EFFECT OF LOW-TEMPERATURES ON IN-VITRO MATURED BOVINE OOCYTES

This research concerned effects of cooling in vitro matured bovine ooc ytes on subsequent fertilization and development in vitro. Oocytes wer e maintained at 39 degrees C (control), 20 degrees C, 10 degrees C or 0 degrees C for 5, 10 or 20 min, then fertilized and cultured in vitro for 7 d. The proportion of fertilized oocytes that cleaved and develo ped to the morula/blastocyst stage was compared between different trea tments. Duration of exposure had no effect on the results. Fertilizati on rate was higher (P<0.05) for oocytes maintained at 39 degrees C (73 .2%) than for oocytes cooled at 20 degrees C (58.6%), 10 degrees C (47 .3%), or 0 degrees C (36.9%). Cleavage rates were 58.3, 45.3, 15.7 and 7.0% for 39 degrees C, 20 degrees C, 10 degrees C and 0 degrees C, re spectively (P<0.05). The lowest development rate to the blastocyst sta ge was obtained with oocytes cooled to 10 degrees C (0.0%) or 0 degree s C (0.9%), followed by 20 degrees C (7.1%) and 39 degrees C (16.5%; P < 0.05). In a second experiment, the zona pellucida was removed after cooling but prior to fertilization (zona-free) from a portion of the i n vitro-matured bovine oocytes in each treatment. When sperm penetrati on rates of zona-free oocytes were compared (percentage of oocytes exh ibiting greater than or equal to 2 pronuclei), there was no difference (P>0.05) between oocytes cooled at 0 degrees C (59.7%) or 10 degrees C (67.9%). However, penetration rates in these 2 groups were lower (P< 0.05) when compared to zona-free oocytes cooled at 20 degrees C (83.1% ) or those maintained at 39 degrees C (83.1%). Zona-free oocytes had h igher penetration rates (P<0.05) when cooled at 0 degrees C (59.7%) or 10 degrees C (67.9%) than zona-intact oocytes cooled at 0 degrees C ( 37.3%) or 10 degrees C (47.3%). However, there was no difference in th e penetration rate when zona-free and zona-intact oocytes were cooled at 20 degrees C or maintained at 39 degrees C. These data demonstrate that cooling in vitro-matured bovine oocytes decreases the percentage of oocytes that undergo fertilization and subsequently develop in vitr o. Moreover, at least part of the decrease in fertilization following oocyte cooling is due to effects on the zona pellucida. (C) 1998 by El sevier Science Inc.