Cryopreservation of embryos plus storage in liquid nitrogen (— 196°C) has several important advantages. First, it eliminates the immediate need for recipients that are closely synchronized in estrus with the donor at the time of embryo recovery. Frozen embryos can be thawed when recipient(s) of suitable breed, size, and estrous synchrony are available. Also, freezing allows convenient movement of embryos over long distances, including internationally, and, in contrast to the transport of live animals, virtually eliminates the chances of transporting infectious microbes.[4] Also, their surrogate mothers endow calves resulting from embryo transfer with significant immunity to local infectious microbes, via colostrum.

Currently, ethylene glycol is usually used as a cryoprotectant, and embryos are transferred immediately following thawing. Embryos of most farm animals can be frozen with high survival rates, but pig embryos have proven very difficult to freeze successfully, and there is currently no commercially practical protocol. Equine embryos less than 300 microns in diameter survive cryopreservation reasonably well, but only small numbers are frozen commercially. Horse embryos are frequently chilled to 4°C and transported for periods of 12 to 24 hours before transfer into suitable surrogates.

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