Effect of co-culture during fertilization of buffalo (Bubalus bubalis) in vitro-matured denuded oocytes vitrified by Cryotop.

Although removal of cumulus cells improves the efficiency of vitrification of buffalo (Bubalus bubalus) in vitro-matured (IVM) oocytes (Gasparrini et al. 2007 Anim. Reprod. Sci. 98, 335–342), the lack of cells impairs the fertilization process. Therefore, the aim of the present work was to evaluate the influence of a somatic support during in vitro fertilization (IVF) of buffalo vitrified denuded matured oocytes. Since IVF on a cumulus cells monolayer was inefficient, we verified the effects of co-culture with cumulus-enclosed oocytes (COCs). IVM buffalo oocytes (n = 316) were vitrified by the Cryotop® method (Kuwayama and Kato 2000, J. Assist. Reprod. Genet. 17, 477 abst) that was recently proven suitable for buffalo oocyte cryopreservation (Attanasio et al. 2006 Reprod. Domest. Anim. 41, 302–310). Denuded buffalo oocytes were equilibrated in 10% ethylene glycol (EG) and 10% dimethyl sulfoxide (DMSO) for 3 min, transferred into 20% EG and 20% of DMSO in TCM199 with 20% fetal calf serum (FCS) + 0.5 m sucrose, loaded on Cryotops, and plunged into liquid nitrogen within 25 s. For warming, oocytes were exposed for 1 min to 1.2 m sucrose and then to decreasing concentrations of the sugar (0.6, 0.4, 0.3 m for 30 s) in TCM199 + 20% FCS. Oocytes were rinsed and allocated to IVM drops for 1.5 h. Survival rate was evaluated at this point and the oocytes that had survived (292/316 = 92.4%) were split into 2 fertilization groups: (A) approximately 5 buffalo oocytes per 50-µL drop of IVF medium, and (B) approximately 3 buffalo oocytes + 3 bovine fresh COCs per 50-µL drop of IVF medium. Since buffalo COCs easily lose their cells following IVF, for better identification we used bovine COCs that have a brighter and more compact cumulus mass. In vitro fertilization and culture were carried out as previously described (Gasparrini et al. 2007). As control, buffalo oocytes (n = 104) were in vitro-matured, fertilized, and cultured up to the blastocyst stage. On Day 1, survival rate was evaluated in the two vitrification groups; cleavage and blastocyst rates were recorded on Days 5 and 7, respectively, in all groups. The experiment was repeated 4 times. Differences in the percentages of survival, cleavage, and blastocyst formation among treatments were analyzed by chi-square test. Within vitrification groups, despite similar survival rates on Day 1 (90.6% v. 93.3%, respectively, in Groups A and B), cleavage rate was significantly improved in Group B compared to Group A (59.2% v. 45.4%, respectively; P < 0.01). Interestingly, the cleavage rate in Group B was not significantly different from that recorded in the control group (71.0%). Although blastocysts were produced in both vitrification groups (3.6% v. 4.1%, respectively, in Groups A and B), the yield was significantly lower than that of the control group (29.0%, P < 0.01). In conclusion, co-culture with bovine COC during fertilization improves the capability of buffalo denuded vitrified oocytes to cleave.