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Cryopreservation is a special procedure in which cells and tissue can be stored for long periods (max. 5 years) to be used later. These are frozen and stored in liquid nitrogen at -196° Celsius. The term used for this technique comes from the ancient Greek word ‘kryos’, which means ‘cold’.
Cryopreservation is also a common method used in fertility treatment:
• Unfertilised eggs
With unfertilised eggs, there is still a relatively high risk that they can be damaged during the cryopreservation process. The reason for this is that the unfertilised eggs have a much greater sensitivity to the freezing process due to their higher water content (compared to fertilised eggs). Water crystals that form during the freezing process can therefore cause more damage. Nevertheless, the frozen, unfertilised eggs are still a valuable egg store which can be used to conceive children in the future.
• Fertilised eggs in the pronuclear stage
By using special cryopreservation procedures in which cells are mixed with a membrane-protecting medium, the cryoprotectant, and computer-assisted temperature programs that control the freezing process, the cells are put in a state that ensures that they can be stored over long periods (years) without impairing their viability and then thawed as required.
As part of infertility treatment, cyropreservation of sperm may be necessary due to the prolonged absence of the partner, a health condition, or difficulty on the part of the man to provide sperm on demand (psychological impotence).
• Testicular tissue
Testicular tissue obtained in a biopsy (see TESE), can be frozen for later sperm retrieval. The availability of this previously surgically removed tissue allows for hormonal stimulation of the ovaries in the woman without time restraints. In addition, multiple biopsy samples can be individually frozen for several ICSI attempts.
• Ovarian tissue
Thanks to state-of-the-art cryopreparation techniques, it is possible to carefully freeze tissue without impairing the physiology of the cells. In order to fulfil a desire to have a child, the thawed, intact ovarian tissue can be re-implanted after several years of cryopreservation (autotransplantation). It is also possible to remove the eggs that have matured in the ovarian tissue from the cryopreserved tissue at a later date and use these for artificial Insemination.
1. As part of artificial or assisted fertilisation
It is often useful to freeze fertilised eggs in the pronuclear stage and store them for later intracytoplasmic sperm injection (ICSI) or in vitro fertilisation (IVF).
If necessary, these eggs can be further cultivated and used, for example, if the parents want another child later. This process is also referred to as cryotransfer. In this case, the advantage of cryopreservation is that the woman does not have to undergo any more hormone treatment or egg collection (follicular aspiration).
2. Before treating severe illnesses
In some serious diseases, there is a very high risk of becoming infertile due to the treatment of the disease. This can be the case, for example, in the treatment of cancer by chemotherapy, radiotherapy or surgery. After a thorough medical consultation and assessment of the risk, those who want to have children can take precautions to prepare for a loss of fertility and have their sperm or testicular tissue or their eggs or ovarian tissue cryopreserved before the Treatment.
3. In social freezing
Some young women may want a child, but they may not be able to or want to become pregnant at their current stage of life for personal reasons. It may be, for example, that her own career does not allow for this or that she has not yet found the right partner. In Denmark, these women are able to have their eggs frozen for a later date.
Once a couple has a desire to use a frozen fertilised egg to get pregnant, this egg can be further cultivated and transferred to the uterus. This is referred to as cryotransfer. Unfertilised eggs that have been frozen are fertilised in advance by an intracytoplasmic sperm injection.
The cryotransfer can be carried out at optimal cycle conditions without any hormonal support. Another possibility is to assist the build-up of the lining of the uterus with oestrogen and to optimally prepare it for the implantation of the egg. The doctor can measure the thickness of the lining of the uterus with an ultrasound scan. Once it is thick enough, ovulation is simulated by administering progesterone. This changes the structure of the lining, preparing it to receive embryos. The third option is hormonal stimulation with clomiphene or FSH to support follicle maturation prior to embryo transfer. The doctor can then insert the embryo/embryos into the uterus using a soft catheter.
Cryopreservation has been used for many years, and most studies show no adverse effects. However, an individual consultation is always included as part of the treatment.