Manipulation of early embryonic development

Murine pre-embryo oxygen consumption and developmental competence

PURPOSE

In search for a new marker of preimplantation embryo viability the present study investigated oxygen consumption of individual cleavage stage murine embryos, and evaluated the predictive value regarding subsequent development to expanded blastocysts.

METHODS

In all, 248 embryos were investigated from 2 cell stage until blastocyst stage with individual measurement of oxygen consumption and recording of developmental stage. Cleavage stage embryos and morula were divided in groups according to their oxygen consumption, and odds ratios (OR) for subsequent development to expanded blastocyst were calculated.

RESULTS

Cleavage stage (2-8 cell) individual oxygen consumption was 0.16-0.20 nl O(2) h(-1), with a significant increase to 0.21-0.23 nl O(2) h(-1) at the morula stage followed by a more than twofold increase for the expanded blastocyst 0.47 nl O(2) h(-1). A significantly higher chance of reaching the expanded blastocyst stage was found in 4-cell embryos with high oxygen consumption, than embryos with low consumption (OR 2.25, 95% CI 1.04-4.90). Among 2-cell embryos the chance of low and high consumers was not significantly different. The method used in the present study somewhat compromised embryo development (51% blastocyst rate) compared to controls (80% blastocystrate) which could make our results less robust.

CONCLUSION

Preliminary data from the present study suggest that oxygen consumption in cleavage stage embryos may be an indicator, but a not a strong predictor, of subsequent development to expanded blastocysts.

Respiration rates correlate with mRNA expression of G6PD and GLUT1 genes in individual bovine in vitro-produced blastocysts

Quantification of embryo respiration is a promising procedure to assess embryonic metabolism and possibly select viable embryos. At the blastocyst stage, ATP is produced by glycolysis and oxidative phosphorylation, processes that require uptake of oxygen and glucose, which is regulated by the expression of GLUT1 and G6PD. The purpose of the present study was to investigate the relationship between respiration rates and relative abundances of G6PD and GLUT1 transcripts in individual bovine blastocysts produced in vitro. Respiration rates of 104 bovine in vitro-produced blastocysts were measured individually using the nanorespirometer technology. Real-time RT-PCR was employed to determine the relative abundance of G6PD and GLUT1 mRNA in individual embryos. The mean respiration rates were similar for male and female blastocysts of the same developmental stage, but the sex ratio was skewed towards males. GLUT1 expression was down-regulated in female versus male embryos. In contrast, a approximately 1.8-fold increase in the expression of G6PD mRNA was observed in female blastocysts when compared to male blastocysts, indicating that dosage compensation for this gene had not yet occurred. Both GLUT1 and G6PD expression levels were affected by morphological quality and stage of development. Expression of GLUT1 and G6PD mRNAs was correlated with respiration rates, indicating that, in metabolically active blastocysts, uptake of oxygen and glucose are jointly increased. These findings suggest that expression of genes for oxidative phosphorylation and glycolysis are both involved in oxygen demanding ATP production.

Investigation of respiration of individual bovine embryos produced in vivo and in vitro and correlation with viability following transfer

BACKGROUND

Quantification of oxygen consumption by individual preimplantation embryos has the potential to improve embryo selection. This study investigated whether respiration rates of individual embryos are useful indicators of embryo viability. The effect of the Nanorespirometer on embryo viability was also evaluated.

METHODS

The respiration rates of individual day 7 bovine in vivo- (n=44) and in vitro-produced (n=156) embryos were measured using the Nanorespirometer. In vivo-produced embryos were individually transferred to recipients.

RESULTS

The respiration rates of in vivo-produced embryos increased with increasing morphological quality and stage of development (P < 0.05). Pregnancy rates on days 35 and 60 were 65 and 60%, respectively. The mean respiration rate did not differ significantly between embryos producing and not producing a pregnancy, but the transfer of embryos with respiration rates <0.78 nl/h, between 0.78 and 1.10 nl/h, and >1.10 nl/h resulted in 48, 100 and 25% pregnancy rate, respectively. The mean respiration rate of in vitro-produced embryos was higher than that of in vivo-produced embryos because of differences in the morphological quality and stage of development.

CONCLUSION

The Nanorespirometer does not adversely influence embryo viability, but the sample size was too small to confirm the significance of the correlation observed between respiration rates and viability.

Respiration rates of individual bovine in vitro-produced embryos measured with a novel, non-invasive and highly sensitive microsensor system

Oxygen consumption is a useful parameter for evaluating embryo quality, since it provides a valuable indication of overall metabolic activity. Over the years, several approaches have been used to measure the respiration rates of individual embryos, but a convincing method has not yet been reported. In this study, we introduce and have validated a novel high resolution microsensor technology to determine the respiration rates of individual embryos at different developmental stages. We have employed this technology to investigate the correlation between respiration rate and embryo morphology, diameter and sex. Following morphological evaluation, individual respiration rates of day 3 (n= 18) and day 7 (n= 60) bovinein vitro-produced embryos were determined. Of the measured embryos, 64 were lysed for sex diagnosis by PCR. Average respiration rates of day 7 embryos (1.30 ± 0.064 nl/h) were 3.4-fold higher than day 3 embryos (0.38 ± 0.011 nl/h). On day 7, the average respiration rate of quality 1 blastocysts was significantly higher than the respiration rates of the lower qualities. For both day 3 and day 7 embryos, respiration rates were directly influenced by embryo diameter but did not differ between sexes. These results have demonstrated that the novel microsensor technology can be used to accurately and rapidly (8 min) measure the respiration rates of individual embryos at different developmental stages. Respiration rates were only in partial agreement with embryo morphology, suggesting a slight discrepancy between these two methods in assessing embryo quality. It is likely that a combined assessment of embryo respiration and morphology would improve embryo classification and subsequent selection.

Statistical models predicting embryo survival to term in cattle after embryo transfer

Embryo survival to term in recipient cattle is highly variable. We examined calving data in the published literature to determine whether a model of binomial independence or a model which includes an embryo (e) and recipient term (r), adequately explain observed embryo survival rates following attempts to induce twin calving using transfer of two embryos. To achieve this we examined 32 published papers which provided us with 47 sets of data concerning 4560 recipients with either 0, 1 or 2 calves born. In each set of data, the observed embryo survival rate to term (p) (number of calves born/number of embryos) was calculated and the expected number of recipients with either 0, 1 or 2 calves born was determined, assuming a binomial distribution. Parameters for the second model were estimated using maximum-likelihood procedures. The model of embryo independence was rejected in 85% of the sets of data, suggesting that factors other than the embryo are important sources of variation in embryo survival or loss. The proposed e and r model of embryo survival adequately describes the published data in recipients receiving either single or twin embryos. In general, only 50-70% of embryos and recipients are sufficiently competent to result in a calving. Variation among laboratories producing either in vitro or in vivo derived embryos was due to variation in recipient and not embryo competence. It is argued that e rather than observed embryo survival rate, and r rather than observed pregnancy rate, should be used to compare differences among embryo treatments and groups of recipients, respectively. Acceptance of this proposition should permit faster progress in identifying the biology of superior embryos and recipients, which is a prerequisite to improving embryo survival rate in cattle. Collectively, the published data are not consistent with a model of embryo independence, and that a model of embryo survival to term which recognises recipient as well as embryo contributions to embryo survival may be more appropriate in cattle.