Cracks in the ice: biological challenges and incremental progress in equine oocyte vitrification

Equine oocyte vitrification could enhance the flexibility of in vitro embryo production, yet developmental competence is reduced. Only vitrification of oocytes matured in vivo yields foaling rates comparable to fresh oocytes. Recent advances in vitrification at the germinal vesicle (GV) stage, supported by systematic comparisons of cryoprotective agents and exposure times, has enabled consistent blastocyst production and birth of several foals. Comparative studies of GV and metaphase II (MII) stages suggest biological advantages of vitrification after in vitro maturation (IVM), warranting similar systematic optimization of MII-stage protocols. Outcomes at both stages may improve further when combined with enhanced IVM systems incorporating antioxidants, lipid-modulating strategies and conditions mimicking in vivo maturation. Emerging ultrafast vitrification approaches and novel cryoprotective molecules from human research offer promising translational opportunities. This review summarizes advances in horse oocyte vitrification from the past five years and highlights translational strategies to move equine oocyte vitrification toward reliable clinical application.