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.
