Cryopreservation of equine embryos : current state-of-the-art

Abstract

During the past 15 years, embryo transfer (ET) has become increasingly widespread within the sport-horse breeding industry. At present, however, the vast majority (>95%) of horse embryos are transferred fresh or after chilled storage for up to 24 h, whereas cryopreservation is rarely employed despite its obvious potential for simplifying recipient mare management and facilitating long-term storage and international transport of embryos. A number of inter-related factors have contributed to the slow development and implementation of equine embryo cryopreservation, these include; 1) the absence of commercially-available products for reliably stimulating superovulation; 2) very poor pregnancy rates following cryopreservation of embryos >300 mm in diameter; 3) difficulty in recovering embryos at early developmental stages amenable to cryopreservation; and 4) inter-embryo variation in susceptibility to cryodamage. However, acceptable success rates (> 55% pregnancy) have been reported for both slow-frozen and vitrified small embryos (<300 mm), and there is renewed interest in cryopreservation, not only in the context of standard ET programmes, but also because it would facilitate pre-implantation genetic testing and allow wider access to techniques for producing embryos in vitro, such as intra2 cytoplasmic sperm injection and nuclear transfer. This article will review the current status of equine embryo cryopreservation.