The effects of meiotic stage on viability and developmental capability of goat oocytes vitrified by the Cryoloop method

In order to evaluate the effects of meiotic stage on survival of vitrified goat oocytes, the Cryoloop method was used to cryopreserve goat immature germinal vesicle (GV) and meiosis II (MII) oocytes following in vitro maturation (IVM). The oocytes collected from local slaughter house were divided into two parts. In the first part, the immature cumulus oocyte complexes (COCs) were further divided into three groups: (1) untreated (control), (2) exposed to the vitrification and dilution solutions but without being plunged into liquid nitrogen (toxicity), or (3) vitrified by the Cryoloop method (vitrification). In the second part, the MII oocytes produced by IVM of immature COCs were also divided into three groups as the above immature COCs. Oocytes survival was assessed by morphological appearance, nuclear maturation, and developmental capability following parthenogenetic activation (PA). The experiment was repeated three times. Our data indicated the rates of oocytes with normal appearance and developmental capability in the toxicity or vitrification group were decreased as compared to the control oocytes. In the toxicity group, the rate of GV oocytes with normal morphology was 76.25% ± 3.37% and significantly less than that of MII oocytes (92.18% ± 1.94%, P < 0.05). However, the cleavage rate of MII oocytes was not significantly different from that of GV oocytes in the toxicity group (68.23% ± 1.71% vs 67.59% ± 3.51%, P > 0.05). Additionally, the percentage of GV oocytes developing to blastocyst was significantly less than that of MII oocytes in the toxicity group (13.84% ± 2.81% vs 29.78% ± 4.17%, P < 0.05). The rate of vitrified/thawed GV oocytes with normal morphology was 60.37% ± 2.91% and significantly less than that of MII oocytes (82.91% ± 3.01%, P < 0.05). Additionally, the cleavage rate of GV oocytes was also significantly less than that of MII oocytes in the vitrification group (42.81% ± 2.94% vs 57.91 ± 1.06%, P < 0.05). However, the blastocyst rate of vitrified/thawed GV oocytes was not significantly different from that of MII oocytes (8.51% ± 1.46% vs 12.41% ± 3.74%, P > 0.05). In conclusion, although vitrification can greatly damage the structure and development capability of goat oocytes, the Cryoloop method can result in acceptable levels of survival and development of goat oocytes. Additionally, compared to immature GV goat oocytes, mature MII oocytes may be more tolerant to the vitrification process.