Cryopreservation of human oocyte can be an alternative to circumvent many of the ethical issues associated with embryo cryopreservation. For oocyte cryopreservation, it is very suitable to use vitrification method. Oocyte cryobanking is a new more efficient approach in oocyte donor-recipient treatment. On the basis of guideline from the Practice Committees of the American Society for Assisted Reproductive Medicine (from 2013) and, in March 2012, European Society of Human Reproduction and Embryology (ESHRE), it is indicated that mature oocyte vitrification and warming are not experimental and should no longer be considered as experimental procedures. This progress in the field of cryopreservation opens new perspectives in assisted reproduction. Recent effective oocyte vitrification systems have a significant impact on clinical practice. It is a possible way in countries where the law forbids the cryopreservation of embryos. Indeed, efficient oocyte vitrification technology eliminates synchronization between donor and recipient. It enables the establishment of egg banks by eliminating the logistics of coordinating egg donors with their recipients. Progress in oocyte vitrification brings new possibilities mainly for women, who are trying to postpone childbearing from professional or social reasons. The process was originally developed as a way to preserve the fertility of cancer patients undergoing possibly sterilizing chemotherapy, and it is relatively simple.
Oocyte cryopreservation is less successful than embryo cryopreservation for many reasons. Oocytes have small surface to volume ratio, temperature-sensitive metaphase spindle, zona pellucida as very specific structure and susceptibility to parthenogenetic activation. Oocytes are one of the biggest cells with high likelihood of intracellular ice formation, Oocytes are very unique cells, because of their developmental capacity to be fertilized and then to support early embryonic development. This capacity derives from maternal legacy of the myriad of transcript, proteins and energetic substrates and also cytoplasmic organelles, which facilitate early mitotic divisions of the embryo until embryonic genome activation occurs. this highly organized structure often incurs serious damage after cryopreservation. the volume of mammalian oocyte is much bigger than that of spermatozoa, thereby substantially decreasing the surface to volume ratio and making them sensitive to chilling and highly susceptible to intracellular ice formation. In fact, in a developing embryo, cleavage division occurs without any increase in volume until blastocyst stage, leading to higher nucleus-cytoplasmic ratio of embryo blastomeres compared with the oocyte. Oocytes are substantially more prone to cryo damage than are embryos. Number of blastomeres in early embryos provides great flexibility to compensate for any detrimental effects of cryopreservation, because missed blastomeres can be replaced by the daughter cells of dividing intact ones. Oocytes contain one-half of the genetic material of the future individual, and so any damage to its chromatin structure may result in deleterious defects in the developmental competence of the resulting embryos. Damage of meiotic spindle can result in chromosomal abnormalities after thawing. the permeability of oocyte plasma membrane to cryoprotective agents is low compared with embryo .
Although mature oocytes in metaphase II are sensitive to cryopreservation (detrimental effect on meiotic spindle or premature cortical granule release) and immature oocytes on prophase I (GV oocytes) look that are more suitable for cryopreservation. It is well known that oocytes frozen at GV stage exhibited decreased affectivity of in vitro maturation and increased spontaneous parthenogenetic activation . For this reason in case of immature oocytes, it is recommended to use in vitro maturation and after that perform their subsequent vitrification.
It was presented that highly organized structure of fresh oocyte changes dramatically (at cellular, ultrastructural, molecular and developmental levels) after cryopreservation. Cryopreserved oocytes have cellular characteristics that differ from those of the fresh oocytes.