Dextran is a complex polysaccharide that functions as a non-penetrating (extracellular) cryoprotectant (CPA). It is widely used in cryopreservation protocols, often in combination with a penetrating CPA like dimethyl sulfoxide (DMSO), to protect cells and tissues from freezing damage.
Mechanism of Cryoprotection
As a large, hydrophilic polymer, dextran remains in the extracellular space and protects biological material through several interrelated physical mechanisms.
- Vitrification and Glass Formation: Dextran promotes the formation of an amorphous, glassy state rather than crystalline ice during freezing. It has an optimal glass transition temperature, and its presence helps suppress the formation of damaging ice crystals and eutectic salt mixtures.
- Ice Recrystallization Inhibition: During the warming phase, small ice crystals can merge into larger, more damaging ones. Dextran helps inhibit this recrystallization process, reducing mechanical damage to cells.
- Membrane and Cellular Protection: By forming a viscous, protective matrix around cells, dextran can reduce the mechanical action of ice on cell membranes. Its presence also helps to stabilize the osmotic environment, reducing hypertonic stress during freezing when combined with a penetrating CPA.
- Synergistic Effect with Penetrating CPAs: Dextran is often combined with DMSO. This combination allows for the use of lower, less toxic concentrations of DMSO while maintaining or even improving cryoprotective efficacy.
