DETRIMENTAL EFFECTS OF SODIUM DURING MOUSE OOCYTE CRYOPRESERVATION

Cryopreservation is an established way of storing embryos, but effecti ve methods are not available for freezing eggs. Most freezing damage i s caused by high solute concentration (solution effects) and intracell ular ice. Sodium salts are the major components of cryopreservation me dia, and the main contributor to the solution effects. The present exp eriments 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. Survi val, fertilization, and development were inversely related to the conc entration of sodium in the freezing medium. Oocytes frozen in a cholin e-based medium had the highest (p < 0.001) survival and development ra tes. The absence of serum during thawing inhibited fertilization, wher eas exposure to serum or opening the zona allowed fertilization to rea ch the control level. Dimethyl sulfoxide was as effective as 1,2 propa nediol for obtaining high survival and fertilization rates. These resu lts support the hypothesis that the high concentration of sodium in co nventional freezing media is detrimental to cells and show that cholin e is a promising replacement for sodium. Reducing or eliminating sodiu m may allow oocytes and other cells to be frozen more efficiently.