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How are embryos frozen?

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Embryo freezing has shown to be an effective technique for allowing fertility patients to store excess embryos for later use. The technology involved in freezing embryos is quite fascinating and merits discussion on this blog.

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.

Step 1. 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 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.

Step 2. When most of the water has been removed the embryo is inserted into a carefully labeled vial, or more typically a small straw, and placed in the cooling chamber of a controlled rate freezer.

Step 3. The embryo is then cooled very slowly at -0.30C per minute. Slow cooling like this allows the embryologist to have precise control over the freezing process, to maximize water extraction from the embryo and to prevent formation of large ice shards that could pierce the embryo.

Step 4. The cooled straw is placed into carefully labeled metal canes and lowered into the tank with other frozen embryos. The entire process takes several hours and the embryo(s) are stored frozen at –1960C in liquid nitrogen. Liquid nitrogen is a safe and effective coolant, which is easy to work with in the laboratory.
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About the Author

Dr. Herbert is a fertility expert and an innovator in the field.

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