Cryopreservation is a method in which are eggs, sperms but also whole embryos preserved by deep freeze for future use. Samples that are intended to be frozen are carefully tested in the quality, and for infectious diseases such as HIV and hepatitis B and C. This method is performed by laboratories in centres of assisted reproduction. The way in which will be tissue frozen depends on its type and stage of its development.
Development of eggs and sperms
Development of human sex cells is complex process. Male sex cells are formed in process of spermatogenesis and female eggs are created during the oogenesis. Male sex cells, i.e. sperms, are formed by fission of germ cells which are located in the walls of the convoluted seminiferous tubules. Female sex cells, the eggs, are formed in the ovaries. The origin and development of gametes in men and women is controlled by sex hormones. The follicle stimulating hormone (FSH) and luteinizing hormone (LH) undertake a leading function of this developmental process. Besides them, there are also other hormones produced in the gonads. Oestrogen is produced by the ovaries and testosterone is created in the testes. Germ cells are being divided during their development. The division continues until each cell contains only half of its genetic information. If it comes to fertilization of the egg by sperm in the fallopian tube, the both halves of genetic information will merge and create the embryo.
Indications of cryopreservation
Donated sperms and as well as sperms taken from men who will undergo cytostatic treatment in which might be spermatogenesis impaired can be used for cryopreservation, or sperms which are collected with demanding surgical method from testicle that the new harvest does no not has to be repeated. With regard to the embryos, embryos from donated eggs and extra embryos created in in vitro fertilization (IVF) in which are taken eggs fertilized by collected sperms can be used to cryopreservation. If there are more fertilized eggs or embryos than it is needed for the transfer to uterus, it is possible to preserve them with Cryopreservation for repeated use.
Process of cryopreservation
Basically, two cryopreservation methods are normally used, i.e. rapid freezing and slow freezing of tissue. During the process of slow freezing, the sample are inserted into the device that gradually reduces its temperature to about -195 °C over several hours. However, the freezing process is not so simple. If there are any water particles left after the freezing process, ice crystals which would destroy the tissue will be formed. Therefore, special substances which drain off the sample are used, for example, propylene glycol. Afterwards, the frozen samples are stored at the temperature of -195 °C which is maintained by cooling with liquid nitrogen. Tissues can be stored for 5 years but on request of infertile couple can be their embryos kept even longer.
Defrosting in cryopreservation
If we want to use cryopreserved samples again, it is necessary to let them thaw. Preservative solutions are gradually washed out from them during defrosting process. Afterwards, it is necessary to determine how severely was sample damaged by deep freeze. The eggs tolerate cryopreservation the worst. Only about 18% of the eggs is suitable for further use after defrosting process. Cryopreservation of sperms and the embryos is far more successful. About 85% of the embryos is successfully cryopreserved. But the question is whether cryopreservation reduces the likelihood of conception or not. Scientists assume that this method has no influence on the probability of success in conception by this way of fertilization. It is true that the number of successfully nidated embryos which were cryopreserved is lower than that of newly developed. This can be explained so that for the cryopreservation are used only embryos of lower quality because the best ones are normally given to patient immediately. For future, it is discussed about possible cryopreservation of ovarian tissue and testes.
Application of cryopreserved sperms and embryos
It is possible to introduce cryopreserved sperms into cervix uteri after the defrosting process. However, we use mainly one of the assisted reproduction techniques. That one is intrauterine insemination (IUI) in which are sperms injected directly into the woman's uterus. Accordingly, the path which must sperms pass to reach the eggs is shortened and the chances of fertilization are greatly increased. Intracytoplasmic sperm injection (ICSI) can be used as alternative method in case of low number and quality of sperms. Sperm of the most quality is carefully selected under a microscope and afterwards injected to the egg which was collected in advance.
Developmental disorders related to sex cells are nowadays more and more common. In women, the problem is in the formation and maturation of eggs or in obstructed fallopian tubes. In men, the amount of sperms and their quality is deteriorated. This both unpleasant situations can be caused by hormonal disorders, overcome infectious diseases, previous chemotherapy or radiotherapy and other underlying diseases. If the couple can not conceive naturally within two years, it is advisable to see a doctor and start thinking about methods of assisted reproduction.