Cryopreservation

A tank of liquid nitrogen, used to supply a cryogenic freezer (for storing laboratory samples at a temperature of about −150 °C)

Controlled-rate and slow freezing, also known as slow programmable freezing (SPF),[15] 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.[16]

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. To minimize the growth of extracellular ice crystal growth and recrystallization,[17] biomaterials such as alginates, polyvinyl alcohol or chitosan can be used to impede ice crystal growth along with traditional small molecule cryoprotectants.[2] 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 sulfoxide, 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.[18]

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 indicate that using frozen embryos and eggs rather than fresh embryos and eggs reduced the risk of stillbirth and premature delivery though the exact reasons are still being explored.

Researchers Greg Fahy and William F. Rall helped to introduce vitrification to reproductive cryopreservation in the mid-1980s.[19] As of 2000, researchers claim vitrification provides the benefits of cryopreservation without damage due to ice crystal formation.[20] The situation became more complex with the development of tissue engineering as both cells and biomaterials need to remain ice-free to preserve high cell viability and functions, integrity of constructs and structure of biomaterials. Vitrification of tissue engineered constructs was first reported by Lilia Kuleshova,[21] who also was the first scientist to achieve vitrification of oocytes, which resulted in live birth in 1999.[22] For clinical cryopreservation, vitrification usually requires the addition of cryoprotectants prior to cooling. Cryoprotectants are macromolecules added to the freezing medium to protect cells from the detrimental effects of intracellular ice crystal formation or from the solution effects, during the process of freezing and thawing. They permit a higher degree of cell survival during freezing, to lower the freezing point, to protect cell membrane from freeze-related injury. Cryoprotectants have high solubility, low toxicity at high concentrations, low molecular weight and the ability to interact with water via hydrogen bonding.

Instead of crystallizing, the syrupy solution becomes an amorphous ice—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.[23]

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 a 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.[24]

Blood can be replaced with inert noble gases and/or metabolically vital gases like oxygen, so that organs can cool more quickly and less antifreeze is needed. Since regions of tissue are separated by gas, small expansions do not accumulate, thereby protecting against shattering.[25] A small company, Arigos Biomedical, "has already recovered pig hearts from the 120 degrees below zero",[26] although the definition of "recovered" is not clear. Pressures of 60 atm can help increase heat exchange rates.[27] Gaseous oxygen perfusion/persufflation can enhance organ preservation relative to static cold storage or hypothermic machine perfusion, since the lower viscosity of gases, may help reach more regions of preserved organs and deliver more oxygen per gram tissue.[28]

Cryopreservation for embryos is used for embryo storage, e.g., when in vitro fertilization (IVF) has resulted in more embryos than is currently needed.

Pregnancies have been reported from embryos stored for 16 years.[31] Many studies have evaluated the children born from frozen embryos, or “frosties”. The result has been uniformly positive with no increase in birth defects or development abnormalities.[32] A study of more than 11,000 cryopreserved human embryos showed no significant effect of storage time on post-thaw survival for IVF or oocyte donation cycles, or for embryos frozen at the pronuclear or cleavage stages.[33] 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.[33] Rather, oocyte age, survival proportion, and number of transferred embryos are predictors of pregnancy outcome.[33]

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,[34] for example in hematologic malignancies or breast cancer.[35] 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),[35] where it starts to produce new eggs, allowing normal conception to occur.[36] 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.[37]

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. Since 1999, when the birth of the first baby from an embryo derived from vitrified-warmed woman's eggs was reported by Kuleshova and co-workers in the journal of Human Reproduction,[21] this concept has been recognized and widespread. This break-through in achieving vitrification of woman's oocytes made an important advance in our knowledge and practice of the IVF process, as clinical pregnancy rate is four times higher after oocyte vitrification than after slow freezing.[38] Oocyte vitrification is vital for preservation fertility in young oncology patients and for individuals undergoing IVF who object, either for religious or ethical reasons, to the practice of freezing embryos.

Semen can be used successfully almost indefinitely after cryopreservation. The longest reported successful storage is 22 years.[39] It can be used for sperm donation where the recipient wants the treatment in a different time or place, or as a means of preserving fertility for men undergoing vasectomy or treatments that may compromise their fertility, such as chemotherapy, radiation therapy or surgery.

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 offspring 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.[40]

Four different ecotypes of Physcomitrella patens stored at the IMSC.

Cryopreservation of whole moss plants, especially Physcomitrella patens, has been developed by Ralf Reski and coworkers[41] and is performed at the International Moss Stock Center. This biobank collects, preserves, and distributes moss mutants and moss ecotypes.[42]

MSCs, when transfused immediately within a few hours post-thawing, may show reduced function or show decreased efficacy in treating diseases as compared to those MSCs which are in log phase of cell growth (fresh). As a result, cryopreserved MSCs should be brought back into log phase of cell growth in in vitro culture before these are administered for clinical trials or experimental therapies. Re-culturing of MSCs will help in recovering from the shock the cells get during freezing and thawing. Various clinical trials on MSCs have failed which used cryopreserved products immediately post-thaw as compared to those clinical trials which used fresh MSCs.[43]

Fungi, notably zygomycetes, ascomycetes and higher basidiomycetes, regardless of sporulation, are able to be stored in liquid nitrogen or deep-frozen. Crypreservation is a hallmark method for fungi that do not sporulate (otherwise other preservation methods for spores can be used at lower costs and ease), sporulate but have delicate spores (large or freeze-dry sensitive), are pathogenic (dangerous to keep metabolically active fungus) or are to be used for genetic stocks (ideally to have identical composition as the original deposit). As with many other organisms, cryoprotectants like DMSO or glycerol (e.g. filamentous fungi 10% glycerol or yeast 20% glycerol) are used. Differences between choosing cryoprotectants are species (or class) dependent, but generally for fungi penetrating cryoprotectants like DMSO, glycerol or polyethylene glycol are most effective (other non-penetrating ones include sugars mannitol, sorbitol, dextran, etc.). Freeze-thaw repetition is not recommended as it can decrease viability. Back-up deep-freezers or liquid nitrogen storage sites are recommended. Multiple protocols for freezing are summarized below (each uses screw-cap polypropylene cryotubes):[45]

Many common culturable laboratory strains are deep-frozen to preserve genetically and phenotypically stable, long-term stocks. Sub-culturing and prolonged refrigerated samples may lead to loss of plasmid(s) or mutations. Common final glycerol percentages are 15, 20 and 25. From a fresh culture plate, one single colony of interest is chosen and liquid culture is made. From the liquid culture, the medium is directly mixed with equal amount of glycerol; the colony should be checked for any defects like mutations. All antibiotics should be washed from the culture before long-term storage. Methods vary, but mixing can be done gently by inversion or rapidly by vortex and cooling can vary by either placing the cryotube directly at −50 to −95 °C, shock-freezing in liquid nitrogen or gradually cooling and then storing at −80 °C or cooler (liquid nitrogen or liquid nitrogen vapor). Recovery of bacteria can also vary, namely if beads are stored within the tube then the few beads can be used to plate or the frozen stock can be scraped with a loop and then plated, however, since only little stock is needed the entire tube should never be completely thawed and repeated freeze-thaw should be avoided. 100% recovery is not feasible regardless of methodology.[46][47][48]

The microscopic soil-dwelling nematode roundworms Panagrolaimus detritophagus and Plectus parvus are the only eukaryotic organisms that have been proven to be viable after long-term cryopreservation to date. In this case, the preservation was natural rather than artificial, due to permafrost.