Principles of Genetics and Breeding

9.1.2.1 Prevention of risks

• Controlled-rate and slow freezing are well established techniques pioneered in the early 1970s which enabled the first human embryo frozen birth in 1984. Since then machines that freeze biological samples using programmable steps, 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 deep frozen, or cryopreserved in liquid nitrogen.
• Programmable freezers are used for freezing oocyte, skin, blood products, embryo, sperm,stem cells and general tissue preservation in hospitals, veterinary practices and research labs around the world.
• Lethal intracellular freezing can be avoided if cooling is slow enough to permit sufficient water to leave the cell during progressive freezing of the extra cellular fluid. That rate differs between cells of differing size and water permeability .
• A typical cooling rate around1°C/min is appropriate for many after treatment with cryoprotectants such as glycerol or dimethyl sulphoxide(DMSO), but the rate is not a universal optimum.