Cow embryos are easily cryopreserved (frozen or vitrified) in the blastocyst stage (7-8 day old embryos).
The equine blastocyst is much larger than the cow’s and develops a cellular capsule that keeps the round shape of the embryo, allowing it to move in the uterus up till day 16 of gestation, a process necessary for the establishment of a successful pregnancy in this species. However the blastocyst’s capsule interferes with the protective action of the cryopreservation extender, and the large amount of fluid in the embryo may produce ice crystals during cryopreservation, which damages the embryonic cells. Thus, despite the fact that it is possible to have highly successful results with the transfer of vitrified/warmed 8 to 9 day old embryos (blastocysts), most commercial operations cryopreserve 6 to 6.5 day embryos before the capsule is formed, and when there is no need of micromanipulation with expensive equipment.
Working with Dr. Lisa Maclellan from Australia, a world-renowned scientist in equine assisted reproduction, cryopreservation protocols of equine embryos were standardized in our laboratory. In the process we have produced what to the best of our knowledge is the first equine born in the country after the transfer of a cryopreserved embryo. Coincidentally, the embryo recipient mare is Josefina, the first horse in the world obtained through the transfer of an embryo produced after artificial insemination with frozen/thawed epididymal sperm.
The advantages of cryopreserving embryos are many. The full genetics of the progenitors (stallion and mare) are conserved, instead of only one of the progenitors (as happens with frozen semen). The embryo can be kept in liquid nitrogen basically indefinitely, and thus, can be used at the desired timing, even years after the progenitors are not around, or can no longer be ridden or bred. Additionally, no recipient mares need to be available at the site, at the time of embryo collection, as cryopreserved embryos are easily transported to be transferred elsewhere. One can also collect and keep embryos from young mares, and transfer them only after/if the mare proves herself as a valuable horse.