Cumulus-oocyte communications in the horse: role of the breeding season and of the maturation medium.

The research investigates the communication between cumulus cells and oocytes (eggs) in horses based on the breeding season and the type of maturation medium used. The researchers examined how seasonal changes and different fertilization procedures affect the quality and functionality of horse oocytes.

Objectives of the Study

• The study aims to determine the effect of the horse’s breeding season on the quality of the oocytes. Horses are seasonal breeders, and information about the quality of oocytes outside of their breeding season is limited.
• The study also seeks to understand the impact of the maturation medium (either bovine serum albumin [BSA] or oestrus mare serum [EMS]) on the cumulus-oocyte communication.

Research Methodology

• A total of 645 oocytes from 158 and 154 ovaries collected during the breeding and non-breeding seasons, respectively, were used in the study.
• The oocytes were exposed to a maturation period of 30 hours in TCM 199 medium supplemented either with 10% EMS or 4 mg/ml BSA.
• The presence of permeable gap junctions between cumulus and oocytes was determined by injecting 3% of the fluorescent dye Lucifer yellow into the ooplasm, the cytoplasm of an oocyte.

Findings of the Study

• The study found no significant difference in the efficiency of oocyte retrieval or oocyte meiotic competence (the ability of the oocyte to undergo meiosis, the process that reduces the number of chromosomes in reproductive cells) between oocytes collected during the breeding and non-breeding seasons.
• 90% of the oocytes obtained during the breeding season demonstrated functional communications with their adjacent cumulus cells. However, these connections were entirely disrupted in 55.3% of the oocytes isolated during the non-breeding season.
• When BSA was used in the maturation medium instead of EMS, fewer oocytes exhibited closed or interrupted communications with cumulus cells during the non-breeding season, and it marginally influenced the breeding season as well.
• BSA usage resulted in incomplete cumulus expansion during both seasons.

Conclusion

• Oocytes collected from horses during the non-breeding season do not show any meiotic deficiencies but experience a lack of active communication with the surrounding cumulus cells at the moment of being isolated from the ovary.
• While the research suggests that these interrupted communications with cumulus cells might impact the oocytes’ developmental competence, further studies are required to assess this fully.