A simple, non-invasive, rapid and sensitive oxygen microsensor system was developed to investigate correlations between oxygen respiration rates of individual bovine embryos and their morphology, sex, diameter and developmental stage. Bovine IVP-embryos (n = 78; Holm et al. Theriogenology 52, 683–700) were analysed around the 8-cell stage (Day 3; n = 18) and at various blastocyst stages (Day 7; n = 60). Each embryo was morphologically evaluated, its outer diameter measured and was then loaded into a glass tube (i.d. 0.68 mm, length 3 mm). After 1 h, oxygen concentration gradients generated by the embryo’s respiration were measured over app. 8 min with an oxygen microelectrode (www.unisense.com). Five embryos were measured in one round together with an empty tube as control. The procedure was repeated twice for each embryo with app. 1 h interval. Individual respiration rates in nL O₂/embryo/h (nL/h) were calculated from these gradients. The measurements were performed at 38.5°C under constant flow of humidified 5% CO₂ in air (app. 19% O₂). After this, 64 embryos (14 Day 3; 50 Day 7) were lysed for sex diagnosis by PCR. Values are given as mean ± SD. The sensitivity of the oxygen measurement system was high (controls: 0.034 ± 0.035 nL/h, n = 15) and its repeatability from 1st to 2nd measurement was 99.7 ± 9.8% (n = 71). The average embryo respiration rate was 0.39 ± 0.05 nLl/h on Day 3 (n = 18) and 1.31 ± 0.52 nLl/h on Day 7 (n = 60). For Day 7 embryos, the respiration rates varied according to their morphological quality, being 1.87 ± 0.46^a (n = 18), 1.17 ± 0.32^b (n = 23), 0.95 ± 0.27^b,c (n = 14) and 0.72 ± 0.24^c (n = 4) nL/h for quality 1, 2, 3, and 4 embryos, respectively (Proc Mixed,^a,b,c: P < 0.05; values with different superscripts differ significantly). The sex ratio (male:female) was 9:5 (Day 3) and 32:18 (Day 7), and on Day 7 this ratio varied between qualities: 11:2, 12:8, 8:4, and 1:3 for quality 1, 2, 3, and 4, respectively. The average respiration rate on day 3 was the same for males and females, as it was on day 7 (1.22 ± 0.43 nL/h (females) and 1.31 ± 0.58 nL/h (males), P > 0.05). There was a correlation between embryo diameter and respiration rate (r² = 0.65, n = 74), which was even stronger for Day 7 male embryos (r² = 0.72, n = 32). In conclusion, a highly reliable, repeatable and sensitive system was established for measuring respiration rates in single bovine embryos, even at early developmental stages. The respiration rate was lower on day 3 compared to Day 7 embryos, and it was correlated with the morphological embryo quality on Day 7. Oxygen consumption could be a valuable supplementary indicator of embryo viability, especially in difficult evaluations (e.g. quality 2 and 3 after IVP). It remains to be demonstrated if such measurements can also reveal quality differences already at Day 3, which would be of interest in, e.g. the human field.

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