Cryopreservation of Equine Embryos

• Small equine embryos can be successfully frozen.
• Larger (>300 μm) embryos must be deflated before freezing.
• Methods for superovulation are needed to enhance embryo recovery.
• Vitrification and transfer of small embryos result in good pregnancy rates.

Equine embryo transfers have increased dramatically in the past decade, but in spite of the advantages of cryopreservation of equine embryos, this technology has not increased proportionally. Lack of a superovulation protocol for mares and the inability to freeze embryos >300 μm have been the limiting factors impeding equine embryo cryopreservation. Data from both controlled laboratory settings and commercial embryo transfer facilities have shown that small embryos can be slow cooled or vitrified and, after thawing and transfer, provide pregnancy rates of 50%–70% similar to that obtained with bovine embryos. In contrast, studies have shown that embryos >300 are damaged more during slow cooling or vitrification than those <300 μm and result in low pregnancy rates after transfer. The presence of the acellular capsule in the equine blastocyst and the large volume of blastocoele fluid were thought to be the major reason for poor survival of cryopreserved large equine embryos. However, deflating the embryo before freezing has been shown to improve the survival of cryopreserved large equine embryos dramatically. Unfortunately, this must be done using very expensive equipment. Developments that could improve the success of equine embryo cryopreservation are as follows: having hormones available for superovulation, a means of hastening the embryo into the mare's uterus to consistently collect <300 μm embryos, or the development of a simple means of collapsing embryos >300 μm.