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Cryopreservation is a process where cells, whole tissues, or any other substances susceptible to damage caused by chemical reactivity or time are preserved by cooling to sub-zero temperatures. At low enough temperatures, any enzymatic or chemical activity which might cause damage to the material in question is effectively stopped. Cryopreservation methods seek to reach low temperatures without causing additional damage caused by the formation of ice during freezing. Traditional cryopreservation has relied on coating the material to be frozen with a class of molecules termed cryoprotectants. New methods are constantly being investigated due to the inherent toxicity of many cryoprotectants.
Water-bears (Tardigrada), microscopic multicellular organisms, can survive freezing by replacing most of their internal water with the sugar trehalose, preventing it from crystallization that otherwise damages cell membranes. Mixtures of solutes can achieve similar effects. Some solutes, including salts, have the disadvantage that they may be toxic at intense concentrations. In addition to the water-bear, wood frogs can tolerate the freezing of their blood and other tissues. Urea is accumulated in tissues in preparation for overwintering, and liver glycogen is converted in large quantities to glucose in response to internal ice formation. Both urea and glucose act as "cryoprotectants" to limit the amount of ice that forms and to reduce osmotic shrinkage of cells. Frogs can survive many freeze/thaw events during winter if not more than about 65% of the total body water freezes. Research exploring the phenomenon of "Freezing frogs" has been performed primarily by the Canadian researcher, Dr. Kenneth B. Storey.
Freeze tolerance, in which organisms survive the winter by freezing solid and ceasing life functions, is known in a few vertebrates: five species of frogs (Rana sylvatica, Pseudacris triseriata, Hyla crucifer, Hyla versicolor, Hyla chrysoscelis), one of salamanders (Hynobius keyserlingi), one of snakes (Thamnophis sirtalis) and three of turtles (Chrysemys picta, Terrapene carolina, Terrapene ornata). Snapping turtles Chelydra serpentina and wall lizards Podarcis muralis also survive nominal freezing but it has not been established to be adaptive for overwintering. In the case of Rana sylvatica one cryopreservant is ordinary glucose, which increases in concentration by approximately 19 mmol/l when the frogs are cooled slowly;
One of the most important early theoreticians of cryopreservation was James Lovelock (born 1919) of Gaia theory fame. He suggested that damage to red blood cells during freezing was due to osmotic stress. Lovelock during the early 1950s had also suggested that increasing salt concentrations in a cell as it dehydrates to lose water to the external ice might cause damage to the cell. Cryopreservation of tissue during recent times began with the freezing of fowl sperm, which during 1957 was cryopreserved by a team of scientists in the UK directed by Christopher Polge. The process was applied to humans during the 1950s with pregnancies obtained after insemination of frozen sperm. However, the rapid immersion of the samples in liquid nitrogen did not, for certain of these samples–- such as types of embryos, bone marrow and stem cells–- produce the necessary viability to make them usable after thawing. Increased understanding of the mechanism of freezing injury to cells emphasised the importance of controlled or slow cooling to obtain maximum survival on thawing of the living cells. A controlled rate cooling process, allowing biological samples to equilibrate to optimal physical parameters osmotically in a cryoprotectant (a form of anti-freeze) before cooling in a predetermined, controlled way proved necessary. The ability of cryoprotectants, in the early cases glycerol, to protect cells from freezing injury was discovered accidentally. Freezing injury has two aspects–- direct damage from the ice crystals and secondary damage caused by the increase in concentration of solutes as progressively more ice is formed. During 1963 Peter Mazur, at Oak Ridge National Laboratory in the USA, demonstrated that lethal intracellular freezing could be avoided if cooling was slow enough to permit sufficient water to leave the cell during progressive freezing of the extracellular fluid. That rate differs between cells of differing size and water permeability: a typical cooling rate around 1°C/minute is appropriate for many mammalian cells after treatment with cryoprotectants such as glycerol or dimethyl sulphoxide, but the rate is not a universal optimum.
Cryogenic storage at very cold temperatures is presumed to provide an indefinite longevity to cells, although the actual effective life is rather difficult to prove. Researchers experimenting with dried seeds found that there was noticeable variability of deterioration when samples were kept at different temperatures–- even ultra cold temperatures. Temperatures less than the glass transition point (Tg) of polyol's water solutions, around −136 °C (137 K; −213 °F), seem to be accepted as the range where biological activity very substantially slows, and −196 °C (77 K; −321 °F), the boiling point of liquid nitrogen, is the preferred temperature for storing important specimens. While refrigerators, freezers and extra cold freezers are used for many items, generally the ultra cold of liquid nitrogen at −196 °C (77 K; −321 °F) is required for successful preservation of the more complex biological structures to virtually stop all biological activity.
