Embryo selection in equine in vitro embryo production: current practices, limitations and future directions

Abstract

Equine in vitro embryo production has become widely commercialized due to refinements in ovum pick up, oocyte maturation and the reliability of intracytoplasmic sperm injection (ICSI). Although ICSI remains the primary fertilization method in horses, recent advances in sperm capacitation have enabled reliable in vitro fertilization with outcomes approaching those of ICSI, positioning in vitro fertilization as a complementary technology in equine embryo production. These techniques allow embryo production from subfertile mares or stallions with limited semen availability; however, accurate identification of embryos with predictable outcomes remains a limitation. In vitro-produced embryos exhibit higher rates of early pregnancy loss and monozygotic twins than in vivo-derived embryos, yet selection in many programs relies on traditional morphological assessment adapted from other species. Such assessments are weak predictors of foaling rates and lack standardization across laboratories, unlike systems established in cattle. In vitro-produced embryos display structural and nuclear deviations, including reduced cell numbers, poorly compacted inner cell mass, failure to form a glycoprotein capsule, and increased micronucleus formation. Consequently, clinical programs prioritize developmental rate during culture as a more reliable prognostic marker than morphology alone. Time lapse imaging further identifies abnormal first cleavage patterns as markers of poor outcome, supporting serial, dynamic assessment over subjective point-in-time evaluation. Current evidence supports a stepwise approach, prioritizing developmental timing, followed by equine-specific morphology and morphokinetic risk markers. Emerging technologies such as metabolic profiling, artificial intelligence and noninvasive preimplantation genetic testing require further validation against foaling outcomes before routine clinical adoption.