Preliminary report on pregnancy rates after transfer of cryopreserved and fresh embryos produced in vivo and in vitro in sheep

Continuous improvements to assisted reproductive technology (ART) have facilitated the advancement and preservation of animal genetics as well as animal reproductive efficiency. One of the ART techniques is cryopreservation which facilitates embryo transfer procedures. In the present experiment embryos produced in vivo or in vitro were transferred to the recipient ewes just after collection (fresh) or after freezing/thawing on day 5 after fertilization. Pregnancy was verified 40 or more days after embryo transfer (ET) by ultrasonography. For in vivo produced embryos, transfer of fresh embryos resulted in a 43% (3 out of 7) pregnancy rate, but transfer of frozen/thawed embryos resulted in 11% (1 out of 9) pregnancy rate. For in vitro produced embryos, transfer of fresh embryos resulted in a 50% (2 out of 4) pregnancy rate, but transfer of frozen/thawed embryos resulted in no pregnancies (0 out of 6). These data demonstrate that the cryopreservation method and ET procedures need additional improvements, since pregnancy rates remained at a low level.