DIRECT TRANSFER OF EQUINE BLASTOCYSTS FROZEN-THAWED IN THE PRESENCE OF ETHYLENE-GLYCOL AND SUCROSE

The present study was designed to examine the suitability of ethylene glycol as a cryoprotectant for equine embryos. Blastocysts recovered n onsurgically from Day 6 donor mares were cryopreserved by conventional 2-step freezing in the presence of 10% ethylene glycol (EG) 10% glyce rol (Gly), or 10% ethylene glycol + 0.1M sucrose (EG + Sue). After tha wing, the EG and Gly were removed by a S-step manner, and the EC + Suc was diluted to one fourth in the freezing straw. The postthaw blastoc ysts were transferred nonsurgically into the uteri of recipient mares on Days 4 to 7 after ovulation. Pregnancy rates, based on Day 15 ultra sonography, were 25.0% (2/8) and 37.5% (3/8) for the blastocysts froze n in EG and Gly, respectively. Direct transfer following thawing and i n-straw dilution of blastocysts frozen in EG + Suc resulted in a pregn ancy rate of 63.6% (7/11). In fresh Day 6 blastocysts (control group), the pregnancy rate was 70.0% (7/10). These results indicate that the combined use of ethylene glycol and sucrose in a 2-step freezing regim en allows for the direct transfer of frozen-thawed blastocysts into re cipient mares, with an acceptable pregnancy rate.