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Protein sources used as supplements of IVF culture media are known to have several implications for the function and stability of embryo culture environment. In fact, they i) transport biologically active molecules ii) chelate heavy metals, iii) regulate media pH, iii) scavenge reactive oxygen species (ROS) and iv) attenuate osmotic stress to which cells are exposed in sub-optimal culture conditions. Instead, their specific relevance to the formulation of cryopreservation solutions used for gamete and embryo cryopreservation remains uncertain. In the present work, we tested the hypothesis that different protein supplements present in cryopreservation solutions, serum or plasma protein solution (PPS), or different concentrations of the same supplement (serum), are associated with different types and/or magnitude of cryopreservation-derived cell damage. To this end, using cryopreservation solutions containing serum or PPS, donated supernumerary human mature oocytes were frozen-thawed by slow freezing and compared with fresh controls. Ultrastructural markers of oocyte quality were adopted as objective measure to assess possible damage from cryopreservation. The study results indicate that the adoption of serum minimises cell damage induced by cryopreservation. Indeed, typical hallmarks of cryodamage in human oocytes, i.e. loss of cortical granules, zona pellucida hardening and above all vacuolization, were largely reduced in oocytes cryopreserved with solutions containing serum, especially if used a higher concentration. This suggest that oocyte cryopreservation still has significant margins of improvement that may derive also from composition of cryopreservation media., (Copyright © 2020. Published by Elsevier Inc.)

The term "cryopreservation" refers to the process of cooling cells and tissues and storing them at subzero temperatures in order to stop all biological activity and preserve their viability and physiological competences for future use. Cooling to subzero temperatures is not a physiological condition for human cells; this is probably due to the high content of water in the living matter, whose conversion to ice crystals may be associated with severe and irreversible damage. Among reproductive cells and tissues, metaphase II oocytes are notably vulnerable to cryopreservation, mainly because of their large size, low surface area to volume ratio, relatively high water content and presence of the meiotic spindle. As human biological systems lack efficient internal defense mechanisms against chilling injuries, it is of the utmost importance to supply adequate external support, in terms of cryoprotectant additives, appropriate cooling/warming rates, and suitable long-term storage. Over the years, scientists have proposed different cryopreservation strategies in the effort to achieve an optimized recipe ensuring cell survival and, at the same time, maintenance of the physiological functions and abilities necessary to continue life. However, despite the first success obtained in the 1980s with frozen oocytes, it was not until recently that notable improvements in the cryopreservation technique, thanks to the advent of vitrification, allowed a breakthrough of this fine procedure., (© 2019 Nordic Federation of Societies of Obstetrics and Gynecology.)

Cryopreserved gametes and embryos are a major feature of human-assisted reproduction and patient care services, accounting for an increasing number of births worldwide. Since the first success obtained using frozen human spermatozoa, cryopreservation technology has been successfully extended to include oocytes and embryos, in a variety of both medical and nonmedical indications. Over the years, the available procedures have become widely implemented and the increasing evidence of its efficacy has contributed to acceptance of the technology. Nevertheless, a gold standard protocol that would be universally shared by clinics has yet to be definitively established and, therefore, research into cryopreservation of gametes and embryos cannot be considered concluded. Moreover, much effort should be committed to the definition and resolution of safety issues, the establishment of automation, and investigations about the potentiality of immature germ cells or stem cells., Competing Interests: Disclosure The authors report no conflicts of interest in this work., (Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.)

Cryopreservation of the human embryo has been successfully achieved at the zygote (day 1), cleavage (day 2/3), and blastocyst (day 5) stages; however, each stage presents specific advantages and disadvantages. During the past decades, two major methods have been applied: slow freezing (equilibrium procedure) and vitrification (nonequilibrium procedure). The overwhelming majority of published data prove that the latest vitrification methods induce less cellular trauma and are a more effective cryopreservation technique of human embryos than any other versions of slow freezing. For this reason, fragmented and slow-cleaving embryos that normally would not be recommended may be revaluated for cryopreservation by using the vitrification method. Furthermore, if laser-assisted necrotic blastomere removal is associated with the slow-freezing/thawing procedure, good clinical results can be obtained. Finally, the most proper embryo cleavage stage at which to perform cryopreservation has to be assessed according to clinical indications and laboratory experience., (© 2011 New York Academy of Sciences.)