Effect of osmotic stress on the developmental competence of germinal vesicle and metaphase II stage bovine cumulus oocyte complexes and its relevanceto cryopreservation

The effects of osmotic stress on germinal vesicle (GV) and metaphase II (MI I) stage bovine cumulus oocyte complexes (COCs) were evaluated by first exp osing them to various anisotonic NaCl solutions (75, 150, 600, 1200, 2400, and 4800 +/- 5 m0sm/kg) for 10 min and then returning them to isotonic TL-H epes solution (270 +/- 5 m0sm/kg) at 20 +/- 2 degrees C. Percentages of ooc yte maturation, fertilization, polyspermy, cleavage, and blastocyst formati on were measured as endpoints. Exposure to anisotonic conditions had a sign ificant (P < 0.05) effect on the developmental competence of both GV and bo vine MII COCs. Oocytes at the GV stage were more sensitive to anisotonic st ress than MII oocytes (P < 0.05). None of the GV oocytes developed to the b lastocyst stage after exposure to hypertonic conditions (2400 or 4800 m0sm solutions), while exposure to hypotonic conditions (75 or 150 m0sm solution s) resulted in significantly lower (P < 0.05) blastocyst formation (9% and 13%, respectively) compared to the isotonic control (25%). A dramatic decre ase to 4% development to blastocyst was observed for MII oocytes following exposure to a 4800 m0sm solution. Blastocyst formation of MII oocytes which were exposed to 75, 150, 600, 1200, or 2400 m0sm solutions were similar (1 5%, 20%, 18%, 14%, and 13%, respectively; P > 0.05), but lower (P < 0.05) t han those in the control group (29%). Exposing GV oocytes to anisotonic con ditions increased polyspermic fertilization (P < 0.05), although MII oocyte s were not similarly affected (P > 0.05). These data support the hypothesis that osmotic stress is detrimental to bovine oocytes and must be considere d when developing optimized cryopreservation procedures for these cells. (C ) 2000 Wiley-Liss, Inc.