Cryopreservation

Cryopreservation, is a process where cells or whole tissues are preserved by cooling to low sub-zero temperatures, such as (typically) -80°C or -196°C (the boiling point of liquid nitrogen). At these low temperatures, any biological activity, including the biochemical reactions that would lead to cell death is effectively stopped. However, when vitrification solutions are not used, the cells being preserved are often damaged due to freezing during the approach to low temperatures or warming to room temperature.

Phenomena which can cause damage to cells during cryopreservation are solution effects, extracellular ice formation and intracellular ice formation. Solution effects are caused by concentration of solutes in non-frozen solution during freezing as solutes are excluded from the crystal structure of the ice. (High salt concentrations can be very damaging.) When tissues are cooled slowly, water migrates out of cells and ice forms in the extracellular space. Too much extracelluar ice can cause mechanical damage due to crushing, but organism and tissues can tolerate some extracellular ice. By contrast, any appreciable intracellular ice is fatal to cells.

Vitrification provides the benefits of cryopreservation without the damage due to ice formation. In vitrification, anti-freeze substances called cryoprotectants are used to prevent ice by replacing water with cryoprotectant prior to cooling. Instead of crystallizing, the syrupy solution hardens like glass. Vitrification of water can be achieved by an extremely rapid drop in temperature (millions of degrees per second).

One of the most important early workers on the theory of cryopreservation was James Lovelock of Gaia theory fame. Dr. Lovelock's work suggested that damage to red blood cells during freezing was due to osmotic stresses.

Water bears (or tardigrada), microscopic multicellular organisms, can survive freezing at low temperatures by replacing most of their internal water with the sugar trehalose. Trehalose is very protective of cell membranes.

Usually, this technique is used for biological materials. Examples include:

• Semen (which can be used successfully almost indefinitely after freezing),
• Blood (special cells for transfusion, or stem cells)
• Tissue samples like tumors and histological cross sections
• Human remains, either the entire body or just the head, which is known as cryonics

External links

• http://www.ivf.com/freezing.html
• http://www.societyforcryobiology.org/
• http://www.sltb.info/
• http://www.carleton.ca/~kbstorey
• Several stories about cryopreservation