Introduction: Melatonin acts as an indirect antioxidant and is a powerful direct free radical scavenger and direct responses to melatonin in the gonads are detected. This study aims to investigate the influence of different doses of melatonin on preantral follicle development and oogenesis of in vitro cultured mouse ovarian follicles.

Materials and Methods: Preantral follicles with diameters of 150–175 μm were mechanically isolated from NMRI mouse ovaries. Follicles were cultured in droplets of α-minimal essential medium (α-MEM) supplemented with 5% FBS, 100 mIU/ml rhFSH, 1% ITS, 100 IU/ml penicillin and 100 μg/ml streptomycin in conjunction with varying doses of melatonin (0, 1, 10, 100 nM and 100, 500 pM) for six days. On day six, in vitro ovulation was induced by the addition of hCG/rEGF to the culture medium and after 16-20 h the maturation state of the oocytes was assessed.

Results: There was a significant (P<0.05) decrease in the number of surviving follicles in the groups that received 10, 100 nM and 500 pM melatonin compared to the other groups. After induction of in vitro ovulation, follicles in groups that received 1, 10, and 100 nM melatonin had higher ovulation rates (P<0.05) compared with the other groups. Oocyte maturation capacity was adversely influenced by five concentrations of melatonin and GV arrest was significantly higher compared to the control group (P< 0.01).

Conclusions: Our data indicates that a dose of 100 pM melatonin has no toxic effects on follicular development and can be used to reduce oxidative stress in follicle culture systems.