Biochemical alterations in the follicular fluid of bovine peri-ovulatory follicles and their association with final oocyte maturation

Abstract

Follicular fluid (FF), a product of vascular transudate and granulosa and thecal cell secretions, is the milieu that has evolved to support oocyte growth and maturation which plays a central role in oocyte quality determination. Therefore, a suboptimal FF composition may be reflected in compromised oocyte progression through maturation, fertilization, or embryo development. To date, the composition of bovine FF remains understudied. To address this, we comprehensively characterized the metabolomic constituency of bovine FF in the period during which the oocyte undergoes meiotic maturation. More specifically, FF from pre (−24 h) and peri (−2 h)-ovulatory follicles was profiled by high-throughput untargeted ultra-HPLC tandem mass spectroscopy. A total of 634 metabolites were identified, comprising lipids (37.1%), amino acids (30.0%), xenobiotics (11.5%), nucleotides (6.8%), carbohydrates (4.4%), cofactors and vitamins (4.4%), peptides (3.6%), and energy substrates (2.1%). The concentrations of 67 metabolites were significantly affected by the stage of follicle development, 33.3% (n = 21) were reduced (P ≤ 0.05) by a mean of 9.0-fold, whereas 46 were elevated (P ≤ 0.05) by a mean of 1.7-fold in peri- vs pre-ovulatory FF. The most pronounced individual metabolite concentration decreases were observed in hypoxanthine (98.9-fold), xanthine (65.7-fold), 17β-oestradiol (12.4-fold), and inosine (4.6-fold). In contrast, the greatest increases were in retinal (4.9-fold), 1-methyl-5-imidazoleacetate (2.7-fold), and isovalerylcarnitine (2.7-fold). This global metabolomic analysis of bovine FF temporal dynamics provides new information for understanding the environment supporting oocyte maturation and facilitating ovulation that has the potential for improving oocyte quality both invivo and in vitro. Lay Summary The ovaries are part of the female reproductive system, and they produce and store eggs in structures known as ‘follicles’. Depending on the species, one or more follicles release an egg from the ovary during ovulation. FF, which is formed from the secretions of follicle cells and substances delivered from the bloodstream, bathes the eggs as they develop within their follicles. For pregnancy to happen, the egg must be capable of being fertilised by a sperm cell, developing into an embryo and implanting it in the womb. FF has evolved to support the egg to achieve this. Using the cow as a model, this study looks at the composition of FF during the final hours before ovulation, when the egg becomes mature and ready for fertilisation. More than 600 different substances were identified, providing new information, that has the potential to improve egg quality.