**9. The composition of freezing extenders**

A number of substances have been added to boar semen during cryopreservation in order to improve FT sperm quality. It has been investigated that egg yolk added to boar semen could protect sperm acrosomes during cold shock and hence reduce cryodamage of FT boar sperm [61]. Protection has been claimed to be due to both phospholipids and the low density lipo‐ protein fraction in egg yolk [62,63]. The mechanism of action is unclear but could be mediat‐ ed by either a less intense cellular dehydration or by stabilization of the sperm plasma membrane [51].

Cryoprotective agents (CPAs) have been divided into those that penetrate the cell and those which remain extracellular. Glycerol considered as penetrating agents and other non-pene‐ trating agents such as various sugars have been evaluated for cryoprotective effect in boar sperm [64,65]. Glycerol in low concentrations (3 to 4%) has been utilized in various techni‐ ques of sperm cryopreservation [47,66]. At these concentrations, glycerol gives maximum post-thaw viability and also in vitro fertilizing capacity of sperm [43]. Both post-thaw motili‐ ty and acrosome integrity of boar sperm would be decreased when glycerol concentration reached 5%. Glycerol and other penetrating agents could improve FT sperm survival by penetrating sperm and reduce the shrinkage of the cells developed during cooling [8]. They could also lower the freezing point of extra-cellular fluid via action of non-penetrating CPAs [67]. Therefore, the damage of sperm from the formation of intracellular ice occurred during freezing is reduced.

The success of the boar sperm cryopreservation was dramatically increased when the deter‐ gent Sodium Dodecyl Sulphate (SDS; later known as Equex STM paste) was included in the cryopreservation protocol [68,69]. The addition of SDS to semen extenders decreases freezethaw damage to sperm in several species, including boar [70-72]. Pursel and co-workers stat‐ ed that the use of 0.5% Orvus Es Paste, a commercial preparation of SDS, in the BF5 extender significantly enhanced the preservation of fertilizing capacity concomitant with an increase in post-thaw percentages of normal acrosome morphology and motility of boar sperm [69]. The beneficial effect of SDS on the sperm membrane is not fully understood, but it has been suggested that its protective effect is mediated through a change in the extending medium, by solubilization of the protective lipids in the egg yolk contained in the extenders. This effect enhanced the cold shock resistance of sperm [73,74]
