Sequential nutrient restriction and provision during bovine in vitro embryo culture differentially affect blastocyst development and quality with oocytes from varied sources

We have demonstrated that bovine blastocyst development was improved after culture in medium with only 6.25% of standard carbohydrate and amino acid concentrations, supplemented with fatty acids. However, these blastocysts had lower cell numbers. We hypothesised that this was due to deficiencies in embryo metabolism at the time of blastocyst formation. Thus, our objectives were to (1) determine whether using a sequential combination of nutrient concentrations could rescue blastocyst cell number; and (2) investigate the efficacy of reduced nutrient medium in 2 sources of oocytes. Oocytes were in vitro matured in identical medium either in our laboratory or during shipment from a commercial supplier. Oocytes in our laboratory were derived from feedlot heifers while purchased oocytes were obtained from culled cows. Zygotes were cultured using sequential medium with fraction V BSA. In step 1/step 2, embryos were cultured using 100% (glucose 0.5 mM/fructose 3.0 mM, pyruvate 0.3/0.1 mM, lactate 10.0/6.0 mM, NEEA 1×/1× MEM, EAA 0.25×/0.5× MEM), 25% or 6.25% of standard nutrient concentrations. On Day 3, embryos were moved to step 2 as follows: 100% to 100%, 25% to 25%, 25% to 100%, 6.25% to 25%, or 6.25% to 100%. Lipid content of single mature oocytes from both sources was determined using gas chromatography coupled to an ISQ-LT MS/MS (GC-MS; Thermo Scientific, Waltham, MA, USA). Data (mean ± s.e.m.) were analysed using ANOVA (P < 0.05). When oocytes from feedlot heifers were used, blastocyst development and cell number did not differ between treatments. When oocytes from culled cows were used, blastocyst development was improved after embryo culture in 25-25% (45.1 ± 3.3%) and 6.25-25% (46.6 ± 3.2%) compared with 100-100% (34.2 ± 3.2%). However, inner cell mass number of blastocysts cultured in 25-25% (25.6 ± 2.5) and 6.25-25% (26.0 ± 2.6) was reduced compared with 100-100% (41.4 ± 4.5); TE and total cell number did not differ. Embryos cultured in 100-100%, 25-100%, and 6.25-100% were equivalent. Metabolomics revealed that 10 lipid compounds (polyunsaturated fatty acids, glycosyldiacylglycerols, and glycerophospholipids) differed in abundance between the two sources of oocytes. These results show that oocytes from different sources lead to different experimental outcomes, likely due to a combination of age, body condition, diet, and hormone treatment of the female. Oocytes from culled cows result in embryos that develop to blastocysts better in a reduced nutrient environment, although these embryos have fewer inner cell masses, suggesting that quality may be reduced. Embryos from feedlot heifer oocytes are relatively immune to nutrient fluctuations. Different endogenous fatty acid reserves in the oocyte may lead to differing metabolic strategies in the subsequent embryo, altering their response to substrate availability during in vitro culture. These results also demonstrate that reduction of nutrients during culture has no detrimental effect on blastocyst development or total cell number in either oocyte source, but that inner cell mass formation requires increased nutrient provision.