In the course of an IVF treatment cycle, more viable embryos may be produced than it is desired for embryo transfer in that same cycle. If so, these "excess" embryos can be preserved by freezing and stored for future use. In some cases, all of the embryos can be frozen without fresh transfer.
Cryopreservation is a series of procedures of freezing cells and tissues. It is a controlled process of freezing to very low temperatures and storage in order to keep cells and tissues for theoretically unlimited period of time and subsequently successfully dissolve the same. At these low temperatures, any biological activity, including the biochemical reactions that would lead to cell death, has been stopped. Specific features of each type of cells undergoing cryopreservation require precise and biologically safe freezing protocol and, subsequently, thawing.
First successful embryo cryopreservation mammal was done in 1972, and the first baby conceived by embryo that was frozen was born in 1984.
Cell survival during cryopreservation depends on the reaction of cellular and extracellular water. At temperatures below zero, when the water is frozen and all metabolic processes are stopped, the cells are able to stand in a state of 'suspended vitality' or they may succumb to fatal damage. Human embryos are kept frozen in liquid nitrogen at -196 ° C.
During the freezing, all chemical reactions within the cell must be stopped. Damage embryos occur during the process of cooling and thawing, and as a result of the creation of ice crystals within the cells, or a change in osmotic pressure as a result of dehydration of the cells.
Cryoprotectant molecules are widely applied in modern cryotechnology, as protective substances that facilitate cell survival and the process of equilibration during phase transition from a liquid to a solid state and vice versa. Cryoprotectant substances have a lower freezing point and prevent making ices crystals that would at such low temperatures damage the cell. Different types of cryoprotectants are used for different stages of embryonic development. Embryos can be frozen on the stage of pronuclear to the blastocyst stage (5-7 days after fertilization). The most commonly used cryoprotectants are propanediol, dimethyl-sulfoxide or glycerol.
Two techniques used to freeze eggs are slow freezing and vitrification. As technology progresses, today is increasingly considered that vitrification is preferable to slow freezing in terms of higher pregnancy rates.
Frozen embryos are kept in special lab dishes, placed in liquid nitrogen. For each individual embryo, its origin, division phase, and the exact time and date of freezing were recorded.
The embryos will be stored in the frozen condition until the patient and spouse request their use (longest 5 years, as current law allows), and the physician responsible for your care determines that appropriate conditions exist in the patient for transfer of the embryos into the patient's uterus. At that time, some or all of the embryos will be thawed. Each embryo will be examined to determine whether it is potentially viable, and if so, the transfer into the patient's uterus will occur.
Risks and Limitations
The primary concern with the use of cryopreservation techniques is the possible loss of embryos, meaning some healthy embryos may not survive the stress of freezing and thawing. The exact number of embryos lost to cryoinjury varies, but it is very likely that freezing will cause loss of some embryos, perhaps 25-50%. However, it could have many good points, because in this way, the selection is made for more viable embryos.
Sometimes, not all embryos obtain adequate quality to be frozen, which is why this option cannot be offered to all couples.
Another concern with cryopreservation is the potential risk of birth defects in children produced from frozen/thawed embryos. But numerous studies have not shown any significant increase in abnormalities when compared to pregnancy outcomes in the rest of the population.
Advantages of embryo freezing procedure:
This procedure reduces the number of preparatory procedures, and reduces the complications that carry the processes of stimulation and aspiration of oocytes. Freezing embryos is important to avoid ovarian hyperstimulation syndrome, since all embryos can be frozen until the transfer to the uterus in the next cycle, and used for later transfer in a non-stimulated natural cycle. Ovarian hyperstimulation has shown negative effects on the endometrial receptivity. Presently, the highest success rates in reproductive medicine are seen in the women who have not had ovarian stimulation - their endometrial tissue has not been exposed to high hormone levels, and they are not at risk of ovarian hyperstimulation syndrome. Freezing embryos for later transfer might therefore improve implantation and pregnancy rates and increase the safety of IVF.
In this way, it also increases the possibility of pregnancy in each cycle of in vitro fertilization.
This way avoids the medical and social problems associated with multiple pregnancies, providing that the appropriate cycle transmits only a small number of embryos.
Studies show that cryopreservation is associated with significantly reduced risk of birth defects, especially in relation to ICSI. This may be due to developmentally vulnerable embryos who fail to survive the process of freezing and thawing.
Embryo freezing procedure allows control of complications of in vitro fertilization process, so when due to certain circumstances the process has to be determinated, the obtained material can be saved.
This method also provides significant financial savings (costs stimulation and intervention).
Assisted reproduction is significantly improved in recent years, while the cryopreservation of ovarian and testicular tissue is significantly more complex than in the case of embryos and gametes. However, technology is constantly improving and it will be offered in many IVF centers.