Understanding Stallion Sperm Capacitation: Species-Specific Mechanisms, Cryocapacitation, and Implications for IVF

Until recently, no repeatable protocol for equine in vitro fertilization (IVF) had been established, despite decades of trying. Recent studies have demonstrated reproducible IVF outcomes under specific experimental conditions, bringing equine IVF into a new perspective. Prolonged incubation of stallion spermatozoa in a TALP based medium supplemented with penicillamine, hypotaurine and epinephrine (PHE) appears to be important factors of protocols that support sperm capacitation and markedly improved IVF outcomes. The sperm capacitation procedure has now been applied successfully using both fresh and frozen-thawed stallion semen, with the latter requiring a shorter pre-incubation period. Here, we review the molecular, structural, and metabolic processes that drive capacitation in stallion spermatozoa, with comparisons to better-characterized mechanisms in other mammals, focusing on changes in sperm membrane composition, energy metabolism, cytoplasmic content, and on the influences of seminal plasma and the female reproductive tract. Because the newly developed IVF protocol appears to be shorter with frozen-thawed semen, special attention is given to capacitation-like changes triggered by cryopreservation, and their consequences for sperm function and fertilizing capacity. Importantly, although successful fertilization can be achieved under these conditions, this does not constitute direct evidence that the molecular mechanisms governing stallion sperm capacitation have been fully elucidated. Rather, these protocols provide a robust experimental platform for investigating these mechanisms in the horse.