DEVELOPMENTAL COMPETENCE OF BOVINE OOCYTES FROZEN AT VARIOUS MATURATION STAGES FOLLOWED BY INVITRO MATURATION AND FERTILIZATION

In the present study, we examined the ability of bovine oocytes, at various stages of maturation. to survive after cryopreservation as well as their subsequent development following in vitro maturation and fertilization. This study was conducted with oocytes frozen as morphologically immature (germinal vesicle), maturing (germinal vesicle breakdown to telophase I) , and matured (metaphase II) oocytes by two different cryopreservation methods (Method A: oocytes were cooled at a rate of -0.7-degrees-C/min from 20-degrees-C to -7-degrees-C,seeded and held at this temperature for 10 min, cooled at a rate of -0.3-degrees-C/min to -30.0-degrees-C. maintained at this temperature for 10 min and plunged into liquid nitrogen; Method B: oocytes were seeded at -5.1-degrees-C and held at this temperature for 15 min, cooled at a rate of -0.5-degrees-C/min to -32.5-degrees-C and plunged into liquid nitrogen) during culture for maturation. At the beginning of culture (0 hours), almost all of the fresh oocytes were at the germinal vesicle stage. and more than 90% of these oocytes reached at metaphase II after culture. Immature oocytes frozen at 0 hours of culture had significantly lower numbers of morphologically normal oocytes and cleavage rate after thawing compared with the maturing and matured oocytes. The post-thaw maturation and fertilization rate of frozen immature oocytes was also lower than that of the other groups, but did not differ significantly. Except for the oocytes frozen between 0 and 12 hours using cryopreservation Method B, developmental capacity beyond the eight-cell stage did not differ among groups. We concluded that 1) the freezability of immature bovine oocytes was inferior to that of maturing and matured oocytes and 2) frozen-thawed methods affected the post-thaw survival and developmental competence of the frozen oocytes.