[Study and applications of human oocyte cryopreservation]

The oocyte cryopreservation is more promising than embryo freezing in clinical applications, preservation of female fertility,legal and ethical aspects in this decade. The redundant oocytes retrieved from IVF cycles can potentially donate oocytes, which have unique advantages in economy and feasibility. We have achieved success in embryo freezing, but just obtained poorer results in oocyte cryopreservation which was mainly because of the low rates of survival, fertilization, and cleavage. The character of the plasma membrane and the time of cortical granules present at the metaphase of meiosis II with the spindle system consist of the major difference between oocytes and embryos. Moreover, the oocytes should be fertilized by sperm at the appropriate time. We used a refined slow freezing method to improve the survival rate and increased sucrose concentration to dehydrate oocytes. Vitrification was another approach to prevent harms. Besides, intracytoplasmic sperm injection was used to overcome possible zona hardening after the release of cortical granules. Oocytes after cryopreservation showed serious disturbances of the microtubules immediately after thawing as they were vulnerable to the thermal changes and easy to depolymerize. Fertilization of these oocytes with disorganized spindles led to chromosomal aneuploidy, digyny, and degradation of the cleavage rate. We can improve cryopreserved oocytes to normal fertilization and development by appropriate incubation and timing of insemination, as the microtubules repolymerize in a time dependent way after incubation which is compatible with recovery of the spindles. With the improvement of survival, fertilization, and development, oocyte cryopreservation will play an imperative role.