Osmotic responses and tolerance limits to changes in external osmolalities, and oolemma permeability characteristics, of human in vitro matured MII oocytes.

Background: Oocyte cryopreservation remains a realistic objective, provided that more systematic approaches are applied, such as thorough analysis of the oocyte oolemma permeability to water and diverse cryoprotectants.

Methods: We prospectively investigated volume changes over time at different temperatures (30 degrees C, 22 degrees C and 8 degrees C) of human metaphase II (MII) oocytes (obtained in stimulated ICSI cycles and matured in vitro from the germinal vesicle stage) when exposed to changes in external osmolality. We also investigated human in vitro matured (IVM) oocytes membrane permeability characteristics at 22 degrees C to 1,2-propanediol (PG) and dimethylsulphoxide (DMSO) and at 30 degrees C, 22 degrees C and 8 degrees C to ethylene glycol (EG), and calculated corresponding oocyte oolemma permeability coefficients (Lp and Pcpa). Furthermore, we investigated the osmotic tolerance limits of IVM oocytes exposed to changes in external osmolality as assessed by their developmental competence during the course of 72 h after ICSI.

Results: The results of our studies describe human oocyte membrane permeability coefficients for EG at 30 degrees C (2.85+/-0.15x10(-3) cm/min), 22 degrees C (1.17+/-0.60x10(-3) cm/min) and 8 degrees C (0.37+/-0.15x10(-3) cm/min). Furthermore, at 22 degrees C the EG oolemma permeability coefficient was lower than that of PG and DMSO (1.17+/-0.60x10(-3) cm/min versus 2.15+/-0.70x10(-3) and 1.56+/-0.38x10(-3) cm/min, respectively). Our results also indicate, that human IVM MII oocytes tolerated exposure to solutions in the range of 39-2264 mOsmol/kg H2O as assessed by the oocytes' developmental competence after exposure.

Conclusions: The results of the present study may contribute to a better understanding of the biology and cryobiology of human oocytes, and to the design of better and more robust cryopreservation (freezing or vitrification) protocols.