Both sperm and embryo cryopreservation have become routine procedures in human assisted reproduction and oocyte cryopreservation is being introduced into clinical practice and is getting more and more widely used. Embryo cryopreservation has decreased the number of fresh embryo transfers and maximized the effectiveness of the IVF cycle. The data shows that women who had transfers of fresh and frozen embryos obtained 8% additional births by using their cryopreserved embryos. Oocyte cryopreservation offers more advantages compared to embryo freezing, such as fertility preservation in women at risk of losing fertility due to oncological treatment or chronic disease, egg donation, and postponing childbirth, and eliminates religious and/or other ethical, legal, and moral concerns of embryo freezing. In this review, the basic principles, methodology, and practical experiences as well as safety and other aspects concerning slow cooling and ultrarapid cooling (vitrification) of human embryos and oocytes are summarized.

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SummaryBiochemical modifications of zona pellucida (ZP) result in zona hardening. Zona hardening (ZH) is induced by several factors such as advancing maternal age, in vitro culture conditions and cryopreservation and adversely effects implantation. The objective of the clinical study was to determine whether or not laser-assisted hatching (LAH) applied on day 3 frozen embryos improves the outcome of frozen embryo transfer (FET) cycles in patients with recurrent implantation failure and/or advanced female age. In total, 413 patients of different ages with recurrent implantation failure (maximum three cycles) were involved into the study. Patients were allocated randomly into LAH and control groups. On the day of FET, after thawing and just before FET, the ZP was thinned using a laser system. In the control group no treatment was applied on frozen embryo before transfer. The main outcome measures were clinical pregnancy rate. Overall, the results indicate a tendency that LAH increased (P = 0.08) clinical pregnancy. However, for patients older than 37 years, LAH increased pregnancy rates significantly (P = 0.03). In the LAH and control groups, the age of patients and the number of transferred embryos influenced pregnancy rates (P = 0.01). For patients older than 37 years, no effect of number of transferred embryos was detected (P = 0.14). The incidence of multiple pregnancies also increased in the LAH group (P = 0.01). In conclusion, in older woman, to overcome the negative effect of zona hardening, LAH could be performed on frozen embryos as a routine strategy before FET in frozen cycles in order to increase the possibility of pregnancy formation.

The human mature oocyte is particularly sensitive to cooling and low temperatures in addition to freeze-thaw damage. The efficiency of oocyte cryopreservation including the pregnancy outcome is still low. The aim of our study is to briefly introduce our preliminary clinical results achieved with oocyte cryopreservation (CP). Our work focused on the use of a slow cooling procedure using the cryoprotectants propanediol (1.5 M) and sucrose (0.3 M). Following a short incubation of 4-6 hours thawed oocytes were injected with a single sperm (ICSI) and fertilization was assessed 12-16 hours later. Laser assisted hatching (LAH) was performed on all transferred embryos and embryo transfer (ET) was carried out 48-72 hours after ICSI. One-hundred and ten eggs were thawed and a survival rate of 76% (84/110) was obtained. Of the 84 oocytes which survived, 64 subsequently fertilized (64/84; 76%) following ICSI and on the following day 55 of those had cleaved (55/64; 86%). Fifty-two embryos were transferred in 29 patients (1.8 embryo/patient), and 7 (7/29; 24%) resulted in clinical pregnancy (1 twin pregnancy). One of the pregnancies encountered first trimester abortion (1/7; 14%). Implantation rate of 15.4% per embryo transferred (8/52) and 7.3% per egg thawed (8/110) were obtained. In all cases, chorion biopsy was performed and chromosomal anomalities were not detected. Our results provide further evidence that the procedure can be applied safely and with good success in clinical assisted reproduction. However, more work is needed since the survival and implantation rate should be improved.

By decreasing the volume of the cryoprotective solution it is possible to increase dramatically the freezing speed and -at the same time -reduce the toxicity and osmotic side effects of cryoprotectants (CPA). The objective of our study was to vitrify Day-3 cleavage stage mouse embryos (n = 229) with the cryoloop technology using a new composition of vitrification media. Embryos were exposed to a 2-step loading of CPA, ethylene glycol (EG) and propylene glycol (PG), before being placed on the surface of a thin filmy layer formed from the vitrification solution in a small nylon loop, then they were rapidly submerged into liquid nitrogen. After warming, the CPA was diluted out from the embryos by a 3-step procedure. Survival of embryos was based on morphological appearance after thawing and continued development to expanded blastocysts upon subsequent 48-hour culture. Embryos of the two control groups were either treated likewise except that they were not vitrified, or cultured in vitro without any treatment. Our data show that a high percentage of embryos survived (92.7%) vitrification in the mixture of EG and PG combined with cryoloop carrier and developed normally (89.1%) in vitro after thawing. To our knowledge this is the first report of the successful vitrification of cleavage stage mouse embryos using VitroLoop vitrification procedure.