24 Lipid composition of fresh or frozen sexed bovine blastocysts produced in vivo or in vitro

Abstract

Currently, in vitro embryo production (IVP) is successfully applied commercially in cattle. However, the high sensitivity of embryos to cryopreservation compared with in vivo-derived (IVD) embryos still impairs the dissemination of this biotechnology. Reduced cryotolerance is frequently associated with lipid accumulation in the cytoplasm mainly due to in vitro culture conditions. The objective of this study was to evaluate the lipid content of fresh and frozen sexed bovine grade 1 IVP or IVD embryos. The same 8 Holstein heifers were used in a Latin square design for both IVP and IVD embryo production. Zygotes were cultured in synthetic oviductal fluid (SOF) supplemented with 1% oestrus cow serum. The same bull was used for IVP and IVD. All expanded Day 7 blastocysts (n = 40 IVP and 40 IVD) were biopsied and sexed. Half of the embryos (n = 20 in each group) was slow frozen (1.5 M ethylene glycol, 0.1 m sucrose) and thawed before lipid extraction. Remaining embryos underwent lipid extraction in the fresh state. Briefly, the liposoluble fraction of the embryos was extracted according to the Bligh and Dyer method using chloroform and methanol. Liquid chromatography–high-resolution mass spectrometry (LC-HRMS) analysis was performed and operated in positive ionization mode. Lipids with variance intensities greater than 30% in quality control samples were removed as well as those identified as background noise. Partial least square discriminant analysis (PLS-DA) was used to show the relationship between variance in the data and difference among embryo origin (IVP vs. IVD), state before extraction (fresh vs. frozen), and sex of the embryos (male vs. female). The differentially lipid species groups were identified using Wilcoxon test, and considered significantly different when P < 0.05. LC-HRMS analysis allowed us to identify 75 lipids. PLS-DA showed that embryo origin (IVP vs. IVD) and state before extraction (fresh vs. frozen) can be determined by LC-HRMS profiles by group in PLS-DA plot, despite slight overlaps. Sex of the embryos did not allow us to differentiate the lipid profile. However, 15 lipids varied significantly between male and female IVD, predominantly triglycerides (TG), whereas no lipid varied between the sexes in the IVP homologues. Moreover, 26 lipids varied significantly between IVP and IVD fresh embryos with enrichment of IVP embryos in TG, phosphatidyl choline, cholesteryl ester, and less diglyceride and lysophospholipid (LP) compared with IVD embryos. The comparison of the lipid profiles before and after freezing for IVP embryos showed that only 7 lipids varied significantly between fresh and frozen states with a decrease in LP for the frozen embryos. For the in vivo counterparts, 13 lipids varied significantly, including the same LP as those identified for IVP embryos in the same way. Our results showed that the embryonic lipid profile is mainly affected by IVP and slow freezing protocols and, to a lesser extent, by sex. Further studies are needed to improve IVP protocols and optimize the cryotolerance of IVP embryos in cattle.