Meiotic maturation and developmental capability of ovine oocytes at germinal vesicle stage following vitrification using different cryodevices.

In order to assess effects of vitrification on ovine oocytes at the germinal vesicle (GV) stage, the conventional plastic straw (CS), the open-pulled straw (OPS), and Cryoloop were used to vitrify ovine oocytes. Oocytes were randomly divided into five groups: (1) Control; (2) Oocytes exposed to vitrification and dilution solutions without any cryopreservation (toxicity); (3) Oocytes vitrified using CS (CS); (4) Oocytes vitrified using OPS (OPS), and (5) Oocytes vitrified using Cryoloop (Cryoloop). The viability, cumulus cell expansion, nuclear maturation after in vitro maturation (IVM), and developmental capability of vitrified oocytes following parthenogenetic activation (PA) or in vitro fertilization (IVF) were assessed. The pretreatment in the vitrification and dilution solutions without any freezing or thawing did not adversely influence oocytes. The viability of vitrified oocytes were significantly declined compared to unfrozen oocytes (P < 0.05). The viability of oocytes vitrified using open-pulled straws or Cryoloop was significantly higher than that in the CS group (P < 0.05). After IVM, the percentage of oocytes reaching to the metaphase II (MII) stage was significantly higher with Cryoloop and OPS following by CS. However, the in vitro maturing percentage of vitrified oocytes was significantly less than that of unfrozen oocytes (P < 0.05). After PA, the developmental capability of vitrified oocytes was significantly decreased compared to unfrozen oocytes. The cleavage rate of oocytes vitrified using conventional plastic straws was significantly less than those of the other freezing groups (P < 0.05). The cleaving capability of oocytes vitrified using Cryoloop was significantly increased compared to the OPS group. However, there was no significant difference existing amongst the freezing groups as concerning the blastocyst rate. Following IVF, the developmental capability of vitrified oocytes was severely damaged compared to that of unfrozen oocytes. The cleavage rate of oocytes vitrified with Cryoloop was similar to that of oocytes vitrified with open-pulled straws. However, the cleavage rate of vitrified oocytes in the CS group was significantly less than that in the OPS or Cryoloop group (P < 0.05). None of oocytes vitrified using conventional plastic straw developed to the blastocyst stage following IVF. There was no significant difference existing between OPS and Cryoloop with respect to the blastocyst rate. After staining with cFDA and PI, cumulus cells surrounding oocytes were partly damaged by vitrification and thawing while the membrane of vitrified oocyte still remained intact. In conclusion, vitrification can seriously damage ovine immature oocytes and cumulus cells surrounding oocytes, which may subsequently affect their developmental capability. Finally, this study further proves that increasing the freezing and thawing velocity benefits survival of vitrified immature oocytes.