Study question: Is O2 tension in the pig oviduct and uterus affected by the estrous cycle stage and the animal's age, and can the outcome of in vitro embryo development be improved by mimicking these physiological values?
Summary answer: O2 tension within the pig reproductive organs is affected by the animal's age, and values close to those measured in vivo have a positive impact on embryo development and quality when used during IVF and embryo culture (EC).
What is known already: To obtain a healthy embryo in vitro, it is necessary to adopt a culture microenvironment that approximates physiological conditions. Despite advances in surgical procedures and sensitive probes that allow accurate assessment of in vivo O2 tension, few such studies have been conducted recently in mammals. In addition, no reference values of physiological O2 tension in the reproductive tract exist for large animal models such as pig, and the effect of O2 tension on ART outcomes is unknown.
Study design, size, duration: This study was conducted in pigs. We measured oviductal and uterine O2 tension (n = 29 and 13, respectively) and then examined how the use of the physiological values in pig IVF and EC affected pig ART output (n = 1447 oocytes).
Participants/materials, setting, methods: The oviductal and uterine O2 tension at the different stages of the estrous cycle was monitored using a laparo-endoscopic single-site surgery (LESS) assisted approach along with a flexible and thin miniaturized luminescent probe. Two groups of pigs, Large-white × Landrace breed, were used: for the first group, 16 pre-pubertal gilts (5 months old and 95 kg) were induced to ovulate with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG); in the second group 13 mature sows (24-48 months and 185 kg) were used. IVF and EC were performed at two different O2 tensions: Atmospheric O2 (20%) and the mean in vivo value measured (7%). At 18-20 h post-insemination (hpi), a small sample of presumptive zygotes were fixed, stained and examined under epifluorescence microscopy to assess the fertilization rates. At 48 hpi, cleavage was evaluated under the stereomicroscope. Finally, at 180 hpi, development to the blastocyst stage was quantified, blastocyst morphology was assessed, and embryos were fixed and stained to count the mean cell number per blastocyst.
Main results and the role of chance: The mean O2 content within the pig oviduct and uterus was always lower than in ambient air. The average O2 percentage was higher in gilts (10.0%) than in sows (7.6%) (P < 0.0001). The cleavage rate of porcine in vitro fertilized embryos maintained under 7% O2 during IVF and EC was significantly higher (60.0 ± 2.3) compared with those cultured under 20% O2 (32.0 ± 2.2) (P < 0.05). An increase in the number of cells in embryos cultured under the low O2 concentration (88.9 ± 5.9) was observed compared to those cultured under 20% O2 (59.0 ± 5.0) (P < 0.05).
Large scale data: None.
Limitations, reasons for caution: Although minimally invasive surgery was used the effect of anesthesia and manipulations on O2 tension within the organs are unknown.
Wider implications of the findings: Using physiological oxygen concentrations in IVF/EC could improve ART outcomes.
Study funding and competing interest(s): This study was funded by Spanish Ministry of Economy and Competitiveness (MINECO) and European Regional Development Fund (FEDER). Grants AGL2012-40180-C03-01 and AGL2015-66341-R. The authors declare no conflict of interest.