Effect of chilling bovine oocytes on their developmental competence.

Bovine oocytes are damaged when chilled to temperatures near 0 degree 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 degrees C, the development of GV oocytes into blastocysts following maturation and fertilization was unaffected by cooling them to 20 degrees C for 30 min (blastocyst formation: 25% vs 26%, respectively), but development decreased after cooling them to 10 degrees C and 0 degree C (blastocyst: 6% and 1%, respectively). Cooling oocytes after maturation gave similar results, with no difference between controls and oocytes cooled to 20 degrees C (blastocyst: 25% and 26%, respectively). However, cooling them to 10 degrees C and 0 degree C did reduce development (blastocyst: 8% and 3%, respectively). Chilling oocytes to 0 degree 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.