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In some IVF cycles, no fresh embryo transfer in the stimulated cycle is advisable. The cryopreservation of zygotes and the transfer of blastocysts in a cryo-embryo transfer is an option to circumvent an inadequate uterine environment due to risk of ovarian hyperstimulation syndrome, inappropriate endometrium build up, endometrial polyps or uterine myomas. For this strategy, highly secure and safe cryopreservation protocols are advisable. This study describes a protocol for aseptic vitrification of zygotes that results in high survival rates and minimizes the potential risk of contamination in liquid nitrogen during cooling and long-term storage. In mouse zygotes, there was no difference in efficiency as compared with a conventional open vitrification system. In IVF patients, aseptically vitrified zygotes showed no difference in blastocyst formation rate as compared with sibling zygotes kept in fresh culture. A clinical study comprising 173 cryo-cycles with a transfer of blastocysts originating from vitrified zygotes showed an ongoing pregnancy rate of 40.9%. The live birth rate per patient was 36.8%. A combination of good clinical results and increased safety conditions due to aseptic vitrification encourages the use of cryo-embryo transfer for patients with a suboptimal uterine environment in a fresh cycle. In stimulated IVF cycles, high doses of hormones are given to stimulate multifollicular growth. One drawback of the hormonal substitution is that the uterine environment is not at the same time optimally prepared for embryo implantation. A solution, which is increasingly under discussion, is to cryopreserve the embryos obtained in the stimulated cycle and to transfer them back into the optimal uterine environment in a subsequent cryo-cycle. This procedure requires highly secure and safe cryopreservation protocols in order to ensure benefits for both pregnancy and birth rates. We have established a protocol for the vitrification of zygote-stage embryos in aseptic devices, which minimize the potential risk of contamination during cooling and storage. The vitrified zygotes showed the same blastocyst development as compared with sibling zygotes in fresh culture. A clinical study comprising 173 cryo-cycles with transfer of blastocysts originating from vitrified zygotes shows an ongoing pregnancy rate of 40.9%. The live birth rate per patient was 36.8%. A combination of good clinical results and increased safety conditions due to aseptic vitrification conditions contributes to a change in transfer strategy and encourages us to increase the cryo-embryo transfer rate for an optimal uterine environment.

Vitrification with the use of "Open" carrier devices (Cryoloop, cryotop, cryoleaf, Vitriplug) which allowed the contact with liquid nitrogen has become a more popular way to achieve cooling rate superior to 20,000 °C/min. Even though the question of contamination with liquid nitrogen during ultra-rapid cooling and storage remain debatable with the use of "open" devices, it is important to revise the carrier system in a way, which minimizes the risk of contamination. According to the EU tissues and cells directive, it is advisable that the cooling and storage should be carried out in embryo carrier devices ensuring complete separation of the embryos from liquid nitrogen in a way, which minimizes the risk of contamination. The consequence of a reduction in the cooling rate resulting from the heat-insulating barrier of aseptic devices has to be counteracted by gradually increasing intracellular concentrations of cryoprotectants without inducing a toxic effect. We developed an aseptic vitrification method of vitrification for MII oocytes and embryos at different stage of development using the "VitriSafe" as "closed" carrier device.

Not before the introduction of the Nomarski differential interference contrast optics (DIC), we became aware of the presence of so-called vacuoles on the surface of the sperm head. Before the implementation of motile-sperm organelle-morphology examination (MSOME), no manuscript at all mentioned that spermatozoa free of vacuole-like structures should be selected for injection.

To determine the best day for the selection and transfer of a single embryo, a prospective, randomized study was undertaken that compared the ongoing pregnancy rate (PR) after single embryo transfer (SET) on day 3 with that after single blastocyst transfer (SBT) on day 5. Our results show an overall significantly higher PR after SBT (32.8%) compared with SET (23.2%), and a PR of 40.8% after SBT versus 25.6% after excellent-quality embryos became available.

