Assessment of morphological stage grade is a subjective procedure. Stage grade is of vital importance to, among other things, recipient synchrony for the purpose of establishing successful pregnancies. Asynchronous embryo transfer has led to decreases in pregnancy rates (Farin et al. 1995 Biol. Reprod. 52, 676–682) and has been implicated in contributing to large offspring syndrome (Young et al. 1996 Theriogenology 45, 231). Differences in embryo kinetics based on culture conditions have been well documented (Mello et al. 2005 Reprod. Fert. Dev. 17, 221 abst). Whether such differences are the result of species, breed, metabolic stress, sire effects, or separation from an in vivo environment has yet to be determined. The correlation between oxygen respiration rates and embryo morphology as well as embryo diameter in bovine embryos produced in vitro has shown promise in the development of a more objective predictor of embryo quality and perhaps pregnancy initiation (Lopes et al. 2005 Reprod. Fert. Dev. 17, 151 abst). As well, recent examination of gene expression patterns of in vitro-derived bovine embryos seems to indicate that longer periods of in vitro culture are associated with lower rates of embryo survival (Lonergan et al. 2006 Theriogenology 65, 137–152). We hypothesize that differences do exist in the number, rate, and morphological appearance of blastocysts and that these parameters are in large part based on culture conditions in vitro. The objective of this experiment was to determine the timing and distribution of blastocyst formation of in vitro-produced bovine embryos cultured in SOF8, CR18AA, and KSOM8, under a standard incubation environment. Bovine ovaries from a local abattoir were aspirated and matured for 18-22. Oocytes were fertilized with frozen-thawed Percoll-separated semen from a Holstein bull. Presumptive zygotes were vortexed to remove cumulus cells and placed into 3 different culture media in a highly humidified atmosphere containing 20% oxygen, 5% carbon dioxide, and compressed air at 38.5�C. Embryos were evaluated specifically at 168 h post-insemination (Day 7) and assigned a morphological stage grade (IETS) to determine fixed time point differences. A total of 6 complete replicates were performed. Only embryos exhibiting the presence of a blastocoel at this time were documented (early blast, mid-blast, expanded blast). At 168 h post-insemination, there were no significant differences in the total number of embryos reaching early or mid-blast stage in any of the media. However, chi-square analysis revealed an increase in the number of expanded blastocysts in SOF (n = 813) and CR1 (n = 838) treatments compared to KSOM (n = 824; P < 0.0001). Expanded blastocysts in SOF were also greater in number than in CR1 (P < 0.05). Embryo selection based on development to the expanded blastocyst stage on Day 7 may prove useful in increasing pregnancy rates, and may validate qualitative correlations based on oxygen consumption and gene expression profiles for embryos produced in vitro.
Pregnancy rates and births after unilateral or bilateral transfer of bovine embryos produced in vitro