Breakthroughs in Equine Embryo Cryopreservation.

Most equine embryos are collected from the donor mare and transferred immediately as fresh embryos or shipped cooled to a recipient station for transfer within 24 hours. Very few equine embryos are frozen despite the numerous advantages of embryo cryopreservation. There are 2 major hurdles: Only the small embryos (<300 μm) provide good pregnancy rates after freezing/thawing and transfer. Also there is no good procedure for superovulating mares; thus, extra embryos for freezing are not readily available. Using either a slow cool or a vitrification method, pregnancy rates of small equine embryos after freezing/thawing are 50% to 70%.