Cryopreservation of embryos by vitrification: current development

Vitrification as a cryopreservation method has many primary advantages and benefits, such as no ice crystal formation through increased speed of temperature conduction, which provides a significant increase in cooling rates. This permits the use of less concentrated cryoprotectant agents so that the toxic effect is decreased. Additionally, chilling injuries are considerably reduced. Many variables in the vitrification process exist that can profoundly influence its effectiveness and the potential to improve the survival rates of vitrified cells. These include (i) the type and concentration of cryoprotectant (almost every kind of cryoprotectant is toxic), (ii) the temperature of the vitrification solution at exposure, (iii) the duration of exposure to the final cryoprotectant before plunging into LN2, (iv) the type of device that is used for vitrification (which influences the size of the vapor coat and cooling rate), and (v) the quality and developmental stage of embryos. Increasing the speed of thermal conduction and decreasing the concentration of cryoprotectant is an ideal strategy for cryostorage of embryos with vitrification methods. However, the actual rate of heat transfer during vitrification procedures may vary extremely depending on the device used, technical proficiency, and the specific movement at immersion. In addition, it is very important to mention that every cell has its own optimal cooling rate. To date, the "universal" vitrification protocol has yet to be defined. In light of this, it is important for researchers to achieve more consistent results from existing protocols and thereby to establish a standardized vitrification protocol that can be applied for cryopreservation of different developmental stages. Toward this end, it should be noted that vitrification protocols are starting to enter the mainstream of human ART. Protocols successfully applied for bovine oocytes and embryos have been used initially with human oocytes, and initial trials have been undertaken with human embryos and blastocysts, with births achieved. Vitrification is relatively simple, requires no expensive programmable freezing equipment, and relies on the placement of the embryos in a very small volume of vitrification medium that must be cooled at extreme rates not obtainable in regular enclosed cryostraws and cryovials. The more convenient protocols of ultrarapid freezing and vitrification, which eliminate the use of expensive controlled-rate freezers, await cross-over from use in other species, and they require validation from more extensive experimental study in humans. Despite this, the convenience of vitrification will push the development of this technique to higher levels of clinical efficiency and use.