EFFECT OF CHILLING BOVINE OOCYTES ON THEIR DEVELOPMENTAL COMPETENCE

Bovine oocytes are damaged when chilled to temperatures near 0 degrees C. We have determined the temperatures at which this injury occurs, a s well as its kinetics and the functional consequences for oocytes bot h at the germinal vesicle-stage (GV) and after in vitro maturation (IV M). Cooling GV oocytes had no effect on their nuclear maturation or fe rtilization. Compared to control oocytes held at 30 degrees C, the dev elopment of GV oocytes into blastocysts following maturation and ferti lization was unaffected by cooling them to 20 degrees C for 30 min (bl astocyst formation: 25% vs 26%, respectively), but development decreas ed after cooling them to 10 degrees and 0 degrees C (blastocyst: 6% an d 1%, respectively). Cooling oocytes after maturation gave similar res ults, with no difference between controls and oocytes cooled to 20 deg rees C (blastocyst: 25% and 26%, respectively). However, cooling them to 10 degrees and 0 degrees C did reduce development (blastocyst: 8% a nd 3%, respectively). Chilling oocytes to 0 degrees C for 30 sec reduc ed their cleavage and blastocyst formation by >40%; there was a high n egative correlation between the length of exposure and subsequent surv ival, both for GV-stage and for IVM oocytes. The extreme sensitivity o f both GV and IVM oocytes to chilling can explain the limited success obtained for cryopreservation of bovine oocytes by conventional slow-c ooling procedures. (C) 1996 Wiley-Liss, Inc.