Oviductal microvesicles and their effect on in vitro maturation of canine oocytes

The effect of conditioned medium (CM) or microvesicles (MVs), secreted by multicellular spheroids of oviductal cells, and the involvement of some microRNAs (miRNAs) were investigated in canine oocyte maturation. To generate CM, spheroids were cultured for 3 days. MVs were obtained by ultracentrifugation of CM at 100,000 g and measured for size and concentration by NanoSight instrument. Cumulus-oocyte complexes (COCs) were matured at 38.5°C with 5% CO₂ and 5% of O₂ in synthetic oviductal fluid (SOF) in biphasic systems: for 24 h, with 5.0 μg/mL of LH and for other 48 h with 10% oestrous bitch serum. SOF was used as control (CTR) or supplemented with 10% CM or 25-50-75-100-150 × 10⁶ MVs/mL labeled with PKH-26. Results show that multicellular aggregates secreted shedding vesicles. By fluorescence microscopy, the incorporation of labeled MVs was visible only at 72 h in oocyte cytoplasm. These MVs had a positive effect (P < 0.05) on maturation rate (MII) at the concentration of 75 and 100 × 10⁶ MVs/mL compared to CM and CTR (20.34% and 21.82% vs 9.09% and 8.66% respectively). The concentration of 150 × 10⁶ MVs/mL provided only 9.26% of MII. The expression of three specific miRNAs (miR-30b, miR-375 and miR-503) was studied. The lower rate of MII with the higher concentration of MVs is possibly due to the high level of miR-375. In conclusion, the oviductal MVs could be involved in cellular trafficking during oocyte maturation and their possible use in vitro could facilitate the exploitment of canine reproductive biotechnologies.