Cryopreservation of animal and human embryos by vitrification

Vitrification is a method in which not only cells but also the whole solution is solidified without the crystallization of ice. For embryo cryopreservation, the vitrification method has advantages over the slow freezing method. For example, injuries related to ice is less likely to occur, embryo survival is more likely if the embryo treatment is optimized, and embryos can be cryopreserved by a simple method in a short period without a programmed freezer. However, solutions for vitrification must include a high concentration of permeating cryoprotectants, which may cause injury through the toxicity of the agents. Since the development of the first vitrification solution, which contained dimethylsulphoxide, acetamide, and propylene glycol, numerous solutions have been composed and reported to be effective. However, ethylene glycol is now most widely used as the permeating component. As supplements, a macromolecule and/or a small saccharide are frequently added. Embryos of various species, including humans, can be cryopreserved by conventional vitrification using insemination straws or by ultrarapid vitrification using minute tools such as electron microscopic grids, thin capillaries, minute loops, or minute sticks, or as microdrops. In the ultrarapid method, solutions with a lower concentration of permeating cryoprotectants, thus having a lower toxicity, can be used, because ultrarapid cooling/warming helps to prevent ice formation.