Factors Influencing Equine Embryo Outcomes in OPU‑ICSI Systems: Insights from a Large‑Scale Program in Brazil.

Overview

• This study analyzed factors influencing the success rates of equine oocyte pick-up combined with intracytoplasmic sperm injection (OPU-ICSI) in producing embryos.
• It used a large dataset from Brazil encompassing different horse breeds to identify key predictors that affect embryo development and established practical strategies for improving outcomes.

Background and Purpose

• Equine OPU-ICSI (oocyte pick-up and intracytoplasmic sperm injection) is a reproductive technology used to support commercial in vitro embryo production (IVEP) in horses.
• This method shows substantial variation in success rates, partly due to biological and procedural factors.
• The study aimed to evaluate whether previously identified predictors of oocyte and embryo development outcomes were consistent in a large and diverse equine population from Brazil, including breeds such as Quarter Horses, Warmbloods, and Mangalarga.
• Furthermore, it sought to uncover specific factors affecting embryo production and potential strategies to enhance outcomes in different ecological and breeding conditions.

Methodology

• A retrospective analysis was performed on a large dataset from Brazil that recorded oocyte retrieval, maturation, cleavage, and blastocyst formation rates using OPU-ICSI across multiple horse breeds.
• Data were analyzed to explore correlations between mare reproductive status, stallion factors, and embryo development stages.
• Comparisons were made with previously published literature to validate findings under the specific ecological and breeding environment.

Key Findings

• Oocyte and embryo outcomes:
  • General success rates (maturation, cleavage, blastocyst formation) were consistent with trends reported in international studies.
• Mare reproductive status and follicular environment:
  • Oocytes retrieved from mares in estrus (heat) possessing a dominant follicle showed significantly higher maturation rates.
  • These oocytes also had higher blastocyst formation rates, indicating a better developmental potential compared to oocytes from other cyclical phases or smaller follicles.
• Stallion effects:
  • Stallion influence on embryo outcomes was found to cluster into distinct phenotypes affecting specific developmental steps:
  • One group of stallions was associated with low cleavage rates but, among embryos that did cleave, blastocyst development was normal. This pattern points to potential issues with oocyte activation.
  • Another group exhibited normal cleavage rates but reduced blastocyst formation, suggesting problems during embryo genome activation, genome integrity, or epigenetic mechanisms critical for later development.

Implications and Recommendations

• Performing OPU during estrus, specifically when a dominant follicle is present, is advantageous for improving oocyte quality and embryo yield.
• Recognizing stallion-specific reproductive phenotypes can guide tailored semen handling, sperm activation protocols, or other intervention strategies to overcome developmental blocks.
• Customized approaches addressing stallion-induced limitations in either oocyte activation or blastocyst formation stages may significantly enhance blastocyst yields in equine IVEP programs.
• The findings support optimizing timing and semen processing procedures in commercial OPU-ICSI programs to achieve better and more consistent embryo production results.

Conclusion

• This large-scale study from Brazil confirms and extends earlier knowledge about factors influencing equine embryo production using OPU-ICSI.
• Focusing on the mare’s cycle stage and dominant follicle presence, along with stallion-specific management, appears to be key for improving success in commercial equine IVEP.
• Adapting OPU-ICSI protocols based on these insights can lead to more reliable and increased blastocyst yields, benefiting breeders and advancing equine assisted reproduction techniques.