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Sperm Storage: Have You Wondered How Sperm Survive the Thaw?
a blog by Michelle Ottey, PhD, Director of Operations, Fairfax Cryobank and Cryogenic Laboratories, Inc., May 3, 2011
Sperm is said to be able to be stored indefinitely. There have been successful pregnancies using sperm that has been store for almost 30 years.
How is this possible? When sperm is stored in Liquid Nitrogen (LN2), which has a temperature of -196° C, the cells are cryopreserved. Being cryopreserved means that there is no metabolic activity — the cells aren’t aging or degrading over time. The process of freezing and thawing the sperm cells will cause damage to the cells, which renders them unviable after the thaw.
When sperm cells are frozen, the crystallized water has the potential to damage the cell membrane, which is why a cryopreservative such as glycerol is used. The glycerol replaces the water in the cell and prevents this damage. Not all cells take in the glycerol uniformly and so not all cells will survive the freeze/thaw. When the cells are being thawed, the glycerol leaves the cell and is replaced by water, and many of the sperm will start swimming right along when they reach the optimal temperature.
The sperm freezing process may sound precarious, and it is. If sperm cells are maintained at temperatures lower than -135° C they will be stable. This requires careful monitoring of the temperature and liquid nitrogen levels in the tanks. Ideally tanks storing sperm should be monitored constantly, and there should always be an extra liquid nitrogen resource available to supplement the tanks if necessary.