How are embryos frozen?
Embryos can be frozen at different times after fertilization. Most typically, embryos are frozen 1, 3 or 5 days after the sperm and egg were put together. Freezing is a stressful process for an embryo, and only embryos that are growing well in the laboratory will tolerate the freezing procedure. Before an embryo can be frozen, all the water that it contains must be removed. Since water expands in size as it turns to ice, water inside the embryo would burst (kill) the embryo if we simply placed it in the freezer. To prevent the embryo from shriveling as the water is extracted, we replace the water with an antifreeze. Antifreeze is a solution that does not expand in size when it freezes. The embryo is cooled to room temperature as the water is replaced with antifreeze. When most of the water has been removed the embryo is inserted into a vial, or more typically a small straw, and placed in the cooling chamber of a controlled rate freezer. The embryo is then cooled very slowly at -0.30C p
