The influence of cryopreservation on murine oocyte water permeability and osmotically inactive volume

Osmotic experiments were performed on unfrozen (N = 18) and cryopreserved (N = 21) ICR murine oocytes in order to determine whether a standard cryopreservation process alters membrane water permeability (hydraulic conductivity, Lp) and/or osmotically inactive volume (Vb). Oocytes, initially in an isotonic (288 mOsm) NaCl solution, were exposed to 900 mOsm NaCl in a microdiffusion chamber. Cell size changes were videotaped and analyzed using a parameter estimation program. Best estimates for a two-parameter model (Lp and Vb) which includes the osmotically inactive volume as a fitting parameter are presented for the first time. The cryopreservation process produced no significant difference between the mean Lp or the mean Vb values for the unfrozen control population (Lp = 0.64 +/- 0.15 micron/min/atm, Vb = 24.7 +/- 2.9%) and the cryopreserved population (Lp = 0.63 +/- 0.12 micron/min/atm, Vb = 28.0 +/- 10.8%). While the cryopreservation process did not cause significant changes in the mean values of Lp, Vb, or the variability of Lp, it did produce more variability of Vb. The cause of the increased variability of Vb produced by cryopreservation is unknown. These results suggest that the osmotic properties of unfrozen control oocytes can be used as a reasonable approximation for frozen-thawed oocytes. They also suggest that multiple parameter models and parameter estimation methods may be useful in developing a more comprehensive understanding of the more subtle alterations in osmotic properties that were detected here. Statistical tests were also used for the first time to confirm the assumption that all of the experimental populations were derived from normal distributions.