Effects of cooling and warming rates during vitrification on fertilization of in vitro-matured bovine oocytes

The purpose of this study was to clarify the relationship of cooling rates ICR) and warming rates (WR) during vitrification with postwarming viability of in vitro-matured bovine oocytes. In Experiment 1. oocytes: were vitrified in a solution containing 7.2 M ethylene glycol and 1.0 M sucrose by use of open-pulled glass capillaries with five: different outer diameters and were warmed by placement of the capillaries into 0.25 M sucrose solution, The capillaries of 2000-. 1400-. 1000-. 630-, and 440-mum diameters provided CR of 2000. 3000, 5000. 8000. and 12,000 degreesC/min and WR of 5000. 8000, 17,000, 33,000. and 62,000 degreesC/min, respectively. In oocytes vitrified in capillaries of 1400-mum diameter (CR, 3000 degreesC/min: WR. 8000 degreesC/min), the morphological survival rate (86% of vitrified. penetration rate (79% of inseminated), and normal fertilization rare (69% of penetrated) were higher or tended to be higher than those in the other vitrification groups. In Experiment 2. oocytes cooled at 2000, 3000, or 12.000 degreesC/min were warmed at 8000 degreesC/min, and oocytes cooled at 3000"C/min were warmed at 5000, 8000, or 33,000 degreesC/min. Among these CR-WR combinations, cooling of oocytes at 3000 degreesC/min regardless of the WR resulted in higher postwarming survival. These results indicate that survival of in vitro-matured bovine oocytes after vitrification and subsequent warming is improved by a slightly rapid cooling rate in open-pulled glass capillaries compared to that obtained in conventional straws. (C) 2001 Academic Press.