Optimal bovine in vitro oocyte maturation (IVM) is a prerequisite for subsequent optimal blastocyst rates. Ovum pick-up (OPU), by which cumulus–oocyte complexes (COC) are collected in vivo, is performed outside a laboratory and often requires IVM to take place during transportation from the farm to the IVF laboratory. Hashem et al. (2017 Reprod. Fertil. Dev. 29, 179) demonstrated that blastocyst rates are affected by type of vial (glass v. plastic), number of COC per vial, and volume of medium per vial. This was achieved by maturing more than 2500 COC from slaughterhouse material under contrasting conditions, followed by standardised IVF and in vitro culture (IVC) and observation of blastocyst rates, morphology (1: poor; 2: good; 3: excellent), and kinetics (1: blastocyst; 2: expanded blastocyst ; 3: hatching/hatched blastocyst). Here we examined differential staining of a subset of expanded blastocysts (XB) from the previous study to assess the influence of vial material, medium volume, and number of COC per vial on total cell count, number and ratio of inner cell mass (ICM), and trophectoderm (TE) cells. In experiment 1 (4 groups), oocytes were matured in different vials without lids in an incubator at 5.5% CO2 in humidified atmospheric air at 38.5°C to assess plastic toxicity. In experiment 2 (6 groups) and experiment 3 (6 groups), the 2 best performing vials-polypropylene cryovials (Sigma-Aldrich, St. Louis, MO, USA) and glass vials (VWR International, Radnor, PA, USA)-containing 50% (Exp. 2) or 95% (Exp. 3) medium volume per vial and 5, 20, or 45 COC per vial were tested. In experiments 2 and 3, the vials were closed and incubated in atmospheric air at 38.5°C. All groups were evaluated for blastocyst rates, kinetics, and morphology. Because kinetics (range 2.01–2.25) and morphology (range 2.15–2.50) were similar in all groups, only XB were collected from each group. These were fixed and stained with CDX2 antibody and Hoechst (Wydooghe et al. 2011 Anal. Biochem. 416, 228-230) and their ICM and TE cells were counted. The cells were counted manually in blinded groups using an inverted fluorescence microscope and 16× magnification. Counts of total, ICM, and TE cells were compared between treatments by a two-way ANOVA analysis. A total of 240 XB from the 16 different vial groups were counted in the 3 experiments, with average total cell counts of 139 (110–211) and ICM cell counts of 44 (28–75). Even though the blastocyst rates differed between some of the groups, the cell counts within the XB did not differ statistically significantly between groups. In fact, the highest cell count was found in the glass vial group with the lowest blastocyst rate (45 COC per vial in 50% medium volume; blastocyst rate 28%, total cells 211, ICM cells 75). We have previously demonstrated that the type of vial, number of COC per vial, and the volume of medium per vial influence the subsequent blastocyst rates. It is concluded, however, that the embryos able to proceed to the blastocyst stages seem to be of the same quality in all groups, assessed by kinetics, morphology, and cell counts within XB.
Stereomicroscopic and histological examination of bovine embryos following extended in vitro culture
