Degeneration of cryopreserved bovine oocytes via apoptosis during subsequent culture.

Cryopreservation causes a significant proportion of bovine oocytes to undergo degeneration during subsequent culture. We investigated the degeneration mechanism of cryopreserved oocytes. In vitro matured bovine oocytes were vitrified by the open-pulled straw (OPS) method. In each replicate, a group of oocytes were randomly taken after warming to determine oocyte survival by both morphological evaluation and propidium iodide vital staining. The remainders were evaluated by morphological criterion. Morphologically intact oocytes were co-incubated with frozen-thawed spermatozoa for subsequent development. In situ examination of DNA breaks in oocytes and embryos was conducted using a Fluorescein-FragEL DNA fragmentation detection kit. A caspase-3 detection kit was used to detect caspase-3 activity in oocytes and embryos. Most of the oocytes survived cooling and warming processes as assessed by both morphological evaluation and vital stain. During subsequent culture, some degenerating oocytes displayed observable apoptotic morphology, such as cytoplasmic condensation, cytoplasmic fragmentation, and formation of apoptotic bodies. Biochemical markers of apoptosis, such as apoptotic DNA fragmentation and activation of caspases, were detected not only in oocytes having typical apoptotic morphology, but also in oocytes without observable apoptotic morphology. In embryos, positive signals for both biochemical markers were detected in blastomeres. This experiment suggests that cryopreserved bovine oocytes degenerate via apoptosis during subsequent culture.

Effect of meiotic stages and maturation protocols on bovine oocyte's resistance to cryopreservation.

We investigated the effect of meiotic stages and two maturation protocols on bovine oocyte's resistance to cryopreservation. Oocytes at germinal vesicle breakdown (GVBD) and metaphase II (MII) stage as well as oocytes matured for 22 h in media supplemented with FSH or LH were vitrified by the open pulled straw method. After warming, oocytes underwent additional 16 h (GVBD group) or 2 h (MII group) maturation. Then they were subjected to in vitro fertilization and culture. Some oocytes that matured in the medium supplemented with LH were subjected to parthenogenetic activation after vitrification to determine their developmental potential in absence of fertilization. Survival of oocytes after vitrifying/warming was determined after 22 h in fertilization medium. Cleavage and blastocyst formation rates were used to assess their developmental competence. In both experiments, a portion of unvitrified MII oocytes were subjected to in vitro fertilization and culture as control groups. In Experiment 1, similar cleavage rates were obtained for both GVBD and MII oocytes (53.56 versus 58.01%, P > 0.05). However, significantly higher proportion of cleaved embryos from vitrified MII oocytes developed into blastocysts than those from vitrified GVBD oocytes (1.06 versus 8.37%, respectively, P < 0.01). In Experiment 2, vitrified MII oocytes matured in medium supplemented with LH were superior to vitrified MII oocytes matured in FSH supplementation not only in cleavage rates (61.13 versus 50.33%), but in blastocyst formation rates (11.79 versus 5.19%, P < 0.01) as well. Cleavage and blastocyst formation rates of parthenogenetically activated oocytes were similar to those that were fertilized. Nevertheless, the vitrifying/ warming process significantly compromised the oocytes' developmental capacity since the vitrified oocytes showed significant reduction in both cleavage and blastocyst rates compared to those of not vitrified controls in both experiments (P < 0.01). We showed that oocytes at different maturation stages respond to cryopreservation differently and MII stage oocytes have better resistance to cryopreservation than GVBD stage oocytes. The maturation protocols also influence oocyte's ability to survive cryopreservation. Poor developmental potential after vitrification seem to have resulted from the cryodamage to the oocyte itself. These results suggested the importance of maturation on the developmental competence of cryopreserved oocytes.