Treatments
Description of available treatments
Egg collection, freezing & storage – A solution for early menopause
Cryopreservation of oocytes is one of the most promising options of the future in human female gamete preservation and donation. Technology for the long term preservation of human oocytes has improved greatly over the past 25 years and currently is used for supporting various assisted reproductive technologies in human reproductive medicine.
To maintain long-term viability after long-term storage, living cells must be brought into a state of suspended animation in which they remain for indefinite periods of time, and from which they can be brought back to viability at some point in the future. The temperature that generally is used for storage of mammalian cells, −196°C, the temperature of liquid nitrogen, appears to be adequate for these purposes.
The ability to cryopreserve human oocytes confers significant advantages in clinical assisted reproduction. First, this technique does not require partner’s sperm and thus can be used to preserve fertility in single women. Second, some of the ethical quandaries relating to the storage and disposal of surplus embryos are avoided. The technique also can simplify the egg donation process for IVF; if frozen oocytes are fertilized as efficiently as the unfrozen ones, donor oocyte banks can be established. There no longer would be a need to synchronize donor and recipient cycles, and long waiting periods for appropriately matched donors could be avoided. The major drawback is that oocytes are cells which do not freeze-thaw “very well”. Physiologically, oocytes are large cells, and their outer layer is fairly water-resistant. When water is cooled to below its freezing point, it solidifies in a crystalline structure known as ice. Because ice is less dense than liquid water, it necessarily follows that ice crystals occupy a greater volume than does the liquid water from which they were formed. As adjacent volumes of liquid water within a cell solidify, their expansion into ice causes pressure and shearing forces on intracellular organelles, which can suffer considerable damage. Avoidance of ice-crystal formation therefore is one of the principal goals of 2 successful cryopreservation. When water is trapped inside an oocyte, ice crystals formed during the cooling process can damage the oocyte. This makes oocyte cryopreservation challenging.
Schematic of a mature oocyte
Oocyte cryopreservation undoubtedly has potential to improve and extend the current assisted reproductive technologies. Until recently the method of choice for oocyte cryopreservation has been the socalled ‘slow-freeze’ (SF). This technique allows the oocytes to be washed though solutions of cryoprotectant (solution which reduces the formation of crystals that can cause severe damage to the oocyte) and then decrease the temperature of the solution slowly until it reaches -196oC. SF has been found to be of limited potential for the cryopreservation of oocytes due to the low rates of survival of approximately 50-74% (according to Gook and Edgar, 2007) as well as low percentage of live births 1-20% (Oktay et al, 2006).
Recently, a technique named vitrification has shown very promising results in studies carried out all over the world. Vitrification is the process by which water is prevented from forming ice crystals due to the viscosity of a highly concentrated cryoprotectant solution cooled at an extremely rapid rate. To reduce exposure to the toxic cocktail of cryoprotectants and prevent extreme dehydration, cells are exposed to the cryoprotectant solutions for a very short period. Therefore, the procedure is faster, with higher expected yields. It has been suggested that vitrification may be less traumatic to the meiotic spindle (it is a detrimental apparatus within each cell of our body and its function is to help the cells divide normally) 3 than slow freezing and may also have less effect on cell physiology (Gardner et al., 2007). Studies on vitrification have revealed high survival rates of 85-100% (Lucena et al, 2006; Antinori et al, 2007) and live births rates resulting from “vitrified” oocytes reaching 40% (Oktay et al, 2006). It has also been shown that vitrification produces 76-91 embryos per 100 oocytes thawed (Oktay et al, 2006) whereas SF produces only 33-56 embryos per 100 oocytes thawed (Gook and Edgar, 2007)
Disadvantages of oocyte freezing
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Vitrification of human oocytes is still a relatively new treatment. There have been few babies born using cryopreserved oocytes thus not allowing us to predict the chances of success.
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This is a new procedure and may involve, as yet, unrecognised risks, such as increased risk of a congenitally abnormal baby.
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Thawing of cryopreserved oocytes is more expensive than thawing embryos because after thawing, the oocytes have to be fertilized using ICSI.
Risks of ICSI
Possible adverse consequences of ICSI are covered in our associated leaflet on ICSI, but principally relate to the possible transmission of male fertility problems.
References
Antinori M, Licata E, Dani G, Cerusico F, Versaci C, Antinori S. Cryotop vitrification of human oocytes results in high survival rate and healthy deliveries. Reprod Biomed Online 2007;14:72–79.
Gardner DK, Sheehan CB, Rienzi L, Katz-Jaffe M, Larman MG. Analysis of oocyte physiology to improve cryopreservation procedures. Theriogenology 2007;67:64–72.
Gook DA, Edgar DH. Human oocyte cryopreservation. Hum Reprod Update, 2007, 13:591–605
Lucena E, Bernal DP, Lucena C, Rojas A, Moran A, Lucena A. Successful ongoing pregnancies after vitrification of oocytes. Fertil Steril 2006;85:108–111.
Oktay K, Cil AP, Bang H. Efficiency of oocyte cryopreservation: a meta-analysis. Fertil Steril 2006;86:70–80.