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Ovarian cryopreservation presents a valid alternative to egg freezing in some circumstances. The possibility to store oocytes for a later use is also an important consideration for women who choose to postpone motherhood for personal or professional reasons. Any newly developed protocol should consider the biochemical and physical properties of the oocyte. In addition to surviving the cryopreservation/warming process, the oocyte needs to maintain competence to fertilize and develop in vitro to the appropriate embryonic stage without any structural alterations. Slow cooling protocol is characterized by a slow decreasing temperature rate. Several mathematical models define an optimal curve applicable to oocytes since the freezing rate is vital to achieve sufficient and progressive dehydration, and thereby minimize the potential of intracellular ice formation. During fresh cycles only a few oocytes can be inseminated; therefore, cryopreservation is the only option to avoid wastage of surplus eggs and consequent repeated ovarian stimulation.
Supernumerary embryos may be cryopreserved as early as day 1 (pronuclear stage) and up to day 6 or day 7. This chapter focuses on the crypreservation of pronuclear stage embryos on day 1. Two main methods are used for the cryopreservation of human oocytes and embryos: slow freezing and vitrification. Although controlled slow freezing remains the main method of cryopreservation in most in vitro fertilization (IVF) programs, the vitrification technique has entered the mainstream of human assisted reproductive technology (ART) more and more. Successful pregnancies and live births have been achieved by vitrification of human oocytes and embryos using different carrier systems. The total concentration of cryoprotectant ranges from 5 to 8 mol/l in vitrification protocols currently used in the cryopreservation of human oocyte and embryos. Relatively limited data are available regarding the application of vitrification in the cryopreservation of pronuclear stage human embryos.
Protecting the reproductive potential of young patients undergoing cancer therapy is increasingly important. With modern treatment protocols, 80% of patients can be expected to survive. It has been estimated that up to one in 250 young adults will be a survivor of childhood cancer in the future; infertility, however, may be a consequence. As a wide range of fertility preservation methods are increasingly offered by clinicians, this systematic and comprehensive textbook dealing with the cryobiology, technology and clinical approach to this therapy will be essential reading to infertility specialists, embryologists, oncologists, cryobiologists, ObGyns, andrologists, and urologists with an interest in fertility preservation. Fertility Cryopreservation reviews all the techniques of this increasingly important field within reproductive medicine. It covers the basic principles of pertinent cryobiology, and contains major sections on the different therapies available, written by international specialists combining experience from both academic centers and commercial industries.
Oocyte cryopreservation represents an attractive and the least invasive option of fertility preservation strategy for patients wishing to retain their choice of a male partner. Recent advances in vitrification techniques have markedly improved the efficacy of oocyte cryopreservation, demonstrating that vitrification may be more effective than the conventional slow-cooling method. The proposed mechanism of vitrification uses a high concentration of cryoprotectant (CPA) and extremely rapid cooling and warming rates in order to avoid intra-and extracellular ice formation. The commonly used CPAs for cryopreservation of oocytes and embryos are small neutral solutes such as ethylene glycol (EG), 1,2-propanediol (PROH), dimethyl sulfoxide (DMSO), and glycerol. Some devices used for vitrification include the McGill cryoleaf, cryotip, cryotop, nylon loop or cryoloop, thin capillary or open pulled straws, hemi-straws, and electron microscope copper/gold grids. The oocyte vitrification technique offers a realistic hope of fertility preservation for women without partners.
Cryopreservation of human embryos is an important tool for in vitro fertilization (IVF) practice. Human embryos can be cryopreserved efficiently from the pronuclear to blastocyst stage. For day 2 or day 3 embryos, it is better practically to culture the thawed embryos further for a period of time before transfer in order to confirm that these embryos resume development. The selection method for post-thawed embryos for transfer directly affects the clinical outcome in terms of pregnancy and live birth rates. Two basic techniques have been employed for the cryopreservation of human embryos: slow freezing and vitrification. Slow freezing was used first and is still the most common method for cryopreservation of human embryos. Vitrification is a relatively new technique and has been introduced to cryopreserve human embryos in recent years. Many studies have reported improved success rates in terms of clinical pregnancy and live birth rates with the vitrification technique.