en Original Article Original Article - Background: In vitro maturation (IVM) of oocytes is a promising technique to reduce the costs and avert the side-effects of gonadotropin stimulation for in vitro fertilization (IVF). The pregnancy rates from oocytes matured in vitro are much lower than those of in vivo stimulation cycles, indicating that optimization of IVM remains a challenge. Objective: In this study, we investigated the effect of cumulus cells on maturation and fertilization rate of immature oocytes (Germinal vesicle). Materials and Methods: Germinal vesicle (GV) oocytes were recovered from 6-8 weeks old Balb C female mice 48hr after injection of 10 IU pregnant mare serum gonadotropin (PMSG). Collected oocytes were divided into two groups. Group A: GV oocytes without cumulus (denuded oocyte). Group B: GV oocytes with cumulus cells (cumulus-oocyte complex). The oocytes in both groups were cultured in TCM-199 medium in a humidified atmosphere of 5% CO2 in air at 37�C. The maturation, fertilization and developmental rates were recorded after 24hr. Results: Maturation, fertilization and developmental rates in denuded oocytes (DO) were 65.1%, 68.02%, 78.63% respectively, and in cumulus-oocyte complex (COC) were 78.20%, 85.57% and 85.05%, respectively. The maturation, fertilization and developmental rates of COC were significantly higher than those of DO (p<0.05). Conclusion: The results show that cumulus cells have beneficial effects on maturation, fertilization and cleavage rates of mice oocytes. Immature oocyte, In vitro maturation, Mouse, Cumulus cells 74 78 http://ijrm.ir/browse.php?a_code=A-10-100-49&slc_lang=en&sid=1 Reza Mahmoudi Yes Aligholi Subhani No Parichehr Pasbakhsh No Farideh Etesam No Iraj Amiri No mozhdeh Salehnia No Farid Abolhasani No