and key metabolic enzymes (Gapdh, Cpt1a and Sod2) did not differ between DiOF and C blastocysts, but that of metabolite receptors (Slc2a1 and Ldr) was down-regulated in DiOF. To limit the role of chance, the experiments were conducted in a defined laboratory setting with the proper controls, and the animals were randomly assigned to each experimental group. Moreover, a P-value of < 0.05 was chosen to determine whether the differences observed between the groups were statistically significant.

LIMITATIONS AND REASONS FOR CAUTION

The results obtained may not fully extrapolate to humans. Also, as follicular activity was not monitored while breeding, so the extended breeding period for DiOF group might be explained by behavioral abnormalities occurring in normal cycling animals.

WIDER IMPLICATIONS OF THE FINDINGS

DiO alters the estrous cycle in the mouse model and demonstrates a role of obesity in infertility. The data also suggest that in an outbred, genetically diverse population, such as the human, individual susceptibility to obesity and associated infertility induced by diet exists. The apparently normal development to blastocyst observed in fertile, obese females suggests that preimplantation embryos can resist potentially adverse outcomes caused by an oversupply of fatty acids and glucose under in vivo conditions. This metabolic plasticity may, in part, be due to an ability to down-regulate metabolite transporters, thereby preventing excessive nutrient uptake.

STUDY FUNDING/COMPETING INTEREST(S)

The research was supported by funds from the University of Missouri, grants from the National Institutes of Health and by a fellowship from the Lalor Foundation. There were no competing interests.

TRIAL REGISTRATION NUMBER

Not applicable.