Cryostoring surplus embryos: freezing

After embryo replacement, surplus good and high quality embryos can be frozen and stored in liquid or vapor nitrogen for future use. Frozen embryo cycles contribute to the overall chance of getting pregnant, without needing to have a further stimulated cycle of injections, frequent ultrasonographic monitorization of follicular size and egg collection. In a cryocycle the endometrium (uterine lining) needs to be precisely synchronized with the developmental stage of the thawed embryos, and in that way is much closer to a natural cycle. Embryo freezing may also be done for clinical reasons, such as the patient having a thin endometrium, or if there is a high risk of ovarian hyper stimulation syndrome (OHSS), which can occasionally occur if a patient overreacts to hormonal drugs given during the fresh cycle (the hormonal regimen may stimulate the ovaries too much, and they may become very swollen).

Vitrification

Embryos can be vitrified at every stage of their development from the very first day to the 6th day. Unlike the slow-freezing method, vitrification uses very rapid cooling rates (2000-20000oC/ minute). This decreases the risk of cryoinjury caused by the high concentrations of cryoprotectants used with the aim of protecting the embryos during the freezing process. One of the most important criteria that prove the success of the vitrification method is the post-thaw survival rate, which is above 95% in our center. In addition to this high survival rate, vitrification is also the most successful freezing method in terms of pregnancy rates.

Although, technically, embryos can be stored for much longer times, regulations issued by the Turkish Ministry of Health restrict this period to 5 years. Also, according to this directive, embryo freezing and thawing can only be performed with the written consent of both spouses, meaning that in the case of divorce or the death of a partner, embryos are discarded. 

Embryo vitrification steps
Embryo thawing steps

Freezing the embryos in nitrogen (-196°C) and storing them in nitrogen tanks

Fertility preservation in cancer patients

Surgical, medical, and technological advances in cancer treatments have led to improvement in the quality of life and an increase in the survival rates of cancer patients, making preservation of reproductive function a more important priority than ever before. Fertility preservation aims to maintain the fertility potential in adults or children who are at risk of sterility due to anti-cancer treatment. Current therapies, including conservative surgery, chemotherapy, radiotherapy, and bone marrow transplantation, can offer increased cure rates. However, aggressive chemotherapy and radiotherapy affect the ovaries, usually irreversibly, often inducing premature ovarian failure (early menopause) or even causing permanent amenorrhea and infertility. Fertility preservation should therefore always be considered for prepubertal girls and for women of child-bearing age undergoing potentially gonadotoxic treatment. Options for fertility preservation methods depend on the age of the patient at the time of diagnosis and treatment, the cancer type, severity and location, and the type of treatment: 

• Embryo freezing is very well established and available for women with a male partner (see above). 
• Oocyte vitrification is a useful option for fertility preservation in young women who do not have a partner. Oocytes have a high water content, hence a higher risk of ice crystals formation during the freezing procedure that can harm the cytoskeletal and meiotic spindle, on which the segregation of maternal chromosomes rely. In the vitrification procedure the temperature is reduced extremely rapidly, changing the oocyte from a liquid into a vitreous state, thus avoiding meiotic spindle damage.
• Ovarian tissue freezing can be a valuable option when the urgent start of cancer treatment is recommended, leaving no time for ovarian stimulation, and for prepubertal girls. In this method, an ovary, or part of an ovary, is removed. From this, strips measuring 10x1x1 mm are prepared. The ovarian cortex (outer layer), comprising the follicles (containing the oocytes), is detached from the stroma (inner layer) to increase the post-thaw survival rate. Samples are frozen via slow-freezing and stored within cryoprotectants in liquid or vapor nitrogen at -196○C until transplantation.  

Preparing and freezing ovarian tissue