Deep-freezing of cow embryos. A review.

The original methods of storage of cow blastocysts involved slow freezing down to -60 degrees C combined with slow thawing. However, short freezing curves with plunge temperatures between -30 to -40 degrees C will give high survival/pregnancy rates when rapid thawing is applied, i.e. direct transfer into water at 37 degrees C. Early reports on freezing of mammalian embryos utilized DMSO as a cryoprotectant, but recent work seems to indicate that glycerol might be marginally better than DMSO in work with cow embryos. Seventeen out of 29 (59%) non-surgically transferred deep-frozen thawed embryos resulted in established pregnancies (greater than 60 days) (Table I:12) indicating that deep-freezing of cattle blastocysts when optimally applied only lowers results marginally compared to direct transfer.