Abstract

The high level of cyclic adenosine monophosphate (cAMP), which is provided to the oocytes from cumulus cells via gap junctional complexes in cumulus-enclosed oocytes (CEOs), is known to contribute to meiotic arrest at the germinal vesicle (GV) stage of CEOs. However, whether intraoocyte cAMP during the period of in vitro maturation (IVM) affects postfertilization developmental competence of mouse oocytes still remains unclear. The aim of this study was to examine the effects of FSH or dibutyryl cAMP (dbcAMP) treatment during IVM on in vitro development of mouse oocytes after in vitro fertilization (IVF). Whether a junctional association between cumulus cells and the oocyte would be essential for a cytoplasmic maturation-promoting effect was also examined. CEOs were isolated from and eCG-primed 3-week-old ICR mouse by rupturing preovulatory follicles with needles in M16 medium with 5% FCS and essential and nonessential amino acids (basal medium). IVM media used were basal medium without (control) or with 100 µm dbcAMP or 1 IU mL^–1 FSH. Carbenoxolone (100 µm, CBX), an inhibitor of gap junction, was used to inhibit a junctional association between cumulus cells and the oocyte. Denuded oocytes (DOs) were prepared by repeatedly pipetting in basal medium with 0.2% hyaluronidase. CEOs and DOs were cultured in IVM media at 37°C under 5% CO₂ in air for 16.5 h, and then transferred to TYH medium (a modified Krebs-Ringer bicarbonate medium) containing 0.4% BSA, followed by insemination with capacitated sperm. After 6 h of IVF, inseminated oocytes were cultured in KSOM medium with 0.3% BSA. Development to the 2-cell and blastocyst stages was estimated at 24 h and 120 h after IVF, respectively. All experiments were done in 3 replicates, and the statistical analysis was carried out by ANOVA and Fisher's protected least-squares difference (PLSD) test. When CEOs were matured in IVM media, the rates of postfertilization development to the 2-cell and blastocyst stages of oocytes matured in the control medium were very low(29% and 13%, respectively), whereas those of oocytes matured with FSH or dbcAMP significantly increased (FSH: 61% and 52%, dbcAMP: 63 and 57%, respectively, v. control; P < 0.05). Next, when CEOs were matured in basal medium with 1 IU mL^–1 FSH and 100 µm CBX, the developmental rate to the 2-cell stage (56%) was similar to that in medium with FSH alone (61%) but the rate to the blastocyst stage (40%) was little lower compared with that in medium with FSH alone (52%), although not significantly different (P > 0.05). Furthermore, when DOs were matured in IVM media, the developmental rates to the blastocyst stage after IVF of the oocytes matured with FSH or dbcAMP significantly increased (FSH: 25%, dbcAMP: 15%; P < 0.05) compared with those in control medium (7%). Taken together, it is suggested that increasing the concentration of intraoocyte cAMP during the IVM period is important to improve the developmental competence after IVF of mouse oocytes, and that the competence is acquired in part in a cumulus-oocyte junctional communication-independent manner.