DIFFERENTIAL-EFFECTS OF CRYOPRESERVATION ON NUCLEAR OR CYTOPLASMIC MATURATION IN-VITRO IN IMMATURE MOUSE OOCYTES FROM STIMULATED OVARIES

The aim of this study was to develop a maturation protocol for immatur e oocytes and assess the protocol with cryopreserved oocytes, Nuclear maturation (mature spindle and aligned chromosomes) occurred irrespect ive of the treatment regime: 71-89% of oocytes matured in vitro had a normal spindle and chromosomes compared with 87% matured in vivo, Fert ilization rates were not significantly different from those of in-vivo matured oocytes, Of the maturation treatment regimes investigated, th e initial treatment producing best development to blastocyst (cytoplas mic maturation) involved a 2 h incubation in standard maturation mediu m (SMM) containing 7.5 IU follicle stimulating hormone (FSH) followed by 14 h in SMM plus 7.5 IU FSH:luteinizing hormone with follicular cel ls [62% (range 49-69)], The addition of 1 ng/ml epidermal growth facto r (EGF) in this protocol resulted in development [75% (range 71-81)] t hat was not significantly different from invivo matured oocytes [82% ( range 73-90)], Exposure of the oocytes to 1.5 M dimethylsulphoxide (DM SO) did not affect fertilization or development rates. Following a slo w-cool/thaw freezing regime, 81% (range 74-89) of the oocytes were mor phologically normal, i,e, had a spherical shape with an intact zona an d oolemma; they had, however, lost their previously attached cumulus a nd corona cells. Maturation of frozen-thawed oocytes in the presence o f EGF gave good fertilization rates but poor development rates [80% (r ange 77-86) and 37% (range 33-40) respectively], In conclusion, the be st maturation, both nuclear and cytoplasmic. occurred in the presence of a combination of gonadotrophins, EC;F and follicular cells, Oocytes cryopreserved using a slow-cool/thaw regime can be matured to produce blastocysts after in-vitro fertilization.