Effect of Different Cooling Rates on Embryo Survivability and Pregnancy Rates in Freezing Sheep Embryos

In the first stage of the study, oocytes obtained from ovaries from abbatoir (n=2990) were incubated in maturation medium for 24 h. The maturated oocytes were left for in vitro fertilization (IVF) for 20 hours. The cleavaged embryos (n=1305) were left for in vitro culture (IVC) for six days. Then morula-blastocyst stage embryos divided randomly in three groups (Grup I: 0.5°C/min, Grup II: 0.8°C/min, Grup III: 1°C/min). Embryos in each group (n=50) were frozen at different cooling rates in 1.5 M ethylene glycol containing freezing medium. As a result, 0.5°C/min cooling rate group was found as the most successful group (P<0.05). In the second stage of the study, in vivo embryos (morula-blastocyst) derived from donor sheep were frozen at 0.5 °C/min. Frozenthawed nineteen embryos were transferred to hormonally pretreated seventeen recipients. At the 60th day of the transfer, three recipients were diagnosed as pregnant and one of them had twins. One of the three recipients gave birth and two sheeps had early embryonic loss are observed in the later ultrasound scanning. In the study, 0.5°C/min cooling rate in the slow freezing of sheep embryos was found out to be more successful cooling rates and pregnancy following a healthy lambing is achieved.