355 DEVELOPMENTAL COMPETENCE OF IMMATURE BUBALINE CUMULUS–OOCYTE COMPLEXES VITRIFIED BY THE OPEN PULLED STRAW AND CONVENTIONAL STRAW METHODS

The in vitro maturation (IVM), fertilization (IVF), and morphological changes in buffalo cumulus–oocyte complexes (COCs) cryopreserved by ultrarapid freezing using conventional (CON) and open pulled staw (OPS) methods were tested. COCs were cryopreserved using a vitrification solution comprised of Dulbecco's phosphate-buffered saline+0.5 M sucrose+0.4% BSA and two concentrations (4.5 or 5.5 M) of each cryoprotectant ethylene glycol (EG) and dimethylsulfoxide (DMSO) by either the CON or the OPS method. Vitrified COCs were stored in LN for 7 days and then thawed; morphologically normal COCs were used for IVM (n = 1070) and IVF (n = 933) in 2 separate experiments to record morphological damage of COCs due to vitrification, nuclear maturation 24 h after culture (9 replicates), and fertilization 24 h after insemination (10 replicates). The COCs were matured in vitro in TCM-199 media with hormone supplements and fertilized using TALP-BSA as described previously (Purohit et al. 2005 Anim. Reprod. Sci. 87, 229–239). Freshly collected COCs were separately used for IVM (n = 110) and IVF (n = 130) and kept as controls. The arcsin transformed data of the proportions of oocytes matured or fertilized was compared by Duncan's new multiple range test. The highest proportion of morphologically normal oocytes was seen in 5.5 M EG with the CON method (94.5%) and the lowest was seen in 4.5 M DMSO with the OPS method (82.4%). At the end of experiment 1, it was apparent that IVM in all vitrification groups was significantly lower (P < 0.05) compared to the control group (66.4%). Among the various vitrification treatments, the highest IVM occurred in 5.5 M EG with the OPS method (39.2%) and the lowest in 4.5 DMSO with the CON method (19.3%). Comparison of both concentrations of EG and DMSO showed that the proportion of COCs attaining Metaphase-II (M-II) increased with increasing concentration of both of the cryoprotectants. However, at equal concentration of EG and DMSO, the proportion of COCs attaining M-II was significantly higher in the OPS method compared to the CON method. In experiment 2, a significantly higher (P < 0.05) IVF was seen for fresh COCs (45.4%) compared to vitrified COCs. Among the vitrification treatments, the highest fertilization was seen in 5.5 M EG with the OPS method (33.6%) and the lowest in 4.5 M DMSO with the CON method (15.17%). A dose-dependent increase in the proportion of oocytes fertilized was seen with increasing concentration of both EG and DMSO [CON: 4.5 M (15.2%), 5.5 M (25.6%); OPS: 4.5 M (21.3%), 5.5 M (27.5%)] in both CON and OPS methods. Comparison of the 2 cryoprotectants revealed that EG was better compared to DMSO.At equal concentrations of EG or DMSO, a significantly higher (P < 0.05) proportion of fertilized oocytes was seen in the OPS method compared to the CON method. It was concluded that vitrification results in some damage to oocytes, with decrease in their subsequent IVM and IVF. Developmental capacity of vitrified buffalo oocytes can be improved by using OPS instead of conventional straws.