Phenomena which can cause damage to cells during cryopreservation mainly occur during the freezing stage, and include: solution effects, extracellular ice formation, dehydration and intracellular ice formation. Many of these effects can be reduced by cryoprotectants. Once the preserved material has become frozen, it is relatively safe from further damage. However, estimates based on the accumulation of radiation-induced DNA damage during cryogenic storage have suggested a maximum storage period of 1000 years.
- Solution effects
- As ice crystals grow in freezing water, solutes are excluded, causing them to become concentrated in the remaining liquid water. High concentrations of some solutes can be very damaging.
- Extracellular ice formation
- When tissues are cooled slowly, water migrates out of cells and ice forms in the extracellular space. Too much extracellular ice can cause mechanical damage to the cell membrane due to crushing.
- Migration of water, causing extracellular ice formation, can also cause cellular dehydration. The associated stresses on the cell can cause damage directly.
- Intracellular ice formation
- While some organisms and tissues can tolerate some extracellular ice, any appreciable intracellular ice is almost always fatal to cells.
Main methods to prevent risks
The main techniques to prevent cryopreservation damages are a well established combination of controlled rate and slow freezing and a newer flash-freezing process known as vitrification.
Slow programmable freezing
Controlled-rate and slow freezing, also known as slow programmable freezing (SPF), is a set of well established techniques developed during the early 1970s which enabled the first human embryo frozen birth Zoe Leyland during 1984. Since then, machines that freeze biological samples using programmable sequences, or controlled rates, have been used all over the world for human, animal and cell biology – 'freezing down' a sample to better preserve it for eventual thawing, before it is frozen, or cryopreserved, in liquid nitrogen. Such machines are used for freezing oocytes, skin, blood products, embryo, sperm, stem cells and general tissue preservation in hospitals, veterinary practices and research laboratories around the world. As an example, the number of live births from frozen embryos 'slow frozen' is estimated at some 300,000 to 400,000 or 20% of the estimated 3 million in vitro fertilisation (IVF) births.
Lethal intracellular freezing can be avoided if cooling is slow enough to permit sufficient water to leave the cell during progressive freezing of the extracellular fluid. That rate differs between cells of differing size and water permeability: a typical cooling rate of about 1°C/minute is appropriate for many mammalian cells after treatment with cryoprotectants such as glycerol or dimethyl sulphoxide, but the rate is not a universal optimum. The 1°C / minute rate can be achieved by using devices such as a rate-controlled freezer or a benchtop portable freezing container such as CoolCell®.
Several independent studies have provided evidence that frozen embryos stored using slow-freezing techniques may in some ways be 'better' than fresh in IVF. The studies were presented at the American Society for Reproductive Medicine conference in San Francisco, USA, 2008. The studies indicate that using frozen embryos rather than fresh embryos reduced the risk of stillbirth and premature delivery though the exact reasons are still being explored.
Researchers Greg Fahy and William F. Rall helped introduce vitrification to reproductive cryopreservation in the mid-1980s. As of 2000, researchers claim vitrification provides the benefits of cryopreservation without damage due to ice crystal formation. For clinical cryopreservation, vitrification usually requires the addition of cryoprotectants prior to cooling. The cryoprotectants act like antifreeze: they decrease the freezing temperature. They also increase the viscosity. Instead of crystallizing, the syrupy solution becomes an amorphous ice—- i.e., it vitrifies. Rather than a phase change from liquid to solid by crystallization, the amorphous state is like a "solid liquid", and the transformation is over a small temperature range described as the "glass transition" temperature.
Vitrification of water is promoted by rapid cooling, and can be achieved without cryoprotectants by an extremely rapid decrease of temperature (megakelvins per second). The rate that is required to attain glassy state in pure water was considered to be impossible until 2005.
Two conditions usually required to allow vitrification are an increase of the viscosity and a decrease of the freezing temperature. Many solutes do both, but larger molecules generally have larger effect, particularly on viscosity. Rapid cooling also promotes vitrification.
For established methods of cryopreservation, the solute must penetrate the cell membrane in order to achieve increased viscosity and decrease freezing temperature inside the cell. Sugars do not readily permeate through the membrane. Those solutes that do, such as dimethyl sulfoxide, a common cryoprotectant, are often toxic in intense concentration. One of the difficult compromises of vitrifying cryopreservation concerns limiting the damage produced by the cryoprotectant itself due to cryoprotectant toxicity. Mixtures of cryoprotectants and the use of ice blockers have enabled the Twenty-First Century Medicine company to vitrify a rabbit kidney to -135°C with their proprietary vitrification mixture. Upon rewarming, the kidney was transplanted successfully into a rabbit, with complete functionality and viability, able to sustain the rabbit indefinitely as the sole functioning kidney.
