Kinetic and temporal factors influence chilling injury to germinal vesicleand mature bovine oocytes

In this study we examined the effects of low, above freezing temperatures o n the viability and functionality of bovine oocytes. Germinal Vesicle (GV) stage and in vitro matured oocytes (MII) were exposed to various combinatio ns of time (15 and 60 min) and temperature (4, 16, 23, and 39 degrees C). A fter being treated, the ability of oocytes to undergo maturation and fertil ization in vitro was examined as well as their Viability assayed by two flu orescent probes, fluorescein diacetate (FDA) and 5-carboxyfluorescein diace tate (cFDA). Cooling GV oocytes to 16 degrees C for 15 min reduced the fert ilization rate by more than 40%, compared with those left at 39 degrees C. Surprisingly, cooling oocytes to 4 degrees C reduced the fertilization rate by only 10% compared with control. Exposing GV oocytes to temperatures bel ow 23 degrees C reduced their viability. Similar to the reduction in fertil ization, the viability of GV oocytes after exposure to 16 degrees C was red uced by more than 50%, whereas exposure to 4 degrees C reduced it by only 9 %. Viability measurements using FDA and cFDA gave comparable results and sh owed a similar trend. The Viability of MII oocytes and of GV oocytes pretre ated with butylated hydroxytoluene, following exposure to low temperatures, was higher compared with that of GV controls. We interpret these results a s indicating chilling effects on membrane integrity. Improving the chilling resistance of bovine oocytes may facilitate their short- and long-term pre servation. (C) 1999 Academic Press.