Introduction: Usually, during a successful in vitro fertilization (IVF), several embryos are produced, and couples prefer to freeze the surplus embryos for future use.
In assisted reproductive techniques such as IVF and microinjection (ICSI), embryo freezing is one of the new ways to preserve women's fertility.
Social changes have led to the postponement of the age of first pregnancy. The decrease in pregnancy in industrialized countries has led to optimization of fertilization in pregnancy centers. Freezing is a promising method for storing all stages of human embryos, and this method is used to postpone embryo transfer in patients who are at risk of ovarian hyperstimulation syndrome (OHSS) or patients who are ready for radiation therapy. It is also very effective in the egg donation program. Freezing prevents the loss of extra embryos. Evidence shows that embryos that have already been frozen have better perinatal results than fresh embryos. Progesterone level is a good biomarker in this case.
Methods: In the frozen embryo transfer (FET) strategy, the desired embryos are transferred to a stimulated cycle and the remaining embryos are frozen for future use.
There is a newer strategy called "freezing all" in which all embryos are frozen for future transfer in subsequent cycles as long as the uterine environment is favorable. The most common reasons for using this method are to avoid OHSS, as well as to reduce maternal blood pressure disorders, the risk of miscarriage and multiple births It should be noted that this method is not offered in women who have recovered less than 15 eggs in the stimulation cycle. In this method, PGD (Preimplantation Genetic Diagnosis) and PGS (Preimplantation Genetic Screening) are used. PGD and PGS tests are performed through IVF before embryo implantation. In this technology, genetic tests are performed on embryo cells in order to select the best embryo for implantation and continued pregnancy. When one of the parents or both of them has a known genetic disorder, PGD method is used to identify embryos with genetic disorder and thereby prevent the transfer of defective genes to the baby. On the other hand, the PGS (Preimplantation Genetic Screening) method is used to investigate aneuploidy cases in embryos resulting from laboratory fertilization.
Results: Among the egg freezing techniques, we can mention "slow freezing", in this type of freezing, it is a method of programmed step-by-step reduction of temperature and it is long-term, this is a precise but expensive tool, and it is possible that ice crystals inside the cell that We have frozen them, which has harmful effects. Another technique is "glass freezing". In this method, we freeze the sample like glass, providing the possibility of cooling and heating with much less freezing damage than the slow freezing method. It has a high survival rate. In glass freezing, the formation of intracellular and extracellular ice is prevented. It has a lower cost and is efficient in terms of time. This type of freezing makes IVF more efficient. The biggest concern of this method is the toxicity and dangerousness of the process, and high concentrations of antifreeze cannot be used.
Freezing causes extensive damage to the cell membrane, which changes the functional and metabolic state of the cell.
which affects DNA fragmentation, sister chromatid exchange (SCE) and aneuploidy in the egg.
Conclusion: Freezing stress, including osmotic shock, changes in temperature, pH, and freezing toxicity may cause epigenetic and transcriptomic changes. Currently, it is not clear whether these changes affect the health of future children or not.
Freezing with changes in DNA methylation level affects the normal expression of genes and gene regulatory region and depends on factors such as temperature, concentration and type of CPA (Cryoprote agents).
During freezing, CPA creates an osmotic gradient that moves water from the inside to the outside while maintaining the membrane and structure inside the cell.
CPAs are of two categories, 1- permeating agents: they pass through the membrane and are compounds with low molecular weight and protect the cell against damage caused by cold, such as glycerol, butanol-ethylene glycol and methyl sulfoxide.
Other category 2-Non-penetrating agents: They are non-diffusible and have a higher molecular weight, such as serum proteins, sugars and polymers.