Cells Alive System freezers
Cells Alive System freezers are able to vitrify tissue without the use of (usually very toxic) cryoprotectant chemicals. They supercool the tissue while applying magnetic and electronic fields along with sympathetic resonance vibrations to keep water from forming ice crystals. The Cells Alive method was originally designed for food preservation. Periodontal ligament cells in frozen tooth banks showed survival improvements with this method.
Generally, cryopreservation is easier for thin samples and small clumps of individual cells, because these can be cooled more quickly and so require lesser doses of toxic cryoprotectants. Therefore, cryopreservation of human livers and hearts for storage and transplant is still impractical.
Nevertheless, suitable combinations of cryoprotectants and regimes of cooling and rinsing during warming often allow the successful cryopreservation of biological materials, particularly cell suspensions or thin tissue samples. Examples include:
- Semen in semen cryopreservation
- Tissue samples like tumors and histological cross sections
- Eggs (oocytes) in oocyte cryopreservation
- Embryos that are 2, 4 or 8 cells when frozen, in embryo cryopreservation
- Ovarian tissue in ovarian tissue cryopreservation
- Plant seeds or shoots may be cryopreserved for conservation purposes.
Additionally, efforts are underway to preserve humans cryogenically, known as cryonics. For such efforts either the brain within the head or the entire body may experience the above process. Cryonics is in a different category from the aforementioned examples, however: while countless cryopreserved cells, vaccines, tissue and other biologial samples have been thawed and used successfully, this has not yet been the case at all for cryopreserved brains or bodies. At issue are the criteria for defining "success". Proponents of cryonics claim that cryopreservation using present technology, particularly vitrification of the brain, may be sufficient to preserve people in an "information theoretic" sense so that they could be revived and made whole by hypothetical vastly advanced future technology.
Cryopreservation for embryos are used for embryo storage, e.g. when in vitro fertilization has resulted in more embryos than is currently needed.
Pregnancies have been reported from embryos stored for 16 years. Many studies have evaluated the children born from frozen embryos, or “frosties”. The result has been uniformly positive with no increase of birth defects or development abnormalities. A study of more than 11,000 cryopreserved human embryos had no significant effect of storage time on post-thaw survival for in vitro fertilisation (IVF) or oocyte donation cycles, or for embryos frozen at the pronuclear or cleavage stages. Additionally, the duration of storage did not have any significant effect on clinical pregnancy, miscarriage, implantation, or live birth rate, whether from IVF or oocyte donation cycles. Rather, oocyte age, survival proportion, and number of transferred embryos are predictors of pregnancy outcome.
Cryopreservation of ovarian tissue is of interest to women who want to preserve their reproductive function beyond the natural limit, or whose reproductive potential is threatened by cancer therapy, for example in hematologic malignancies or breast cancer. The procedure is to take a part of the ovary and perform slow freezing before storing it in liquid nitrogen whilst therapy is undertaken. Tissue can then be thawed and implanted near the fallopian, either orthotopic (on the natural location) or heterotopic (on the abdominal wall), where it starts to produce new eggs, allowing normal conception to occur. The ovarian tissue may also be transplanted into mice that are immunocompromised (SCID mice) to avoid graft rejection, and tissue can be harvested later when mature follicles have developed.
Human Oocyte cryopreservation is a new technology in which a woman’s eggs (oocytes) are extracted, frozen and stored. Later, when she is ready to become pregnant, the eggs can be thawed, fertilized, and transferred to the uterus as embryos.
Semen can be used successfully almost indefinitely after cryopreservation. The longest reported successful storage is 22 years. It can be used for sperm donation where the recipient wants the treatment in a different time or place, or for men undergoing a vasectomy to have the option to have children.
Cryopreservation of immature testicular tissue is a developing method to avail reproduction to young boys who need to have gonadotoxic therapy. Animal data are promising, since healthy offsprings have been obtained after transplantation of frozen testicular cell suspensions or tissue pieces. However, none of the fertility restoration options from frozen tissue, i.e. cell suspension transplantation, tissue grafting and in vitro maturation (IVM) has proved efficient and safe in humans as yet.
Cryopreservation of whole moss plants, especially Physcomitrella patens, has been developed by Ralf Reski and coworkers and is performed at the International Moss Stock Center. This biobank collects, preserves, and distributes moss mutants and moss ecotypes.
- Cells Alive System freezers
- Chemical brain preservation
- Cryostasis (clathrate hydrates)
- Ex-situ conservation
- Frozen zoo
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