Effect of chilling bovine oocytes on their developmental competence

Bovine oocytes are damaged when chilled to temperatures near 0°C. We have determined the temperatures at which this injury occurs, as well as its kinetics and the functional consequences for oocytes both at the germinal vesicle-stage (GV) and after in vitro maturation (IVM). Cooling GV oocytes had no effect on their nuclear maturation or fertilization. Compared to control oocytes held at 30°C, the development of GV oocytes into blastocysts following maturation and fertilization was unaffected by cooling them to 20°C for 30 min (blastocyst formation: 25% vs 26%, respectively), but development decreased after cooling them to 10° and 0°C (blastocyst: 6% and 1%, respectively). Cooling oocytes after maturation gave similar results, with no difference between controls and oocytes cooled to 20°C (blastocyst: 25% and 26%, respectively). However, cooling them to 10° and 0°C did reduce development (blastocyst: 8% and 3%, respectively). Chilling oocytes to 0°C for 30 sec reduced their cleavage and blastocyst formation by >40%; there was a high negative correlation between the length of exposure and subsequent survival, both for GV-stage and for IVM oocytes. The extreme sensitivity of both GV and IVM oocytes to chilling can explain the limited success obtained for cryopreservation of bovine oocytes by conventional slow-cooling procedures.