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 in vivo and in vitro.