With some of the tissue, they are using conventional cryopreservation. Chemicals to protect cells from the hazards of freezing are added before taking tissue from the refrigerator temperature of 4 degrees Celsius to minus 80 degrees Celsius over two- and one-half hours. Later, liquid nitrogen takes it to minus 196 degrees
“You put it in a control-rate freezer that takes down the temperature one degree centigrade per minute so it drops the temperature very, very slowly,” says Dr. Song.
Slow cooling works well for simple tissue, such as sperm or even embryos, and for blood. “In blood, for example, conventional cryopreservation freezes the liquid part but not the cells inside. Liquid freezes and the water inside the cells moves out gradually so they dehydrate,” Dr. Song explains.
But, for more complex structures, such as a human egg or ovarian tissue, resulting ice formation can be destructive. “Ice crystals break up your inside organelles. That is what hurts eggs, which are very delicate,” he says.
“When you trigger ovulation with a hormone or naturally, you get the last separation of the chromosomes, from 46 to 23,” says Dr. Emmi. That separation enables a future baby to get half his chromosomes from mom and half from dad. Fragile spindles, which line up chromosomes for division, are easily broken during freezing so chromosomes can’t properly divide. Typically the resulting embryo dies. Plus, fertilization is unlikely since freezing often hardens the egg’s outer shell that sperm must penetrate.
“That is why we have tried to develop technology without freezing,” says Dr. Song, who has pioneered use of vitrification in blood vessels, cartilage and heart valves.
Vitrification, which takes tissue from room temperature to minus-100 degrees Celsius in 20 minutes, solidifies tissue into a clear, glass-like structure minus the opacity of ice cubes and frozen meats, a tell-tale sign o
Source:Medical College of Georgia