WORCESTER MEDICINE
Infertility
Planned Oocyte Cryopreservation Armando Arroyo, MD Yanguang Wu, PhD
W
hat is planned oocyte cryopreserva-
tion? Who should consider oocyte freezing? When should they freeze oocytes? How many oocytes should be frozen? How are oocytes frozen? How effective is oocyte freezing? This short article discusses oocyte freezing for planned oocyte cryopreservation based on recent American Society for Reproductive Medicine guidelines. (1, 2) Planned oocyte cryopreservation is an approach that provides young, fertile women the option to conceive later in life when the ability to conceive naturally declines. This process involves ovarian stimulation, oocyte retrieval and oocyte freeze in liquid nitrogen. When a woman is ready, the oocyte is thawed, fertilized and the embryo is transferred to the uterus. The time to freeze oocytes is between the ages of 25 and 40, with the best time being before the age of 35. The ovaries are stimulated using the GnRH antagonist protocol. Usually, 20 oocytes are retrieved and frozen, depending on the woman’s age. The chance of having a child with frozen oocytes is dependent on the age of the woman and the number of mature oocytes cryopreserved. No increased birth risks have been identified compared to standard IVF. Planned oocyte cryopreservation, or OC , is effective and safe. In 2013, the American Society for Reproductive Medicine, or ASRM, recommended OC only for women undergoing gonadotoxic treatments seeking to preserve fertility. In 2018, the ASRM ethics committee stated: “planned OC is an emerging but ethically permissible procedure that may help women avoid future infertility.” (3) Many women are not aware the ability to naturally conceive declines with advancing maternal age. The Society for Assisted Reproduction, or SART, publishes yearly IVF outcome data. Women younger than 35 have a 50% chance of having a child with IVF on the first attempt, at 35-37 years (40%), 38-40 years (30%), 40-41 years (10%), 42 years (4%) and 45 years (1%). This fertility rate decline, when paired with age, is due to a decline in ovarian reserve. Human ovarian reserve – number of oocytes – declines from conception to menopause (Fig 1A). Number of oocytes drop dramatically from 6 million at 20 weeks gestation to
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500,000 at birth, to 300,000 at menarche, to 25,000 at age 35, and 1,000 remain at age 50. Oocyte quality also declines with age, further reducing reproductive potential (Fig 1B). Oocyte quantity is important, however, oocyte quality may be even more important. What is the age-related master switch that determines oocyte quality? The definition of oocyte quality is the ability of an oocyte to mature, fertilize, cleave, implant and form a healthy infant. Surprisingly, after many years of investigation, very little is known about the master switch. Currently, the leading hypothesis is that oocyte chromosome ploidy is the major oocyte quality determining factor. Embryo ploidy is the best proxy marker of oocyte ploidy. The best information on embryo ploidy comes from preimplantation genetic testing of human embryos. Embryo aneuploidy dramatically increases with age: 20% at age 20, 50% at 35, 60% at 40, 80% at 42 and 90% at 44 (Fig 1B). Clearly, the decline in oocyte quantity and quality with advancing age reduces fertility potential. The oocyte freeze procedure is complex. The traditional procedure was known as the slow controlled-rate freezing technique. The first live birth from a cryopreserved human oocyte, in 1987, used the slow freeze technique. Slow freezing requires cooling the oocytes in low concentrations of cryoprotectants, or CP, slowly dropping the temperature at a rate of 0.3-2°C per minute until -196°C. The procedure normally takes 3-4 hours and requires expensive equipment . Besides the complicated manipulations and expensive costs, the oocyte survival, embryo development and live birth rates when using thawed oocytes in IVF were still significantly lower than using fresh oocytes. The unsatisfactory outcome of slow freezing is due to ice crystal formation that occurs during the slow cooling step which results in irreversible oocyte membrane damage. The cytoskeleton of the oocyte specifically spindle organization is impacted by the slow cooling rate. In 1998, a new technique, recognized at the beginning as fast-freezing, then changed to vitrification was deployed for oocyte freezing in clinical IVF. Compared to slow-freezing, vitrification is a fast freeze procedure. The oocytes are placed in high cryoprotectant concentrations, which replaces the water in the oocytes. Then they are directly deposited into liquid nitrogen. Because the temperature drops in a super-fast manner (greater than 25,000°C per minute), the remaining water molecules do not have time to form ice crystals, and instead instantaneously solidify into a glass-like structure. After 10 years of cryoprotectant modifications, replacing single CP with mixture of several CPs, and carrier-device liquid nitrogen changes, replacing closed systems with open systems, the vitrification technique became the preferred oocyte cryopreservation method by the end of the 2000s. (4) Today, planned OC by vitrification is routinely performed in IVF clinics. The IVF outcomes from vitrified oocytes are comparable to fresh retrieved oocytes as we described above. It seems vitrification perfectly resolved the problems of oocyte cryopreservation associated with the slow freeze method. Like all complicated embryology procedures, the success of oocyte vitrification requires support including a highly skilled, experienced embryologist and extremely strict laboratory procedure quality control. Academic fertility clinics have additional administrative, financial, clinical and research support all contributing to a successful oocyte cryopreservation program.
JANUARY / FEBRUARY 2022
