Osmotically inactive volume, hydraulic conductivity, and permeability to dimethyl sulphoxide of human mature oocytes

Controlled ovarian stimulation during an in vitro fertilization cycle usual ly produces large numbers of oocytes and, consequently, it is likely that m ore embryos will be generated than can be transferred in a given cycle. It is desirable to freeze-bank surplus oocytes before insemination to avoid th e ethical and legal complications of disposing of or storing embryos. Altho ugh many attempts have been made to cryopreserve human oocytes, to date, po st-thaw survival has been poor, and viable pregnancies after in vitro ferti lization have been rare. A possible explanation for the lack of success is that the freezing methods have been adapted from animal studies but have no t been optimized for the human oocyte. In this study, video microscopy was used to determine the volumetric responses of mature human oocytes to chang es in osmolarity during preparation for freezing. A Boyle van't Hoff plot o f data collected in static experiments with fresh human oocytes gave a valu e of 0.19 +/- 0.01 (mean +/- SEM) for the osmotically inactive volume. Dyna mic measurements during exposure to dimethyl sulphoxide at room temperature (22 degrees C) were analysed by a two-parameter transport model and produc ed values of 1.30 x 10(-6) cm atm(-1) s(-1) for the hydraulic conductivity of the plasma membrane and 3.15 x 10(-5) cm s(-1) for dimethyl sulphoxide p ermeability (chi-squared = 0.43, df = 20) of fresh human oocytes. Oocytes t hat had failed to fertilize had a slightly lower hydraulic conductivity and dimethyl sulphoxide permeability and, after exposure to 1.5 mol dimethyl s ulphoxide l(-1), these cells appeared to become permeable to normally imper meable solutes. These permeability properties have been used to design a pr otocol for the addition and removal of dimethyl sulphoxide to control the m agnitude of volumetric changes.