ABSTRACT
There is no general recipe for cryopreservation of all cell types, or even for the same cell type in different individuals1,15. Optimal protocols vary according to the water and solute permeability, surface to volume ratio, type of cryoprotectant, activation energy, cooling and freezing rate, and storage temperature. Spermatozoa are often regarded as ideal cells for cryopreservation because they are small, available in large numbers, readily obtainable, contain little cytoplasm, and have a lower relative and absolute water content than most other cells. The recovery of motility is a useful indicator of sperm viability (although not necessarily of fertility). The first prerequisite of cryopreservation is to reduce the intracellular water content to avoid the formation of intracellular ice crystals, which are normally lethal. Partial removal of water at lower temperatures may also help reduce thermally driven metabolic activities, leading to a stable state and the suspension of most cellular activities. This state is safeguarded by the addition of a cryoprotective agent (CPA), such as glycerol, usually in combination with a buffer -based medium containing phospholipids from egg yolk (extender). Once spermatozoa have been cryopreserved in this manner, they can be stored safely in liquid nitrogen dewars for many years. The only reactions that may proceed are the effects of free radicals and associated low-level background radiation, which would take many decades to be regarded as a significant hazard16. Fortunately too, some CPAs actually have free radical scavenging properties.
