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There are many dangers associated with freeze-tolerance and numerous species of insects are unable to survive this process.
- One such danger is the formation of intracellular ice, which is lethal in almost all cases. Intracellular ice disrupts cell membranes and damages subcellular architecture. There are some species of insects that are able to survive the formation of ice in fat body cells, which are involved in lipid, protein, and carbohydrate metabolism. But there is no evidence for why this occurs in certain insects (Storey,1996).
The formation of extracellular ice causes problems as well (Duman, 1991). First off it leads to mechanical stress. Cells trapped between ice fronts may be compressed and the ice matrix obstructs intercellular communication, which is important in the recovery process. Furthermore, the expansion of water when it crystallizes can rupture small vessels, which could lead to problems in tissue reperfusion after thawing (Constanzo,1994).
Osmotic stress is one of the most dangerous threats that freeze-tolerant animals must be able to overcome. During the freezing process an osmotic gradient is established between intercellular and extracellular environments. This occurs because only water molecules are included in the developing crystal lattice therefore all solutes are left behind. A high concentration of solutes in the extracellular space causes an influx of water molecules out of the cell, resulting in severe dehydration and shrinkage of cells. Rapid rates of ice formation are especially injurious because there is not sufficient time for the body to make metabolic adjustments and prevent damage to the cells (Storey, 1996).
Other injuries can result from ischemia, since freezing halts the circulation of hemolymph. Ischemia is a restriction in blood supply that often results in the damage or dysfunction of tissues. The fact that vital functions are stopped during freezing also proposes several risks to the recovery process.
Those species that are able to survive freezing have evolved several adaptations to where, when, and to what extent ice forms (Storey, 1996). |
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