Detrimental effects of sodium during mouse oocyte cryopreservation

Cryopreservation is an established way of storing embryos, but effective methods are not available for freezing eggs. Most freezing damage is caused by high solute concentration (solution effects) and intracellular ice. Sodium salts are the major components of cryopreservation media, and the main contributor to the solution effects. The present experiments examine the effect of substituting choline for sodium as the major extracellular cation in the cryopreservation of mouse eggs. The effects of serum and various cryoprotectants were also examined. Survival, fertilization, and development were inversely related to the concentration of sodium in the freezing medium. Oocytes frozen in a choline-based medium had the highest (p < 0.001) survival and development rates. The absence of serum during thawing inhibited fertilization, whereas exposure to serum or opening the zona allowed fertilization to reach the control level. Dimethyl sulfoxide was as effective as 1,2 propanediol for obtaining high survival and fertilization rates. These results support the hypothesis that the high concentration of sodium in conventional freezing media is detrimental to cells and show that choline is a promising replacement for sodium. Reducing or eliminating sodium may allow oocytes and other cells to be frozen more efficiently.