Characterization of preovulatory follicular fluid secretome and its effects on equine oocytes during in vitro maturation

In vitro maturation (IVM) of oocytes is clinically used in horses to produce blastocysts but current conditions used for horses are suboptimal. We analyzed the composition of equine preovulatory follicular fluid (FF) secretome and tested its effects on meiotic competence and gene expression in oocytes subjected to IVM. Preovulatory FF was obtained, concentrated using ultrafiltration with cut-off of 10 kDa, and stored at -80 °C. The metabolic and proteomic composition was analyzed, and its ultrastructural composition was assessed by cryo-transmission microscopy. Oocytes obtained post-mortem or by ovum pick up (OPU) were subjected to IVM in the absence (control) or presence of 20 or 40 μg/ml (S20 or S40) of secretome. Oocytes were then analyzed for chromatin configuration or snap frozen for gene expression analysis. Proteomic analysis detected 255 proteins in the Equus caballus database, mostly related to the complement cascade and cholesterol metabolism. Metabolomic analysis yielded 14 metabolites and cryo-transmission electron microscopy analysis revealed the presence of extracellular vesicles (EVs). No significant differences were detected in maturation rates among treatments. However, the expression of GDF9 and BMP15 significantly increased in OPU-derived oocytes compared to post-mortem oocytes (fold increase ± SEM: 9.4 ± 0.1 vs. 1 ± 0.5 for BMP15 and 9.9 ± 0.3 vs. 1 ± 0.5 for GDF9, respectively; p < 0.05). Secretome addition increased the expression of TNFAIP6 in S40 regardless of the oocyte source. Further research is necessary to fully understand whether secretome addition influences the developmental competence of equine oocytes.

Description of a new quantitative method to assess mitochondrial distribution pattern in mature equine oocytes

The Mitochondrial distribution pattern or MDP in mammalian oocytes serves as an indicator of their cytoplasmic maturity, with a heterogeneous pattern associated with mature cytoplasm. Currently, MDP assessment involves fluorescent labelling of mitochondria followed by visual evaluation, as no quantitative method exists. Our objective was to develop a quantitative approach to assess MDP in mature equine oocytes. Equine oocytes, obtained by ovum pick up (OPU) were matured in vitro, and only metaphase II oocytes were used in the study (n = 56). Following denudation, oocytes were fixed, stained with MitoTracker™ Red CMXRos (50 nM in TCM-199 with Hank´s salts and 10% FBS) for 15 min at 38 °C, and then incubated with 2.5 µg/ml Hoechst 33342 for 10 min at 38 °C. Confocal microscope images were acquired, and the oocyte's MDP was visually classified as either homogeneous (HoD; n = 17) or heterogeneous (HeD; n = 39). For quantitative analysis, Fiji-ImageJ software was employed. Background subtraction was performed, and a 1-pixel line along the diameter was drawn to calculate the intensity profile. Fluorescence intensities were normalized, and ratios of peripheral to central fluorescence intensity were determined. Student´s t-test was used for comparations; MDP ratio was (mean ± standard error of the mean): 0.8 ± 0.02 for HoD and 0.3 ± 0.02 for HeD (p < 0.001). These results demonstrate concordance between quantitative and qualitative MDP assessment in mature equine oocytes. Our study describes a new approach to quantify mitochondrial distribution pattern and cytoplasmic maturation in mature equine oocytes.