O-014 How effective is oocyte cryopreservation?
Authors: Cobo A 
Published: August 2021

Abstract text
The challenge of cryopreserve, store for prolonged period, and successfully implant the female gamete is nowadays feasible thanks to vitrification. The technology that was initially validated in oocyte recipients is currently applied to a vast population, including women at risk of losing their ovarian function due either to iatrogenic causes as occurs in cancer patients, or due to the natural depletion of the ovarian reserve as a result of age related fertility decline. That is the case of a growing population of women who wish to postpone childbearing and decide on oocyte vitrification as a means of fertility preservation (FP). At present, there is a growing body of evidence regarding the use of vitrified oocytes by many women under different indications, which makes it possible to evaluate the approach from different scenarios. So that vitrification can be evaluated in terms on survival rates, embryo development and the rate at which vitrified oocytes develop into live-born children in IVF cycles using vitrified oocytes which were initially stored due to different reasons. The effects of vitrification at the subcellular level and its impact on oocyte competence is of interest in the evaluation of the efficacy of the technology. Some studies have indicated that vitrification may affect ultrastructure, reactive oxygen species (ROS) generation, gene expression, and epigenetic status. However, it is still controversial whether oocyte vitrification could induce DNA damage in the oocytes and the resulting early embryos. Recent studies show that oocytes survival and clinical outcome after vitrification can be impaired by patients’ age and the clinical indication or the reason for vitrification. These studies show that age at oocyte retrieval strongly affects the survival and reproductive prognosis. In our experience, oocyte survival, pregnancy and cumulative live birth rates are significantly higher when patients are aged 35 years or younger versus patients older than 35 years at oocyte retrieval. Therefore, elective-FP patients should be encouraged to decide at young ages to significantly increase their chances of success. There is also evidence that the reason for vitrification is associated to the success rates. Poorer reproductive outcome was reported in cancer patients, low responders and endometriosis patients when compared to healthy women in age matching groups. Moreover, there are certain individualities linked to specific populations, as occurs when endometriosis patients had cystectomy earlier than the oocyte retrieval for FP. These women achieved lower success rates as compared to non-operated age matching counterparts. In this case, the lower cumulative live birth rates observed in operated women are, most probably, due to the smaller number of oocytes available, as a consequence of the detrimental effect of the surgery on the ovarian reserve. In this regard, several reports show that the number of oocytes available per patient is another variable closely related to the outcome in all populations using vitrified oocytes after FP. Thus, a significant improvement in the cumulative live birth rates can be achieved by adding a few oocytes, especially in healthy young patients. Different populations using vitrified oocytes under several indications achieve differential results in terms of pregnancy rates, when calculated in overall. Nonetheless, when the calculations for the cumulative probability of achieving a baby are made according the number of oocytes used per patient belonging to the same group of age, the results become comparable between different populations, as shown by the comparison between elective freezers versus endometriosis patients. Undoubtedly, vitrification can be recognized as one of the latest brakethrough in the ART field, but certainly the next step forward would be the successfull automatization of the vitrification and warming processes to achieve fully consistency among different laboratories.