Review articleCryopreservation of animal oocytes and embryos: Current progress and future prospects
Introduction
Over the last few decades, cryopreservation techniques have progressed rapidly. This progress has made a significant impact in many fields, with reproductive medicine possibly the most significant. From initial success in cryopreservation of sperm [1], it is now routinely used for the preservation of oocytes, sperm, and embryos within both agricultural systems and in assisted reproductive technology (ART) in humans. Cryopreservation is a process by which biological cells or tissues are preserved at subzero temperatures resulting in a radical decrease in the rate of metabolic processes and the ability to store samples for extended periods [2]. However, as would be expected, freezing cells causes damage and this must be circumvented. The two major causes of cellular damage are the physical damage caused by the formation of ice crystals and the chemical damage that results from changes in intracellular solute concentrations. Both of these damage types can be avoided, or at least ameliorated, by controlling how the temperature is reduced and by modifying the cellular conditions. For instance, the mechanical damage that results from the piercing action of ice crystals can be avoided by making the freezing process very rapid and the significant rise in intracellular solute concentration as the formation of ice crystals increases can be avoided by use of cryoprotectants [3]. Permeating cryoprotectants replace intracellular liquid and decrease ice formation [4]; as such they need to have low toxicity, be capable of penetrating cells, and be able to withstand extremely low temperatures. Examples of commonly used cryoprotectants include glycerol, ethanediol, dimethyl sulfoxide, ethylene glycol, and propanediol [5]. Whilst most cells cannot survive the freezing process without use of a cryoprotectant, it is also important to note that simply using such solutions alone is insufficient for cell survival after freezing (and thawing); survival also depends on the cell type and its ability to withstand various stresses caused by physical and physiochemical changes during the process, as well as rates of cooling and warming [5].
Section snippets
The promise of vitrification
Vitrification eradicates damage caused due to ice crystal formation during the cooling process. The method involves rapid cooling and liquid solidification due to a substantial rise in viscosity and results in the formation of a solid glass-like form [6]. This solid ‘glassy’ layer is amorphous; meaning that it can readjust and take the shape of the cell, hence enabling the cell to maintain its structure and remain intact, unlike in slow-freezing, where the formation of ice crystals during
Open and closed vitrification systems: A comparison
In an open system, the oocytes or embryos come into direct contact with liquid nitrogen, whereas in a closed system, they do not. Direct comparisons between these types of systems have been limited; however, the available evidence suggests that the viability of oocytes and embryos after warming can be similar. For instance, Papatheodorou et al. [20] compared open and closed systems by conducting a randomized trial using human sibling oocytes. Whilst survival rates following vitrification using
The mouse as a model
In some cases, the vitrification of nonhuman oocytes and embryos can be particularly challenging, such as is the case in the pig model; these challenges are discussed later, in addition to a discussion pertaining to oocyte and embryo vitrification of other important agricultural species. The mouse (Mus musculus) is a particularly powerful model for studying mammalian embryo development due to broad morphological similarities [29]. The extensive genome similarities between mouse and human
Embryology and vitrification progress in agricultural animals
Human ART procedures are now used clinically worldwide, and such methods give families the chance to have healthy offspring, which in many cases would not have been possible before such advances. The ‘one child at a time’ report, published by the Human Fertilisation and Embryology Authority in 2006 [43], aimed to reduce the incidence of multiple births following ART; however, the challenges associated with agricultural animal IVF are somewhat different; procedures developed for humans need to
Embryology and vitrification progress in aquaculture
Many of the same challenges around the need to increase levels of production and to improve stock that are found in agriculture also exist in both fish and invertebrate aquaculture. For many aquaculture systems, there is also the additional problem that production depends on the harvest of broodstock or seed from wild populations [77]. Cryopreservation of sperm is relatively common within aquaculture, but widespread implementation of cryopreservation within the industry has however only
Future prospects
Whilst there has been considerable success with vitrification of oocytes, embryos, and even ovaries or ovarian tissue (particularly in humans and the mouse model), advantages of using closed systems instead of open systems, is a topic that is still widely debated in the literature; additionally, more research is required to produce more data, and importantly, more reliable results especially in agricultural animals. The development of automated devices for vitrification is potentially a huge
Acknowledgments
The authors thank Canterbury Christ Church University for supporting Mandawala AA, Harvey SC, and Fowler KE.
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2022, CryobiologyCitation Excerpt :Furthermore, cryopreserved gametes can be used for conservation purposes and genetic or even reproductive material from postmortem animals may still be viable [3]. Cryopreservation of oocytes and embryos has enabled herd management in agriculture systems [22,32]and assisted reproduction treatments in humans [14,35]. The current method for cryopreserving oocytes and embryos requires them to be equilibrated with high concentrations of cryoprotective agents (CPAs) [14,39,46].
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2021, TheriogenologyCitation Excerpt :Storing oocytes with AFP in cold-liquid was shown to be effective for all species tested [57,64,68,69], but a high-dose cytotoxicity should be taken into account, especially in human oocytes [68]. Embryo cryopreservation has been widely used in the fishery and livestock industry, particularly in domestic ruminants, and has helped to accelerate genetic improvement, or to alleviate infertility [2,92,93]. More recently, the interest in embryo cryopreservation has escalated in human medicine, particularly in the context of the increasing practice of single embryo transfer and pre-implantation genetic testing [94,95].
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