STUDY QUESTION What is the intracellular concentration of cryoprotectant (ICCP) in mouse zygotes during vitrification (VIT) and slow-freezing (SLF) cryopreservation procedures? SUMMARY ANSWER Contrary to common beliefs, it was observed that the ICCP in vitrified zygotes is lower than after SLF, although the solutions used in VIT contain higher concentrations of cryoprotectants (CPs). WHAT IS KNOWN ALREADY To reduce the likelihood of intracellular ice crystal formation, which has detrimental effects on cell organelles and membranes, VIT was introduced as an alternative to SLF to cryopreserve embryos and gametes. Combined with high cooling and warming rates, the use of high concentrations of CPs favours an intracellular environment that supports and maintains the transition from a liquid to a solid glass-like state devoid of crystals. Although the up-to-date publications are reassuring in terms of obstetric and perinatal outcomes after VIT, a fear about exposing gametes and embryos to high amounts of CPs that exceed 3-4-fold those found in SLF was central to a debate initiated by advocates of SLF procedures. STUDY DESIGN, SIZE, DURATION Two experimental set-ups were applied. The objective of a first study was to determine the ICCP at the end of the exposure steps to the CP solutions with our VIT protocol (n = 31). The goal of the second investigation was to compare the ICCP between VIT (n = 30) and SLF (n = 30). All experiments were performed in triplicates using mouse zygotes. The study took place at the GIGA-Research Institute of the University of Liege. PARTICIPANTS/MATERIALS, SETTING, METHODS Cell volume is modified by changes in extracellular osmolarity. Hence, we estimated the final ICCP after the incubation steps in the VIT solutions by exposing the cells to sucrose (SUC) solutions with defined molarities. The ICCP was calculated from the SUC concentration that produced no change in cell volume, i.e. when intra- and extracellular osmolarities were equivalent. Cell volume was monitored by microscopic cinematography. ICCP was compared between SLF and VIT based on the principle that a high ICCP lowers the probability of (re)crystallization during warming but increases the probability of over-swelling of the cell due to fast inflow of water. The survival rates of mouse zygotes after SLF or VIT were compared using either (i) various warming rates or (ii) various concentrations of SUC in the warming dilution medium. MAIN RESULTS AND THE ROLE OF CHANCE The ICCP in mouse zygotes during the VIT procedure prior to plunging them in liquid nitrogen was ∼2.14 M, i.e. one-third of the concentration in the VIT solution. After SLF, the warming rate did not affect the zygote survival rate. In contrast, only 3/30 vitrified zygotes survived when warmed slowly but as many as 30/30 zygotes survived when warming was fast (>20 000°C/min). Vitrified zygotes showed significantly higher survival rates than slow-frozen zygotes when they were placed directly in the culture medium or in solutions containing low concentrations of SUC (P < 0.01). These two experiments demonstrate a lower ICCP after VIT than after SLF. LIMITATIONS, REASONS FOR CAUTION The results should not be directly extrapolated to other stages of development or to other species due to possible differences in membrane permeability to water and CPs. WIDER IMPLICATIONS OF THE FINDINGS The low ICCP we observed after VIT removes the concern about high ICCP after VIT, at least in murine zygotes and helps to explain the observed efficiency and lack of toxicity of VIT. STUDY FUNDING / COMPETING INTEREST(S) The study was funded by the FNRS (National Funds for Scientific Research). The authors declare that they have no competing interests.

Intracytoplasmic morphologically selected sperm injection (IMSI) is now a reality in ART practice but still with a lot of questions regarding (1) the terminology of vacuoles, their classification, their location on the sperm head, and their origin and meaning; (2) the application of IMSI instead of in vitro fertilization in cases of unexplained infertility; (3) the age of the woman; and (4) the technical aspects. We have to be aware that this technique is demanding and has to be performed in the best working conditions so as not to impair the oocyte quality. The introduction of IMSI has the advantage that embryologists realize that more attention has to be paid during sperm selection even in case of classical intracytoplasmic sperm injection (ICSI). The application of IMSI leads to more and better quality blastocysts and, as consequence, it increases the chances of selecting the proper embryo for transfer with high implantation potential. Even though there is no real proof in the human species on the abnormal outcome generated by spermatozoa carrying vacuoles, a higher and better-resolution technique has to be added as an additional tool for ICSI knowing the possible consequence of sperm DNA damage for offsprings.

During embryo vitrification, it is advisable that cooling and storage should occur in a carrier device in which there is complete separation of the embryos from liquid nitrogen to ensure asepsis. The consequence of a reduction in the cooling rate resulting from the heat-insulating barrier aseptic devices has to be counteracted by gradually increasing intracellular concentrations of cryoprotectants without inducing a toxic effect. Blastocysts originating from couples with male and/or female factor infertility (group 1) or from oocyte donors (group 2) or from in-vitro matured oocytes (group 3) were gradually exposed to increasing concentrations of dimethylsulphoxide/ethylene glycol (5/5%, 10/10% and 20/20%) before aseptic vitrification using a specially designed carrier (VitriSafe), a modification of the open hemi-straw plug device. A total of 120 aseptic vitrification/warming cycles were performed in group 1, 91 in group 2 and 22 in group 3. Survival rates before embryo transfer, ongoing pregnancy and implantation rates were as follows: for group 1, 73, 43 and 26%; for group 2, 88, 53 and 34%; and for group 3, 69, 50 and 38%, respectively. In spite of reduced cooling rates due to aseptic vitrification conditions, a three-step exposure to cryoprotectant solutions protects the embryos effectively from cryo-injuries and guaranties high survival rates.

Spermatozoa selection at high magnification before intracytoplasmic sperm injection seems to be positively associated with pregnancy rates after day 3 embryo transfers. The aim was to demonstrate an association between the presence of vacuoles in sperm nuclei and the competence of embryos to develop to day 5. Grading of spermatozoa at x 6000-x 12,500 magnification: grade I, no vacuoles; grade II,or=2 small vacuoles; grade III,or=1 large vacuole; grade IV, large vacuoles with other abnormalities. The outcome of embryo development in a group of 25 patients after sibling oocyte injection with the four different grades of spermatozoa showed no significant difference in embryo quality up to day 3. However, the occurrence of blastocyst formation was 56.3 and 61.4% with grade I and II spermatozoa respectively, compared with 5.1% with grade III and 0% with grade IV respectively (P0.001). Spermatozoa selection at high magnification using Nomarski interference contrast is useful to identify more precisely the size and the number of nuclear vacuoles that greatly exert a negative effect on embryo development to the blastocyst stage. These observations confirm previous studies pointing to possible 'early and late paternal effects', both of which may have an impact on early embryonic development.