A simplified grading system for in vivo and in vitro derived vitrified equine embryos.
Authors: Morris L H A, Maclellan L J
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Morris and Maclellan developed a practical three-tier grading system for vitrified equine embryos based on morphological assessment during the cryopreservation process, specifically the volume of extruded material visible between the trophoblast and zona pellucida in day 8 embryos. Their analysis of 254 embryos (109 in vivo-derived and 145 in vitro-derived via intracytoplasmic sperm injection) revealed that embryo grade significantly predicted pregnancy outcomes following vitrification, thawing, and transfer (P ≤ 0.001), with embryo origin also playing a statistically significant role in survival rates (P ≤ 0.05). This simplified classification system offers commercial breeding programmes a reliable pre-transfer tool to identify embryos with the highest viability potential without requiring complex morphometric analysis, potentially improving decision-making around recipient mare allocation and programme economics. For practitioners involved in embryo transfer, this grading framework provides objective, observable criteria that can be readily implemented during routine vitrification protocols to stratify embryo quality and counsel clients on expected success rates based on their embryo's grade and derivation method.
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Practical Takeaways
- •Use this simplified grading system during vitrification to predict embryo viability and counsel clients on expected pregnancy rates before transfer
- •In vitro ICSI-derived embryos show different survival patterns than in vivo embryos—manage client expectations accordingly when using embryos from different production methods
- •Grade 1 embryos (minimal extruded material) will likely have superior pregnancy rates compared to Grade 2 and 3, allowing selection of best candidates for transfer to valuable mares
Key Findings
- •A simplified three-grade embryo grading system based on extruded material between trophoblast and zona pellucida is predictive of survival in vitrified equine embryos
- •Embryo grade significantly affects pregnancy rates after vitrification and transfer (P ≤ 0.001)
- •Origin of embryo (in vivo vs in vitro ICSI-derived) significantly affects post-transfer survival (P ≤ 0.05)
- •The grading system is applicable to both in vivo-derived (n=109) and in vitro ICSI-derived (n=145) day 8 embryos